M.A. Périard and P. Chaloner
(PDF Version - 53 K)
Table of Contents
The article: "Diagnostic X-Ray Imaging Quality Assurance: An Overview" appeared in The Canadian Journal of Medical Radiation Technology, October 1996, 27(4), pgs. 171-177.
A basic diagnostic imaging quality assurance program is a regulatory requirement in many provincesFootnote 1,Footnote 2Footnote 3Footnote 4Footnote 5 and in federal institutions.Footnote 10 An ineffective quality assurance program can lead to poor quality radiograms that can impair diagnosis, increase operating costs and contribute to unnecessary radiation exposure to both patients and staff. Any extension of the basic quality assurance program is the responsibility of each x-ray facility. To achieve maximum benefit, all levels of management and technical staff must support and participate in the operation of a well-defined program. This article outlines the essential aspects of a quality assurance program and is intended to encourage the review of a moderate size hospital's x-ray imaging quality assurance procedures.
Paul Chaloner is Head, Inspection Unit for the X-ray Section, Radiation Protection Bureau, Health Canada. He previously worked as a Radiation Health officer for Workers' Health, Safety and Compensation in Alberta. Prior to that, he was employed as a General Duty Technologist, Special Procedures and Area Supervisor Technologist at the Edmonton General Hospital.
Michel A. Périard is a Radiation Safety Inspector with the X-ray Section. Formerly he was with the National Dosimetry Services of Health Canada where he worked on thermoluminescent extremity dosimeters. Prior to that he worked at the Department of National Defense in the research and development of radiation detection instruments.
The authors wish to thank
- Mr. P. Dvorak,
- Mr. C. Lavoie and
- Dr. P.J. Waight of the Radiation Protection Bureau for their review comments and recommendations. Discussions with and comments by the authors' colleagues are also gratefully appreciated.
Each year, more than 20 million diagnostic x-ray procedures are performed in Canada. Although the radiation exposure connected with these procedures cannot be avoided, there are means to reduce it as much as possible. For the protection of patients, workers and the public for example, federal and provincial government agencies enact legislation and take necessary steps to ensure that only safe and properly installed x-ray equipment is used in Canadian diagnostic x-ray facilities, for the protection of patients, workers and the public. Also, in most provincesFootnote 1Footnote 2Footnote 3Footnote 4Footnote 5 and in federal institutionsFootnote 10 there is a requirement that each diagnostic x-ray facility have in place a basic quality assurance (QA) program to control the quality of diagnostic images. Any extension of the basic program is determined by the radiology department's management and QA committee.
A successful QA program requires that all staff within the radiology department understand the goals set out in the program and take an active part in achieving its objectives. Any program lacking genuine interest from its staff and initiated only to satisfy a regulatory requirement is unlikely to produce optimal results. Meticulous care is required in the quality control of diagnostic imaging equipment to ensure good quality radiograms.
This article is divided into two parts.
- Part I discusses the essential aspects of a QA program recommended for implementation in a moderate-sized hospital's x-ray facilities.
- Part II contains detailed worksheets designed to help radiology personnel charged with reviewing their current diagnostic x-ray imaging QA procedures.
Part I. Definition
It is necessary to define, at least briefly for the purposes of this article, the general concepts of quality assurance and quality control and to review the principal objective of a radiology quality assurance program.
Quality assurance (QA) is a program used by management to maintain optimal diagnostic image quality with minimum hazard and distress to patients. The program includes periodic quality control tests, preventive maintenance procedures, administrative methods and training. It also includes continuous assessment of the efficacy of the imaging service and the means to initiate corrective action.
The primary goal of a radiology quality assurance program is to ensure the consistent provision of prompt and accurate diagnosis of patients. This goal will be adequately met by a QA program having the following three secondary objectives:
- to maintain the quality of diagnostic images;
- to minimize the radiation exposure to patient and staff; and
- to be cost effective.
Quality control (QC) consists of a series of standardized tests developed to detect changes in x-ray equipment function from its original level of performance. The objective of such tests, when carried out routinely, allows prompt corrective action to maintain x-ray image quality. It is important to note that the ultimate responsibility for quality control rests with the physician in charge of the x-ray facility, not with the regulatory agency.
Radiology Department QA Committee
In a hospital x-ray facility, the radiology department should establish a formal Quality Assurance Committee (QAC). It will provide the structure required to plan and evaluate the program and to resolve quality assurance issues and problems. A QAC will also provide management with recommendations for direction to those charged with the various aspects of the program.
The QAC should have an overall documented strategy with clearly defined work plans to achieve the goals and objectives of the radiology department. The committee should include representatives from all levels of the radiology staff, meet at regular intervals and report directly to the department's management. It should recommend program policies to management and outline program specifics such as the duties and responsibilities of the staff. In addition, it should formulate the standards for image quality and regularly review the effectiveness of the program. A formal QAC will promote the importance of and encourage participation in the department's QA program.
Radiology Department QA Program
A documented QA program should be developed, under the guidance of the QAC, specifically to address the needs of the radiology department. The QA program should include a written plan of action outlining policies and procedures. It should clearly define the goals and objectives of the department.
The QA program should cover both QC testing techniques and administrative procedures. The latter are to verify that QC testing is effective, i.e., the tests are performed regularly and correctly, the results evaluated promptly and accurately, and the necessary action taken. They include recommendations regarding the responsibility for quality assurance action, staff training, equipment standards, and the selection of the appropriate equipment for each examination. The quality assurance program should include the means to evaluate the effectiveness of the program itself, e.g., ongoing retake rate and causes, equipment repair and replacement costs and analysis of trends in the equipment performance.
A QA manual should be developed in a format that allows easy reference by staff and permits future revision by management and the QAC. The content of the manual should be determined by management with the advice of the department's QAC but it should contain the following items considered essential:
- a list of the radiology department QAC personnel and an outline of their duties, authority and responsibilities;
- a list of personnel involved in QC testing and an outline of their responsibilities;
- guidelines for equipment specification writing;
- a list of the equipment parameters to be measured and the frequency of monitoring for each x- ray system and system component;
- a description of the performance standards, with specific tolerance limits established for each QC test;
- a description of the method used to measure each parameter;
- sample (blank) forms, worksheets, charts and records used for QC testing;
- guidelines for equipment acceptance testing;
- a schedule of QC testing for each equipment (monitoring frequency);
- guidelines for photographic QC;
- guidelines for recording equipment performance;
- procedures to be followed when equipment failure occurs or when test results fall outside the tolerance limits;
- guidelines for the reject-repeat analysis program;
- a list of the publications where detailed instructions for monitoring and maintenance procedures for each equipment can be found (although separate from the QA manual, service and technical operations manuals should be readily available);
- guidelines for equipment appraisal and replacement;
- guidelines for the standardization of patient exposure, e.g., patient positioning, loading factors and measurement of patient exposure;
- guidelines for quality acceptance of diagnostic radiograms; and
- a schedule of management's review QC reports and the QA program.
