AAPM Medical Physics Practice Guideline 8.b: Linear accelerator performance tests.

The purpose of this guideline is to provide a list of critical performance tests to assist the Qualified Medical Physicist (QMP) in establishing and maintaining a safe and effective quality assurance (QA) program. The performance tests on a linear accelerator (linac) should be selected to fit the clinical patterns of use of the accelerator and care should be given to perform tests which are relevant to detecting errors related to the specific use of the accelerator. Current recommendations for linac QA were reviewed to determine any changes required to those tests highlighted by the original report as well as considering new components of the treatment process that have become common since its publication. Recommendations are made on the acquisition of reference data, routine establishment of machine isocenter, basing performance tests on clinical use of the linac, working with vendors to establish QA tests and performing tests after maintenance and upgrades. The recommended tests proposed in this guideline were chosen based on consensus of the guideline's committee after assessing necessary changes from the previous report. The tests are grouped together by class of test (e.g., dosimetry, mechanical, etc.) and clinical parameter tested. Implementation notes are included for each test so that the QMP can understand the overall goal of each test. This guideline will assist the QMP in developing a comprehensive QA program for linacs in the external beam radiation therapy setting. The committee sought to prioritize tests by their implication on quality and patient safety. The QMP is ultimately responsible for implementing appropriate tests. In the spirit of the report from American Association of Physicists in Medicine Task Group 100, individual institutions are encouraged to analyze the risks involved in their own clinical practice and determine which performance tests are relevant in their own radiotherapy clinics.

AAPM Task Group Report 307: Use of EPIDs for Patient-Specific IMRT and VMAT QA.

PURPOSE: Electronic portal imaging devices (EPIDs) have been widely utilized for patient-specific quality assurance (PSQA) and their use for transit dosimetry applications is emerging. Yet there are no specific guidelines on the potential uses, limitations, and correct utilization of EPIDs for these purposes. The American Association of Physicists in Medicine (AAPM) Task Group 307 (TG-307) provides a comprehensive review of the physics, modeling, algorithms and clinical experience with EPID-based pre-treatment and transit dosimetry techniques. This review also includes the limitations and challenges in the clinical implementation of EPIDs, including recommendations for commissioning, calibration and validation, routine QA, tolerance levels for gamma analysis and risk-based analysis. METHODS: Characteristics of the currently available EPID systems and EPID-based PSQA techniques are reviewed. The details of the physics, modeling, and algorithms for both pre-treatment and transit dosimetry methods are discussed, including clinical experience with different EPID dosimetry systems. Commissioning, calibration, and validation, tolerance levels and recommended tests, are reviewed, and analyzed. Risk-based analysis for EPID dosimetry is also addressed. RESULTS: Clinical experience, commissioning methods and tolerances for EPID-based PSQA system are described for pre-treatment and transit dosimetry applications. The sensitivity, specificity, and clinical results for EPID dosimetry techniques are presented as well as examples of patient-related and machine-related error detection by these dosimetry solutions. Limitations and challenges in clinical implementation of EPIDs for dosimetric purposes are discussed and acceptance and rejection criteria are outlined. Potential causes of and evaluations of pre-treatment and transit dosimetry failures are discussed. Guidelines and recommendations developed in this report are based on the extensive published data on EPID QA along with the clinical experience of the TG-307 members. CONCLUSION: TG-307 focused on the commercially available EPID-based dosimetric tools and provides guidance for medical physicists in the clinical implementation of EPID-based patient-specific pre-treatment and transit dosimetry QA solutions including intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) treatments.

Report of AAPM Task Group 219 on independent calculation-based dose/MU verification for IMRT.

Independent verification of the dose per monitor unit (MU) to deliver the prescribed dose to a patient has been a mainstay of radiation oncology quality assurance (QA). We discuss the role of secondary dose/MU calculation programs as part of a comprehensive QA program. This report provides guidelines on calculation-based dose/MU verification for intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) provided by various modalities. We provide a review of various algorithms for "independent/second check" of monitor unit calculations for IMRT/VMAT. The report makes recommendations on the clinical implementation of secondary dose/MU calculation programs; on commissioning and acceptance of various commercially available secondary dose/MU calculation programs; on benchmark QA and periodic QA; and on clinically reasonable action levels for agreement of secondary dose/MU calculation programs.

Investigation of a 90Sr/90Y source for intra-ocular treatment of wet age-related macular degeneration.