QA Personnel Training
The QA program should include the means to provide appropriate training for all personnel with QA responsibilities and especially those directly involved with QC testing. A continuing education program is necessary to keep personnel up-to-date. Since QC training is expensive, yet proven to be cost effective, effort will be required by hospital management to ensure that adequate financial provision be available to meet this requirement.
All staff in the radiology department should be involved in quality control. However, specific tests are usually performed more effectively by specially trained technologists. The amount of time spent on QC should be adequate to perform the functions required for an effective quality control program. QC technologists should be allowed to devote at least 50 per cent of their time to a QC program in small institutions (200 beds or less) and full time in larger institutions. Institutions with more than 500 beds may require additional help. Among the activities of the QC technologist(s) should be to:
- carry out the day-to-day QC tests on the department's photographic, radiographic and fluoroscopic imaging equipment as prescribed by the QC test schedule;
- record and/or chart the QC test measurement data;
- evaluate the test results;
- report any deterioration or trends in equipment performance to the radiology manager and staff using the equipment;
- initiate prompt corrective action and/or preventive measures when necessary;
- oversee the repair of defective equipment performed by the hospital biomedical or electronic maintenance staff or by private service companies;
- perform the required tests to confirm that defective equipment was repaired and restored to the original level of performance;
- maintain equipment performance records;
- provide monthly reports on QC activities to the radiology manager; and
- develop new QC monitoring and maintenance procedures as required.
The person in the position should report directly to the radiology manager.
Equipment Specification Writing
The QAC should assist in determining the technical specifications of new equipment being purchased, based on the facility's clinical imaging requirements. The department's QA program should provide guidelines for writing equipment specifications to assist management or the procurement committee during the equipment selection phase. The guidelines should cover the general requirements specified for equipment selection with a view to upgrade or maintain the department's standard for diagnostic imaging quality. The content of the guidelines for writing specifications for each type of diagnostic equipment system or system components intended for purchase should be determined by management, with participation from the QAC, but the following elements are considered essential:
- the desired level of quality of equipment, i.e., the system design, construction and performance that can reasonably be achieved and maintained;
- the conformance standards applicable to the equipment specifications and the facility, e.g., international, federal, provincial, local regulatory requirements and occupational health and safety codes;
- the standard of equipment performance and the tolerance limits set for each equipment parameter;
- the specific equipment acceptance testing protocol and conformance standard to be followed during the acceptance phase of the equipment purchase, i.e., the specified equipment performance criteria, specification of the testing equipment, the test methods and schedules, the delegated person(s) with authority to perform the tests and authorize the acceptance;
- the removal, reinstallation and upgrading or disposal of existing equipment;
- the delivery and installation of equipment purchased, e.g., delivery date and delivery co- ordinator, associated responsibilities and liabilities during transit;
- the level of equipment guarantees and warranties, i.e., the extent and duration of warranty, cost of equipment or component replacement and servicing schedule requirements;
- the extent and cost of service contracts with the equipment supplier, i.e., minimum response time, availability of parts and components, qualifications and availability of service personnel and cost of service calls;
- the cost of the system, system components and ancillary equipment, the cost of delivery and installation; and
- the test equipment necessary to measure the system parameters, i.e., exposure meters, irradiation time measuring devices, x-ray volt meters, focal spot test tools, aluminium attenuators, image resolution test tools and patient equivalent phantoms.
The purchase specifications should include all of the important parameters to be monitored throughout the useful life of the equipment. These should be retained for use during the acceptance testing phase.
QC Test Equipment
The adequacy of the department's QC test equipment and radiation measuring instrumentation should be reviewed periodically by the QAC. Specifications for new test equipment intended for purchase should include the following items:
- the equipment specifications (accuracy, precision, sensitivity, range, etc.);
- a calibration reference;
- compatibility with existing equipment;
- the expected useful life of the equipment;
- the availability of parts and service;
- an estimate of maintenance costs; and
- instructions and/or training in the operation of the equipment.
Specific instrumentation required to achieve an effective level of equipment QC monitoring should be determined by management with participation from the QAC, but the following basic items are considered essential:
- Processor QC monitoring equipment: sensitometer, densitometer, thermometer, stop watch or a watch with sweep second hand, and graduated transparent beaker.
- Radiographic and/or fluoroscopic x-ray QC monitoring equipment: exposure/exposure rate meter with a full range of ionisation chambers, electronic x-ray timer, electronic kVp meter, aluminium attenuators for HVL measurements, collimation accuracy test tools, high and low contrast resolution test tools, and patient equivalent phantoms.
Other QC test instruments that should be considered, depending on the complexity and type of diagnostic imaging equipment, are listed in Part II (worksheets) of this article.
Equipment Acceptance Testing
The purpose of post-installation acceptance tests is to insure that the x-ray equipment operates correctly and meets the following criteria:
- the standards of design, construction and functioning, for diagnostic x-ray equipment as specified in the Radiation Emitting Devices Regulations, Part XIIFootnote 6 and applicable provincial regulations;
- the purchase contract specifications; and/or
- the original equipment manufacturer specifications.
Acceptance tests should be performed on every diagnostic imaging system or major equipment system component purchased prior to routine service. The QAC should be directly involved in the equipment acceptance testing phase to ensure that the equipment meets the specifications indicated in the purchase agreement.
The QA program should provide documented guidelines to assist the QAC in developing the appropriate acceptance testing protocol for all major diagnostic imaging equipment purchases. The protocol should be incorporated into every purchase specification. The content of the guidelines should be determined by management with participation from the QAC, but the following elements are considered essential:
- a list of the equipment specifications and tolerances;
- the conformance standards and the tolerance limits for each parameter to be tested;
- a list of the equipment required to test each parameter;
- a detailed method (protocol) of testing for each parameter;
- a schedule for the completion of each test;
- a list of the persons authorized to perform or witness the acceptance tests; and
- a list of the persons responsible for authorizing the acceptance of each test.
The report on the acceptance tests results should contain all of the information listed above including the actual data with graphs, charts and test films for each equipment parameter tested. This report should be retained as part of the equipment performance log book and used to compare with future QC test results to assess the continued acceptability of the equipment's performance and estimate the equipment's remaining useful life.
QC Testing Program
The purpose of a QC testing program is to maintain the quality of diagnostic images. This is done with routine monitoring of photographic and x-ray equipment parameters to detect deviations of equipment performance and take prompt corrective action. Periodic monitoring should not be eliminated if the test results indicate relatively stable equipment performance. Small but progressive changes in image quality, not readily detectable to the eye, will be more easily noticed using standardized test procedures and specific test equipment. The most important objectives of a routine QC testing program are to:
- establish a baseline against which future measurements can be compared to maintain the original level of performance;
- assist in detecting and diagnosing the cause of any deterioration in equipment performance;
- promptly correct any deterioration in equipment performance when the cause is known;
- detect defects on installation, or after major repairs, which may adversely affect image quality or patient dose; and
- enable comparable loading factors to be used on similar x-ray machines, where appropriate.
The QC testing program should be divided into component parts to cover each area effectively. For example:
- general radiography, i.e., photographic, radiographic and fluoroscopic equipment QC;
- mammography, i.e., photographic and radiographic equipment QC; and
- computed tomography, i.e., image quality and radiographic equipment QC.
X-Ray Equipment QC
It is advisable to determine and document the many important aspects of the monitoring and maintenance activities in the QC manual. The manual should provide the following information:
- a list of all x-ray and ancillary diagnostic imaging equipment systems located in each x-ray room or operating room under the control of the radiology department;
- a list of the parameters monitored for each equipment;
- the priority given to each parameter measured, i.e., "desirable" or "essential" based on equipment type, types of examinations and experience;
- the frequency of testing required for each parameter, i.e., daily, weekly, monthly, etc.;
- the acceptable range within which the equipment must function;
- the tolerance levels outside which the equipment should not continue in use;
- detailed QC monitoring protocols (test sheets) clearly describing the title of the test, the purpose of the test, the frequency of testing, the test equipment required, the test procedure to follow, and the tolerance limits for each test of each part or type of equipment; and
- a list of the most suitable person(s) to carry out each routine start-up procedure, routine QC tests, preventive maintenance procedures, corrective action or repairs. For example, only qualified x-ray service personnel should repair equipment.
Photographic Equipment QC
Undoubtedly the most important item in diagnostic imaging quality control is routine (daily7) photographic processor control. Contaminated film processing solutions or small changes in developer temperature affect both the image quality and the patient exposure. For routine processor control to be effective, the sensitometric data must be evaluated promptly and the necessary corrective actions taken before x-rays are taken of patients. This is to ensure that the x-rays will be processed correctly and to minimize retakes. Often, the early morning's processor control test data are evaluated later in the day and the necessary corrective actions are taken too late to be effective. Unless the processor is monitored closely to ensure optimum performance then all other efforts at QC will probably be in vain. Along with processor sensitometry there are many other tests that should be done to ensure proper operation of the film processing unit. These tests are listed in "Radiographic Quality Control, Minimum Standards"Footnote 7 and Table A.1 in NCRP Report No. 99.Footnote 8
The radiology department should adopt an effective silver recovery and chemical effluent control program. Silver recovery can be split into two parts, that from the fixer and that from old or discarded radiograms. Most institutions recognize that the silver in the film is recoverable and may represent as much as 10 per cent of the purchase price of good "green" film, depending on the current silver and film prices. New silver recovery systems currently available will recycle the fixer and developer and minimize the negative effects on the environment by removing silver salts and other toxic substances from the effluent.
The effectiveness of the QC monitoring program should be reviewed annually by the QAC. Guidelines such as "Radiographic Quality Control, Minimum Standards"Footnote 7 or Appendix A: "Summary of Quality Control Tests"Footnote 8 or "Diagnostic X-ray Equipment and Facility Survey"Footnote 9 should be reviewed to determine a list of "essential" and "desirable" QC tests to be performed routinely.
Equipment Performance Records and Record Keeping
The method of record keeping should be determined by management with advice from the QAC. The records may be maintained in a file or individual log books or in a computerized data base. Commercial software is available and can be customized with relative ease to record the data collected. In some cases the measured kilovoltage, exposure time and exposure is transmitted directly from the radiation detection meter to the computer.
The records should be maintained in a ready-to-use form and the information be readily available to all staff. The information should be complete, up-to-date and presented in a form suitable for departmental reviews and provincial radiation safety audits.
The equipment performance record should clearly identify the equipment and its location. It is recommended to keep one log book for each x-ray room. The required equipment performance data should be determined by the QAC but the following elements are considered essential:
- the equipment system identification, i.e., the name of manufacturer, the model designation and serial number, the date and country of manufacture;
- the equipment location;
- a copy of the equipment acceptance test report;
- a copy of the current provincial radiation safety survey;
- a copy the current federal, provincial or territorial registration certificate;
- the QC monitoring records (data, graphs, charts, etc.);
- the equipment service and repair record including service frequency and costs; and
- the equipment down-time record.
Equipment Appraisal and Replacement Policy
The radiology department's financial policy should include provision for the replacement of x-ray equipment. Although x-ray equipment normally has a life expectancy of 10 to 15 years, replacement costs often require a large share of the capital available. Even within this relatively long period there should be plans for the replacement of major components that deteriorate rapidly with use and age, such as image intensifiers, x-ray tubes and ancillary equipment like intensifying screens.
The department's QA program should provide documented equipment appraisal and replacement policy guidelines to assist management in the financial planning related to the replacement of aging equipment. The guidelines should be based on an estimate of the equipment's remaining useful life determined by the following long-term equipment performance items:
- decreasing image quality, increasing exposure to patient and staff, and decreasing patient flow; and
- increasing operating costs, increasing number of service calls, and increasing equipment down time.
Standardization of Exposure
The following three sections relate to patient exposure: radiographic positioning, loading factors and entrance-skin-exposure. The recommendations listed in each section are given with a view to maximize the diagnostic quality of radiograms and minimize patient exposure.
The radiographic positioning manuals should be reviewed regularly by the QC technologist and the information updated and maintained in clear typewritten form. The radiologists should determine which views are required for each examination, e.g., lumbar spine: AP, LAT, L5-S1, etc. The contents of the radiographic positioning manuals should be determined by management with participation from the QAC, but the following information should be included:
- the body part to be examined;
- the projection to be shown;
- the number of projections required;
- the part rotation;
- the image receptor size used for each view;
- the source-to-image receptor distance;
- the tube angle;
- the trajectory of the central ray;
- the details of structures to be shown; and
- detailed instructions to correctly position the patient, the x-ray tube and the image receptor to obtain satisfactory and consistent results. Illustrations are recommended.
The loading factors manual should be reviewed regularly (e.g., monthly) by the QC technologist and each chart updated and maintained in clear printed form. Changes should not be maintained in hand-written form. The contents of the loading factors manual should be determined by management with participation from the QAC but the following information should be included:
- the body part and projection;
- the patient size or body part thickness;
- the optimum kVp;
- the optimum mA and time, or mAs, or AEC including the specific cell;
- the film/screen combination selection;
- the grid selection; and
- the tube focal spot size selection.
Any change made to loading factors, to compensate for decreased diagnostic image quality, should be investigated to determine the cause. Poor film processing is the most frequent cause of a decrease in image quality.