PURPOSE: The purpose of this study is to perform an extensive investigation of an approximately 2.5 mm long 90Sr/90Y source designed for treating wet age-related macular degeneration. METHODS: As part of this investigation, a NIST-traceable absorbed dose to water calibration technique was established, and a source deployment verification test was developed. The influence of treatment cannula construction tolerance on the measurements as well as the dose delivered to the patient was investigated using the Monte Carlo code MCNP5. Variation between production cannulae was quantified experimentally using a well-type ionization chamber, and additional measurements along with Monte Carlo calculations of the collimating insert used for source deployment verification were performed to validate the model. RESULTS: Maximum variation in the integrated target dose was seen when the source was shifted laterally within the treatment cannula. For the well chamber measurements, the observed standard deviation in ionization current for a single source placed in different reference cannulae was +/-0.3%, with a maximum observed range of less than +/-0.5%. Clinical cannulae in the collimating insert showed an average of 17.8% +/-0.4% of the reference signal when sources were fully deployed compared to 18.5% predicted by Monte Carlo calculations. This discrepancy has been attributed primarily to construction of the collimator since the collimation gap was observed to be approximately 0.025-0.075 mm smaller than specified. Construction tolerance of the well chamber insert as well as position tolerance of the cannula tip were both investigated, and their influence on the predicted signal was quantified. Additional measurements along with Monte Carlo based calculations of the collimating insert with polyethylene spacers added to the setup were performed to validate the Monte Carlo model. The shimmed Monte Carlo and measured data agree to within 1%, which is a magnitude difference of approximately 0.1% of the reference signal. CONCLUSIONS: This investigation confirms that the signal for an acceptably deployed source in the collimating insert is between 17.5% and 21.5% of the reference signal, as calculated using Monte Carlo models. Clinical cannulae for which the source deployment verification measurement falls outside the acceptable range should not be used to treat patients.

Ophthalmic applicators: an overview of calibrations following the change to SI units.

Since the NIST dose to water standard for 90Sr/90Y ophthalmic applicators was introduced, numerous sources have undergone calibration either at NIST or at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL). From 1997 to 2008, 222 of these beta-emitting sources were calibrated at the UWADCL, and prior reference source strength values were available for 149 of these sources. A survey of UWADCL ophthalmic applicator calibrations is presented here, demonstrating an average discrepancy of -19% with a standard deviation of +/- 16% between prior reference values and the NIST-traceable UWADCL absorbed dose to water calibrations. Values ranged from -49% to +42%.

Calibration of multiple LDR brachytherapy sources.

A trend is underway toward the use of prepackaged low dose rate brachytherapy sources, which come in the form of strands, coiled line sources, preloaded needles, and sterile cartridge packs. Since the medical physicist is responsible for verification of source strength prior to patient treatment, development of prepackaged source strength verification methods is needed. Existing guidelines are reviewed to establish the situation that medical physicists find with respect to prepackaged sources. This investigation presents an experimental evaluation of the effect of some of these multiseed geometries on source strength measurements. Multiseed strands and coils, whether 125I, 103Pd, or 192Ir can be measured in a chamber with a long, sensitive axial length with a uniform response. Sterile seed cartridge packs can also be measured but require a correction factor to be applied. Sources in needles, however, cannot be measured in the needle since there is too great a variation in needle composition and needle tolerance thickness. Removing these seeds from the needle into a sterile measurement insert, which maintains sterility is a practical source strength verification method, similar to those done for multiple seed configurations in a well chamber with adequate axial uniformity. Values are compared with individual air kerma strength calibrations, and correction factors, are presented' where needed. In each case, care must be taken to maintain sterility as multiple seeds are measured in well chamber inserts.

Experimental determination of the radial dose function of 90Sr/90Y IVBT sources.

A series of measurements were undertaken using both high sensitivity radiochromic film and new lithium fluoride thermoluminescent dosimeters in a liquid water medium to define the radial dose function of 90Sr/90Y beta emitting intravascular brachytherapy sources more accurately. These measurements of a single 5 French source pellet served to verify current Monte Carlo transport models and extrapolation chamber measurements of the radial dose function, thus providing the recommended independent published measurements for g(r) of these sources. A slight deviation in the published radial dose function at depth leads the authors to recommend that treatment planning be performed using updated g(r) values from current Monte Carlo transport models verified by measurements such as those shown in this investigation.