The method used to derive loading factors, i.e., fixed kVp or fixed mAs, should be determined by the radiologist. Loading factors derived from the subjective evaluation of patient size are unreliable and do not provide consistent radiographic images All patients should be measured and the loading factors adjusted accordingly. For example, to compensate for variations in patient sizes, using fixed mAs techniques, the method of increasing or decreasing the kilovoltage by approximately 2 kVp per centimetre of body part can be employed.
Commercial software is available to generate loading factor charts for the examples listed above in both DOS and Windows. The loading factor charts can easily be updated on the computer and printouts made.
Where phototiming (automatic exposure control) is used, the specific photocell(s) to be utilized should be specified and the density setting defined for each range of patient sizes. Each range of patient sizes (small, medium or large), for both adults and children, should be defined and the selection derived from actual physical measurements of the patient.
The entrance-skin-exposure (ESE) should be measured and recorded for standard radiographic examinations or projections. The standard projections recommended for routine QC ESE measurement should be based on those found in Tables 1 and 2 of Safety Code-20AFootnote 10 and are listed in Table 1.
The recommended upper limits for ESE given in the table above are based on:
- the third quartile levels from unpublished (Canadian) Nationwide Evaluation of X- Ray Trends (NEXT) data; and
- a reference patient having the anthropometrical characteristics shown in Table 2.
Note: In practice, it should be feasible to have actual ESE substantially lower than these limits.
This series of ESE measurements should be performed at least semi-annually for every radiographic equipment system or each time the radiographic system is repaired or serviced. The ESE measurement results should be used to update the information contained in the radiographic technique chart.
Commercial software is available and may be used to calculate the ESE. After the basic information is entered into the computer program, the computer is able to calculate the ESE for any given examination or loading factors or patient size.
The measurement of standard fluoroscopic exposure rates for the average-sized patient should be performed regularly for all fluoroscopic equipment systems. The frequency of measurement should be monthly and always after service or repair of the fluoroscopic system. The measurements should include all exposure rate delivery modes where applicable and measured in both the manual and automatic exposure rate control mode operation. The radiologist should be informed regularly about the results.
The maximum exposure rate should be measured at least every six monthsFootnote 7 for every fluoroscopic equipment system. Measurement should include both the manual and the automatic brightness control mode operation.
The loading factors used during fluoroscopic procedures (time, kilovoltage and tube current) should be recorded on the patient requisition form or the patient's chart.
Acceptance Criteria for Diagnostic Radiograms
Guidelines should be developed to determine the minimum level of diagnostic quality acceptable to the radiologist. Comprehensive acceptance criteria should be established for all radiographic views. The department's radiologist should be directly involved in developing these guidelines. The following additional diagnostic quality acceptance criteria are recommended for consideration:
- the visibility of predetermined landmarks clearly defined for each radiographic view;
- the acceptable film density range measured at predetermined anatomical landmarks; and
- three limits of acceptability that clearly define whether the x-ray technologist forwards the radiogram to the radiologist for reporting, or the x-ray technologist consults with the radiologist, or the radiogram is rejected and a repeat is done.
These acceptance criteria should be further refined, in an effort to closely approximate the radiologist's subjective impressions of image quality. They should be very useful to "pre-screen" radiograms of questionable diagnostic quality before being viewed by the radiologist. This information is necessary, e.g., when new staff are introduced to the department, when staff are working alone on weekends or evenings, etc., and will provide guidelines to follow when the radiologist is not available.
Reject-Repeat Analysis Program
Guidelines for an effective reject-repeat analysis program (RRAP) should be documented and included into the department's QC protocol manual. Such guidelines are described in "Quality Control in Diagnostic Imaging Equipment."Footnote 11 Staff must be made aware that the object of a reject-repeat analysis program is not to embarrass anyone, but to identify problem areas and train or retrain those who are unable to perform certain radiographic examinations. In turn this will reduce the number of rejected radiograms and reduce patient and occupational dose. The RRAP guidelines should also include documented standards to aid in the analysis and classification of rejected radiograms. Such guidelines are necessary for consistent classification and comparison of data.
The basic RRAP should be refined to determine how many rejected radiograms or repeats were acceptable and did not need repeating and the reasons for this determination. The radiologist should be involved in this aspect of the program. A thorough analysis may find that some rejected radiograms were repeated unnecessarily. The information derived from the reject analysis program can be of benefit and should be communicated to all x-ray staff, for example during in-service training workshops.
QA Program Review
The diagnostic imaging QA program should be reviewed regularly by the department's QAC to determine the effects of each quality assurance action. Among the items recommended for review are:
- the reports on monitoring and maintenance techniques should be reviewed at least quarterly to ensure that these are being performed effectively;
- The monitoring and maintenance techniques and their schedules should be updated at least annually to ensure that they continue to be appropriate and in step with the latest developments in QA;
- the standards for image quality should be reviewed at least annually to ensure that they are consistent with the state of the art and the needs and resources of the facility;
- the effectiveness of QA procedures should be reviewed at least annually to determine where improvements are required; and
- the approach and effectiveness of the QA program compared to that of outside groups (scientific and/or professional societies, national authorities, etc.), to identify areas where improvements can be made.
- Seibert JA et al., eds. Specification, acceptance testing and quality control of diagnostic x-ray imaging equipment. Woodbury, New York: American Association of Physicists in Medicine, (American Institute of Physics, Inc.), 1994.
- Quality assurance in diagnostic radiology. Geneva: World Health Organization, 1982.
- Guidelines for a radiology department. Ontario Medical Association and Ontario Hospital Association, 1984.
- McKinney WEJ. Radiographic processing and quality control. Philadelphia: J.B. Lippincott Co., 1988.
- International Electrotechnical Commission, Technical Report, Evaluation and Routine Testing in Medical Departments. (1223-1) Part 1: General Aspects, (1993-07); (1223-2-1) Part 2-1: Constancy Tests -Film Processors, (1993-07); (1223-2-2) Part 2-2: Constancy Tests - Radiographic Cassettes and Film Changers - Film-screen Contact and Relative Sensitivity of the Screen-cassette Assembly (1993-07); (1223-2-3) Part 2-3: Constancy Tests -Darkroom Safelight Conditions (1993-07).
|Examination (Projection)||Entrance Skin Exposure|
Recommended Upper Limits
|Cervical Spine (A/P)||137||35.0|
|Thoracic Spine (A/P)||380||98.0|
|Full Spine (A/P)||263||68.0|
|Lumbo-Sacral Spine (A/P)||614||158.0|
|Retrograde Pyelogram (A/P)||539||139.0|
|Body Part||Reference Patient Thickness (cm)|
- Date modified:
Division of Radiation Oncology, Medanta Cancer Institute, Medanta-The Medicity, Gurgaon, Haryana 122001, India
Copyright © 2014 Shikha Goyal and Tejinder Kataria. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In modern day radiotherapy, the emphasis on reduction on volume exposed to high radiotherapy doses, improving treatment precision as well as reducing radiation-related normal tissue toxicity has increased, and thus there is greater importance given to accurate position verification and correction before delivering radiotherapy. At present, several techniques that accomplish these goals impeccably have been developed, though all of them have their limitations. There is no single method available that eliminates treatment-related uncertainties without considerably adding to the cost. However, delivering “high precision radiotherapy” without periodic image guidance would do more harm than treating large volumes to compensate for setup errors. In the present review, we discuss the concept of image guidance in radiotherapy, the current techniques available, and their expected benefits and pitfalls.
Radiotherapy has always required inputs from imaging for treatment planning as well as execution, when the treatment target is not located on the surface and inspection and visual confirmation are not feasible. Traditional radiotherapy practices incorporate use of anatomic surface landmarks as well as radiologic correlation with two-dimensional imaging in the form of port films or fluoroscopic imaging.
Broadly, imaging has two major roles in radiotherapy:(a)Sophisticated imaging techniques such as contrast enhanced computed tomography (CECT) scans, magnetic resonance imaging (MRI), positron emission tomography (PET) scans, and angiography obtain three-dimensional (3D) structural and biologic information which is used to precisely define the target and thus enable precise and accurate treatment planning with shaped beams in isocentric or non-isocentric geometry.(b)“In-room” imaging methods (planar, volumetric, video, or ultrasound-based) obtain periodic information on target position and movement (within the same session or between consecutive sessions), compare it with reference imaging, and give feedback to correct the patient setup and optimize target localization. They also have the potential to provide feedback that may help to adapt subsequent treatment sessions according to tumor response.
More specifically, modern day radiotherapy regards the latter application with “in-room” imaging as “image guided radiation therapy” (IGRT).
Modern external beam radiotherapy techniques such as intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), or stereotactic radiotherapy (SRT) have helped reduce the safety margin around the target volumes thus allowing for lower normal tissue doses without compromising delivery of tumoricidal doses. However, there is a great deal of uncertainty in accurately defining of the position of targets during the delivery of fractionated radiotherapy, both during a given fraction and between successive fractions. Targets that may move during treatment due to respiratory or peristaltic movements or with cardiac pulsations create an even bigger challenge. Hence, there is need to develop and implement strategies to measure, monitor, and correct these uncertainties. This has led to evolution of various in-room imaging technologies which enable evaluation and correction of setup errors, anatomic changes related to weight loss or deformation, or internal organ motion related to respiration, peristalsis, or rectal/bladder filling.
Brachytherapy treatment planning also incorporates orthogonal X-ray imaging and fluoroscopy for guiding brachytherapy catheter/applicator placement, volumetric imaging with CT or MRI for applicator identification and reconstruction, and plan optimization in three dimensions based on imaging. Isodose distribution is reviewed and optimized on visualizing dose distribution to the target as well as critical structures. This adds to treatment efficacy and safety.
2. The Concept of IGRT
Though technically complex, surgical procedures enable the operating surgeons to directly visualize and handle their targets, thus eliminating the ambiguity in identification and appropriate management. Radiotherapy, despite being a local therapy that aims to achieve similar goals, inherently carries the disadvantage of making a significant number of assumptions when using traditional treatment techniques. The 3D image dataset acquired at simulation is a snapshot of the tumor, its relation to normal structures, and the patient’s shape and position at a single time point, and it is this model that is used for plan development and dose calculation. During the planning stage, a lot of assumptions based on prior experience and literature are used with regard to clinical target volume (CTV) margins to define the microscopic spread around the tumor and planning target volume (PTV) margins to incorporate the expected range of internal organ motion and setup errors. The treatment is then carried out with the belief that all those assumptions would hold true for any given patient during daily treatment sessions, the foremost being that the patient and tumor anatomy and their positions with respect to the positioning devices have remained unchanged since the time of simulation. However, the assumption that the dose calculated on the CT dataset on the planning system matches the dose delivered through each fraction or through the entire radiation therapy course is grossly in error. Additionally, the internal organs and targets move with respiration and peristalsis and planning radiotherapy on a static image dataset is unable to account for errors due to this motion. To ensure that all of these assumptions do not compromise the dose delivery to the CTV, wider PTV margins are taken. This causes a large volume of normal tissues to be included in the irradiated volume. IGRT gives a method to capture this information regularly during the treatment course in the form of serial “snapshots” and is a means of verifying accurate and precise radiation delivery. In simple terms, the IGRT process ensures that the delivered treatment matches the intended treatment in accurately targeting the tumor while minimizing “collateral damage.” Changes to the composite delivered dose and their impact on disease control as well as toxicity may be minimized by use of appropriate localization devices and PTV margins. Occasionally, replanning may be required if gross deviations beyond predetermined tolerances are observed [1–3].
IGRT allows assessment of geometric accuracy of the “patient model” during treatment delivery. It provides a method whereby deviations of anatomy from initial plan are determined and this information is used to update dosimetric assumptions. Correction strategies may include daily repositioning to register patient position in accordance with the base plan or recalculation of treatment delivery in real time to reflect the patient’s presentation during a given fraction. This philosophy of reevaluating treatment and accounting for the differences between actual patient anatomy on a given day and the snapshot of planned treatment is known as adaptive radiotherapy . The eventual goal is to reevaluate and in certain situations redefine daily positioning for treatment to keep it on the same track as the intended treatment. Future applications may include dose titration for maximizing effect or mitigating side effects.
3. Errors and Margins
An error in radiotherapy delivery is defined as any deviation from intended or planned treatment. A great degree of uncertainty is inherent to radiotherapy practices and may be in the form of mechanical uncertainties related to treatment unit parameters such as couch and gantry motion, patient uncertainties related to ability to lie comfortably in a certain position and cooperate during the treatment time, geometric uncertainties related to position and motion of target, and dosimetric uncertainties. IGRT deals with the geometric uncertainties, which may be either intrafractional or interfractional [5, 6].
Both inter- and intrafractional uncertainties may be a result of a combination of systematic and random errors.
A systematic error is essentially a treatment preparation error and is introduced into the chain during the process of positioning, simulation, or target delineation. This error, if uncorrected, would affect all treatment fractions uniformly. A random error, on the other hand, is a treatment execution error, is unpredictable, and varies with each fraction. Systematic errors shift the entire dose distribution away from the CTV, while random errors blur this distribution around the CTV. Of the two, systematic error is more ominous since it would have a much larger impact on treatment accuracy and hence the therapeutic ratio.
Margins are added to the CTV to take these errors into account. These margins are geometric expansions around the CTV and may be non-uniform in all dimensions depending on the expected errors. These margins ensure that dosimetric planning goals are met despite the variations during and between fractions. ICRU 62 defines the expansion of PTV around the CTV as a composite of two factors—internal target motion (internal margin) and setup variations (setup margin) . Depending on observed systematic and random errors in a given setup for a particular treatment site, a variety of recipes for calculating PTV margins exist in literature [8, 9]. To enhance the therapeutic ratio, a host of correction strategies may be applied to reduce these margins and may include online or offline correction of interfraction errors or real time correction of intrafraction motion. Tracking and correcting organ motion helps reduce internal margin while improved accuracy of positioning reduces setup margins, thus reducing the required PTV margin.
4. Offline versus Online Corrections
Offline and online IGRT correction strategies refer to whether the patient is on the treatment couch while the verification is being done and whether the correction would be applied to the same or subsequent sessions.
In the offline strategy, images are acquired before treatment and matched to the reference image at a later time point. This strategy aims to determine the individual systematic setup error and thus reduce it. When combined with setup data of other patients treated under the same protocol, it helps define the population standard error for that treatment in that institution. Widely used offline correction protocols include Shrinking action level and No action level protocols [10, 11]. PTV margins in an institution depend on these determinations of individual and population systematic errors.
An online strategy, on the other hand, employs acquisition of images and their verification and correction prior to the day’s treatment. It aims to reduce both random and systematic errors. The treatment site and the expected magnitude of error may determine the frequency of online imaging. Sites where large daily shifts are anticipated (abdomen, pelvis, and thorax) or where even slight shifts will alter the dose distribution within adjacent critical structures (paraspinal tumors, intracranial tumors in close proximity to optic structures) are best managed with daily imaging. Our experience with online corrections showed the maximum errors in thorax followed closely by abdomen and pelvis. The minimum errors were observed in head and neck region . Additionally, treatments such as VMAT and SBRT carry the potential to translate minor shifts into major alterations in dose distribution and hence require daily online verification. For daily online correction, systematic and random errors may be calculated from the matched data. Post-treatment imaging is required to quantify both intrafraction motion and residual errors. If evaluated for a patient population, these data may help check the PTV margin for that treatment protocol. In fact, the use of daily online imaging with corrections in conjunction with use of automatic couch with 6 degrees of freedom has obviated the need for invasive frames for SRS treatments .
5. IGRT Technology Solutions
Depending on the imaging methods used, the IGRT systems may broadly be divided into radiation based and nonradiation based systems [14, 15].
5.1. Nonradiation Based Systems [16–22]
These may employ ultrasound, camera-based systems, electromagnetic tracking, and MRI systems integrated into the treatment room.
5.1.1. Ultrasound-Based Systems
These systems (e.g., BAT, SonArray, Clarity) acquire 3D images that help align targets to correct for interfractional errors. Geometric accuracy is 3–5 mm and the greatest advantage is lack of any ionizing radiation. Sites of common application include prostate, lung, and breast radiotherapy.
5.1.2. Camera-Based (Infrared) or Optical Tracking Systems
These systems identify the patient reference setup point positions in comparison to their location in the planning CT coordinate system, which aids in computing the treatment couch translation to align the treatment isocenter with plan isocenter. Optical tracking may also be used for intrafraction position monitoring for either gating (treatment delivery only at a certain position of target) or repositioning for correction. Tools such as AlignRT image the patient directly and track the skin surface to give real time feedback for necessary corrections. These systems have found application in treatment of prostate and breast cancer and for respiratory gating using external surrogates. Geometric accuracy is 1-2 mm, but application is limited only to situations where external surface may act as a reliable surrogate for internal position or motion.
5.1.3. Electromagnetic Tracking Systems
These systems (e.g., Calypso) make use of electromagnetic transponders (beacons) embedded within the tumor, and motion of these beacons may be tracked in real time using a detector array system. Beacons need to be placed through a minimally invasive procedure, their presence may introduce artifacts in MR images, and there are limitations to the patient size. Calypso has a geometric accuracy of <2 mm, but its use at present is limited to prostate radiotherapy.
5.1.4. MRI-Guided IGRT
These systems (e.g., ViewRay) help real time assessment of internal soft tissue anatomy and motion using continual soft tissue imaging and allow for intrafractional corrections. Geometric accuracy of the system is 1-2 mm. However, MRI has certain drawbacks such as motion artifacts, distortion with non-uniform magnetic fields, and cannot be performed for patients with pacemakers or metallic implants. All these limitations of diagnostic MRI apply to this IGRT system as well. A wide application potential exists in treatment of prostate, liver, and brain, as well as for brachytherapy.
5.2. Radiation Based Systems
These include static as well as real time tracking, using either kilovoltage (KV), megavoltage (MV), or hybrid methods.
5.2.1. Electronic Portal Imaging Devices (EPID)
EPID was developed as a replacement of film dosimetry for treatment field verification and is based on indirect detection active matrix flat panel imagers (AMFPIs). They are offered as standard equipment by nearly all linear accelerator (LINAC) vendors as both field verification and quality assurance (QA) tools. Image acquisition is 2D, with a geometric accuracy of 2 mm. Bony landmarks on planar images are used as surrogates for defining positional variations respective to the digital reconstructed radiographs (DRRs) developed from the planning CT dataset (Figure 1). Different systems may use either KV or MV X-rays for imaging, with the image contrast being superior with KV images while there is lesser distortion from metallic implants (dental, hip prostheses) in MV images. EPID systems are unable to detect or quantify rotations. Average dose per image is 1–3 mGy for KV systems while it is as high as 30–70 mGy for MV systems [23–25].
Figure 1: Use of MV EPID for online correction using orthogonal 2D images (anteroposterior and lateral). Both the field and the bony anatomy are matched sequentially to give an estimate of error. The comparison of live image with reference DRR helps assess and correct translational shifts but does not estimate rotational errors. (a) Right parietal glioma. (b) Head and neck cancer. MV: Megavoltage; EPID: Electronic portal imaging device; 2D: two-dimensional; DRR: Digital reconstructed radiograph.
5.2.2. Cone Beam CT (CBCT), KV or MV
These systems consist of retractable X-ray tube and amorphous silicon detectors mounted either orthogonal to (Elekta Synergy, Varian OBI) or along the treatment beam axis (Siemens Artiste). These have capability of 2D, fluoroscopic and CBCT imaging. Another system (Vero, BrainLAB) consists of a gimbaled X-ray treatment head mounted on an O-ring with two KV X-ray tubes, two flat panel detectors, and an EPID. The O-ring can be rotated 360 degrees around the isocenter and can be skewed 60 degrees around its vertical axis. Geometric accuracy is 1 mm or lesser with possibility of 2D and 3D matching with DRRs or X-ray volumetric images generated from planning CT data sets. Scanning is done through a continuous partial or complete gantry rotation around the couch, acquiring the “average” position of organs with respiratory motion. Both interfraction setup changes and anatomical changes related to weight changes or organ filling (bladder, rectum) may be monitored (Figure 2). Repeat scans at the end of treatment may give an estimate of intrafractional changes. For tumors discernible separately from surrounding normal tissue, treatment response may also be monitored and these scans may be used for dose recalculation or treatment plan adaptation after necessary image processing. KV CT gives better contrast resolution compared to MV CT but may be limited by artifacts from prostheses and scatter from bulky patient anatomy. Average dose per image is 30–50 mGy [26–29].
Figure 2: KV CBCT volumetric imaging. Both translational and rotational errors may be estimated. Translational errors are easily corrected whereas few systems have provisions for correcting rotational errors with couch rotations. (a) CBCT compared with reference scan before and after correction of setup error in a case of Carcinoma right breast, post-mastectomy. (b) CBCT correction in a case of Carcinoma larynx. (c) CBCT in a case of Carcinoma prostate not only corrects for setup errors, but also provides an estimate of reproducibility of prostate position with respect to bladder filling. In this particular case, the live image shows negligible bladder filling and treatment was delayed to allow for optimum bladder position for obtaining a reproducible position of prostate as well as moving the bowel out of treatment field. KV: kilovoltage; CBCT: cone beam computed tomography.
5.2.3. Fan Beam KV CT (CT-on-Rails)
This system has an in-room CT scanner and gantry that moves across the treatment couch/patient, which can be rotated towards either the scanner or the gantry for imaging and treatment, respectively. 3D images are taken with the patient immobilized on the couch, the difference from a diagnostic CT being a larger bore size (>80 cm diameter) to accommodate bulky immobilization devices, and a multislice detector. Accuracy and applications are similar to CBCT with average dose of 10–50 mGy per image .
5.2.4. Fan Beam MV CT (TomoTherapy Hi ART II)
This includes an on-board imaging system to obtain MV CT images of the patient in treatment position. The same LINAC is used to generate both the treatment (6 MV) and imaging beam (3.5 MV). A xenon detector located on the gantry opposite the LINAC collects exit data for generation of MV CT images. Patient dose from imaging varies with pitch setting and is typically 10–30 mGy per scan .
5.2.5. Hybrid Systems for Real Time 4D Tracking
2D KV Stereoscopic Imaging (CyberKnife). The Accuray CyberKnife robotic radiosurgery system consists of a compact LINAC mounted on an industrial robotic manipulator arm which directs the radiation beams to the desired target based on inputs from two orthogonal X-ray imaging systems mounted on the room ceiling with flat panel floor detectors on either side of couch, integrated to provide image guidance for the treatment process. Images are acquired throughout the treatment duration at periodic intervals ranging from 5 to 90 seconds, and the couch and robotic head movements are guided through an automatic process. Several tracking methods may be used depending upon the treatment site (Figure 3). Skull, skull base, or brain tumors may be treated using 6D skull tracking, paravertebral tumors whose movement parallels that of spine may be treated with X-Sight spine tracking, and lung tumors that are surrounded by normal lung parenchyma may be tracked with X-Sight lung tracking. Lung tracking may employ automatic generation of internal target volume depending upon visibility of tumor through both, one or none of the X-ray imaging systems in the treatment position. For all other tumors (e.g., prostate, liver, neck nodes, abdominal masses, etc.), internal surrogates or fiducial markers may need to be placed within or in direct contact with the tumor and the tumor motion is tracked and corrected for through monitoring the fiducial position including translations, rotations, and deformation. Respiratory motion is also monitored and accounted for when correcting for target position and motion through a synchrony model generated in real time. The system also has a couch that has 6 degrees of freedom to correct for positional variations. Treatment may be limited by patient position and size, and posterior treatment beams cannot be used. A semi-invasive procedure may be required if fiducial markers are needed for tracking. This system can be employed for both cranial (frameless) and extracranial radiosurgery or SRT [32, 33].
Figure 3: CyberKnife console showing the tumor tracking options in a case of head and neck malignancy. (a) 6D skull for skull base lesions. (b) Spine tracking for paravertebral tumors. (c) Fiducial tracking for all other lesions whose motion is independent of skull or spine position, such as base of tongue or neck nodes.
Real Time Tumor-Tracking (RTRT) System. This system is designed for real time tracking of tumors by imaging implanted fiducials and using this information for gating. It consists of four X-ray camera systems mounted on the floor, a ceiling-mounted image intensifier, and a high-voltage X-ray generator. The LINAC is gated to irradiate the tumor only when the marker is within a given tolerance from its planned coordinates relative to the isocenter [34, 35].
VERO. This system has two X-ray tubes and corresponding flat panel detectors and uses a combination of initial couch motion and a pair of radiographs for patient alignment. The couch is capable of 3D alignment for initial coarse setup and then the on-board imaging subsystem helps fine-tuning. A pair of radiographs is acquired and registered with prior DRRs using bony landmarks to evaluate the translational and rotational shifts. The system can also compensate for organ motion .
5.2.6. Combination Alignment Systems: Optical Imaging and 2D KV Orthogonal Imaging
ExacTrac X-Ray 6-D Stereotactic IGRT System. It uses a combination of optical positioning and KV radiographic imaging for online positioning corrections. There are two main subsystems: an infrared-based system for initial patient setup and precise control of couch movement using a robotic couch and a radiographic KV X-ray imaging system for position verification and readjustment based on internal anatomy or implanted markers. Infrared system may also be used for respiratory monitoring and signaling to LINAC for beam tracking and gating. Novalis Tx combines this system with an additional on-board imaging system (MV, KV X-rays, and KV CBCT) on a multiphoton/electron beam LINAC [37, 38].
6. Guidelines for Medical Personnel and Implementation
American College of Radiology (ACR) and the American Society for Radiation Oncology (ASTRO) jointly developed guidelines for IGRT that define the qualifications and responsibilities of personnel including radiation oncologists, medical physicists, dosimetrists and radiation therapists, QA standards, clinical implementation, and suggested documentation. Similar guidelines have also been proposed by European agencies [39–41]. A summary of the key points is given below.
6.1. Qualifications and Responsibilities
Qualifications. Respective personnel should obtain appropriate certification with specific training in IGRT before performing any stereotactic procedures.
Radiation Oncologist. (i) Conduct of disease-specific treatment, staging, evaluation of comorbid conditions and prior treatments, exploration of all available treatments including discussion of pros and cons of IGRT, treatment, and subsequent follow-up.
(ii) Determination of the most appropriate patient positioning method, recommendation of the appropriate approach to manage organ motion, supervision of simulation paying particular attention to positioning, immobilization and appropriate motion management, determination and delineation of target volumes and relevant normal critical structures using available imaging techniques, communication of expected goals and constraints and collaboration with the physicist in the iterative process of plan development to achieve the desired goals, supervision of treatment delivery and determination of acceptable day-to-day setup variations, and participation in the QA process and subsequent approval.
Medical Physicist. (i) Acceptance testing and commissioning, assuring mechanical, software, and geometric precision and accuracy, as well as image quality verification and documentation in a given IGRT system.
(ii) Implementation and management of a QA program.
(iii) Development and implementation of standard operating procedures (SOPs) for IGRT use, in collaboration with the radiation oncologist.
Dosimetrist. (i) Normal structure delineation under the guidance of radiation oncologist.
(ii) Management of volumetric patient image data (CT and other fused data sets) on radiation treatment planning (RTP) system.
(iii) Generation of a treatment plan under oncologist’s and physicist’s guidance.
(iv) Generation of all technical documentation for IGRT plan implementation.
(v) Assisting with treatment verification.
Radiation Therapist. (i) Understanding and appropriate use of immobilization/repositioning systems.
(ii) Performance of simulation and generation of imaging data for planning, implementation of treatment plan, acquisition of periodic verification images under supervision and periodic evaluation of stability and reproducibility of the immobilization/repositioning system, and reporting inconsistencies immediately.
6.2. IGRT Implementation
Fiducial Markers. These serve as surrogates to soft tissue targets when they are difficult to visualize and their alignment cannot be related to bony anatomy. These may be tracked in real time to obtain 3D coordinates of the target for subsequent corrections.
Moving Targets and Delineation. Intrafraction target motion or interfraction displacement, deformation, or alteration of targets and other tissues should be accounted for during determination of PTVs. Appropriate motion management methods should be chosen depending on available expertise and degree and type of motion. This process starts at the time of simulation and continues throughout till the end of therapy.
Patient Positioning. It is imperative to ensure the accuracy of patient position and its reproducibility for fractionated treatments relative to the chosen IGRT device as well as treatment unit.
Image Acquisition. The IGRT system should be calibrated to ensure high imaging quality with attention to slice thickness uniformity, image contrast, spatial resolution, isocenter alignment between imaging and treatment planning and delivery systems, accuracy of software used for identification, and correction of couch misalignments. Relevant QA procedures should ensure reliability and reproducibility of the entire process.
Treatment Verification. Image review by radiation oncologist at the first fraction and then periodically is necessary to ensure treatment accuracy and reproducibility. Each department should determine its own threshold of couch positioning changes that would necessitate setup review or change before treatment delivery.
Quality Assurance and Documentation. A documentation of all the necessary QA procedures throughout the course of simulation, treatment, and periodic verification should be maintained. These would help determine departmental thresholds for action as well as serve as guides for modification of the processes involved following review of findings.
7. IGRT: Clinical Benefits
Use of the IGRT process has improved our awareness and understanding of daily inter- and intrafractional setup variations and motion. Real time tracking has helped quantify interpatient and intrapatient variations in lung and liver tumor motion related to breathing and complexities of such motion have become clearer. We now understand that even when breath-holds are repeated, the relative position of soft tissue and skeletal structures may vary, rendering use of bony landmarks useless for such endeavors. Changes in prostate position (translation, rotation, and shape) have been quantified and we can better correct for these errors as well as tailor PTV margins to these findings, thus allowing more accurate targeting. Understanding of the various IGRT techniques, their applicability, limitations, and additional radiation hazards helps the radiation oncologist take an educated decision on the method best suited to a particular clinical situation for maximizing benefit from radiation therapy. Changes in parotid position relative to the tumor in head and neck cases, change in body contour due to weight loss, seroma, or body fluid collections, change in prostate position relative to bladder or rectal filling and effect of bowel gas, reduction of tumor size during treatment, and changes in spinal position during spinal or head and neck radiotherapy are situations which were never even considered of significance in the pre-IGRT era and their respective roles and solutions are being developed as we are understanding their role during treatment. With better geometric precision, volume of irradiated healthy normal tissue can be significantly reduced with reduction in toxicity risks. Adaptation to reduction in tumor volume may lead to additional gains in normal tissue toxicity reduction.
Results from ongoing and future trials will hopefully demonstrate the net gain in therapeutic ratio from application of IGRT technologies and the onus lies on the radiation oncology community to take up the challenge of demonstrating the benefit of these potentially expensive approaches.
IGRT is most likely to benefit clinical situations where the tumor is in close proximity to sensitive healthy tissues, when doses required for disease control exceed the tolerance levels of adjacent normal tissues or when large organ motion and setup errors may result in severe consequences of positional errors. All patients treated with conformal radiotherapy, IMRT, and SBRT should, in theory, benefit from IGRT. Thoracic and upper abdominal targets with significant respiratory motion, obese patients, head and neck cancers, paraspinal and retroperitoneal sarcomas, and prostate cancer are situations that are expected to derive maximum benefit with some clinical experience forthcoming. Clinical situations where even low dose irradiation produces excellent local control, palliative radiotherapy delivered using large fields, and superficial tumors that are amenable to direct visual inspection are likely to derive least benefit from IGRT.
8. Concerns with IGRT
Limited availability of experienced trained staff is a major hurdle in wide application of the technique despite its demonstrable benefits, even with the simplest approaches. Other factors that need consideration include quality control, algorithms that define the decisions whether to change a plan or continue with original plan, and need for commercial development of software as well as hardware to match clinical needs and demands. Another major concern regarding frequent on-treatment imaging is the radiation dose to normal tissues. Although the doses from IGRT appear insignificant, only long term follow-up will define any potential risk of second malignancies from low dose exposure. Thus, there is an ongoing debate on the necessary frequency of verification imaging especially when using ionizing radiation. Recent developments in MR-LINACs have tried to address these concerns while allowing daily imaging for treatment verification. Another concern is that of treatment safety since the technologies available in the clinic require integration of hardware and software from different vendors. Clinical use of any system should be preceded by proper acceptance testing, commissioning, and routine QA used to assure accurate regular functionality. Education of all users (oncologists, physicists, and technologists) on safe use and clinical utility is mandatory, along with knowledge of additional dose and possible risks associated with use. No single technology is ideal in every scenario and no single institution can manage to integrate all or most technologies in one place. Only time will tell which of these methods gain wider popularity and acceptance, based on clinical relevance and ease of use.
9. Clinical Applications: Current and Future
Use of IGRT systems is essential to treatment of any site where setup deviations and organ motion are anticipated. Additional gains are monitoring of treatment response, weight changes, and organ filling on day-to-day basis. With improved precision of planning systems, use of SRS or SRT, and high dose hypofractionated regimens, the chances of small deviations leading to significant errors in treatment delivery are much higher, and the use of IGRT is far more critical in these situations. Integration of LINACs with MR-based soft tissue imaging and PET-based biological imaging may help even further improve targeting accuracy in the future [42, 43]. However, it is mandatory to ensure proper training of staff and QA at all steps for optimum use of such technology and its integration into routine use.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.