J Lowenstein

Accelerator beam data commissioning equipment and procedures: Report of the TG-106 of the Therapy Physics Committee of the AAPM

For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no-scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors (<1%), beam data processing and detector size convolution for accurate profiles. The TG-106 also provides a brief discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.

Report of AAPM TG 135: Quality assurance for robotic radiosurgery

The task group (TG) for quality assurance for robotic radiosurgery was formed by the American Association of Physicists in Medicines Science Council under the direction of the Radiation Therapy Committee and the Quality Assurance (QA) Subcommittee. The task group (TG-135) had three main charges: (1) To make recommendations on a code of practice for Robotic Radiosurgery QA; (2) To make recommendations on quality assurance and dosimetric verification techniques, especially in regard to real-time respiratory motion tracking software; (3) To make recommendations on issues which require further research and development. This report provides a general functional overview of the only clinically implemented robotic radiosurgery device, the CyberKnife. This report includes sections on device components and their individual component QA recommendations, followed by a section on the QA requirements for integrated systems. Examples of checklists for daily, monthly, annual, and upgrade QA are given as guidance for medical physicists. Areas in which QA procedures are still under development are discussed.

Comparison between TG‐51 and TG‐21: Calibration of photon and electron beams in water using cylindrical chambers

A new calibration protocol, developed by the AAPM Task Group 51 (TG‐51) to replace the TG‐21 protocol, is based on an absorbed‐dose to water standard and calibration factor (ND,w), while the TG‐21 protocol is based on an exposure (or air‐kerma) standard and calibration factor (Nx). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG‐51 may be somewhat different from those based on TG‐21. The Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG‐51 report, covering the majority of current manufacturers. Measured ratios between absorbed‐dose and air‐kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG‐21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed‐dose to water determined by TG‐51 and that determined by TG‐21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG‐51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV photon beam. On the other hand, the results for 9 and 16 MeV electron beams show larger changes up to 2%, perhaps because of the updated electron stopping power data used for the TG‐51 protocol, in addition to the inherent 1% discrepancy presented in the calibration factors. The results also indicate that the changes may be dependent on the electron energy. PACS number(s): 87.66.–a, 87.53.–j

Technical Report: Reference photon dosimetry data for Varian accelerators based on IROC‐Houston site visit data

PURPOSE Accurate data regarding linear accelerator (Linac) radiation characteristics are important for treatment planning system modeling as well as regular quality assurance of the machine. The Imaging and Radiation Oncology Core-Houston (IROC-H) has measured the dosimetric characteristics of numerous machines through their on-site dosimetry review protocols. Photon data are presented and can be used as a secondary check of acquired values, as a means to verify commissioning a new machine, or in preparation for an IROC-H site visit. METHODS Photon data from IROC-H on-site reviews from 2000 to 2014 were compiled and analyzed. Specifically, data from approximately 500 Varian machines were analyzed. Each dataset consisted of point measurements of several dosimetric parameters at various locations in a water phantom to assess the percentage depth dose, jaw output factors, multileaf collimator small field output factors, off-axis factors, and wedge factors. The data were analyzed by energy and parameter, with similarly performing machine models being assimilated into classes. Common statistical metrics are presented for each machine class. Measurement data were compared against other reference data where applicable. RESULTS Distributions of the parameter data were shown to be robust and derive from a students t distribution. Based on statistical and clinical criteria, all machine models were able to be classified into two or three classes for each energy, except for 6 MV for which there were eight classes. Quantitative analysis of the measurements for 6, 10, 15, and 18 MV photon beams is presented for each parameter; supplementary material has also been made available which contains further statistical information. CONCLUSIONS IROC-H has collected numerous data on Varian Linacs and the results of photon measurements from the past 15 years are presented. The data can be used as a comparison check of a physicists acquired values. Acquired values that are well outside the expected distribution should be verified by the physicist to identify whether the measurements are valid. Comparison of values to this reference data provides a redundant check to help prevent gross dosimetric treatment errors.

Radiotherapy quality assurance of gynecologic oncology group (GOG) protocol 165, a cooperative group study of carcinoma of the cervix

ABSTRACT: GOG protocol 165 was a randomized phase III trial to evaluate radiation vs. radiation plus weekly cisplatin vs. radiation plus protracted venous infusion 5-FU in patients with stage II-B, III-B and IV-A carcinoma of the cervix. Protocol treatment included external beam therapy and high-dose rate or low-dose rate brachytherapy. Historically, GOG has performed an extensive review of patient treatments on their clinical trials. As HDR had no previously been used in GOG trials, credentialing of institutions and physicians was required prior to entering patients onto the study for the use of HDR, but was not required for external radiation therapy or LDR. Credentialing consisted of a review of the institution’s HDR physics and QA, and a clinical and dosimetric review of the brachytherapy treatment of two patients treated by the same radiation oncologist in a manner similar to the protocol guidelines. The credentialing process not only evaluated the quality of HDR procedures at the institution, but also assured that the institution and participating radiation oncologist had HDR experience. At the same time, it educated the institution as to the specific requirements of the protocol. Retrospective review of radiotherapy of 326 patients entered on the study was performed. A recalculation of patient dose and a review of the records and all planning and verification films were performed by the Radiological Physics Center in conjunction with the GOG HDR subcommittee and the protocol study co-chairs, respectively. Deviations from protocol guidelines were assessed according to predefined criteria. 100% of the patients treated at credentialed institutions were treated without major protocol deviations. In contrast, 81% of patients from non-credentialed institutions completed treatment without major deviations. Minor deviations occurred in both groups with the result that 75% and 50% of patients from credentialed and non-credentialed intuitions respectively were treated in strict compliance with the protocol. A breakdown of protocol deviations appears in table 1. Table 1. Summation of Deviations for Non-Certified and Certified Institutions

Rationale of technical requirements for NRG-BR001: The first NCI-sponsored trial of SBRT for the treatment of multiple metastases

INTRODUCTION In 2014, the NRG Oncology Group initiated the first National Cancer Institute-sponsored, phase 1 clinical trial of stereotactic body radiation therapy (SBRT) for the treatment of multiple metastases in multiple organ sites (BR001; NCT02206334). The primary endpoint is to test the safety of SBRT for the treatment of 2 to 4 multiple lesions in several anatomic sites in a multi-institutional setting. Because of the technical challenges inherent to treating multiple lesions as their spatial separation decreases, we present the technical requirements for NRG-BR001 and the rationale for their selection. METHODS AND MATERIALS Patients with controlled primary tumors of breast, non-small cell lung, or prostate are eligible if they have 2 to 4 metastases distributed among 7 extracranial anatomic locations throughout the body. Prescription and organ-at-risk doses were determined by expert consensus. Credentialing requirements include (1) irradiation of the Imaging and Radiation Oncology Core phantom with SBRT, (2) submitting image guided radiation therapy case studies, and (3) planning the benchmark. Guidelines for navigating challenging planning cases including assessing composite dose are discussed. RESULTS Dosimetric planning to multiple lesions receiving differing doses (45-50 Gy) and fractionation (3-5) while irradiating the same organs at risk is discussed, particularly for metastases in close proximity (≤5 cm). The benchmark case was selected to demonstrate the planning tradeoffs required to satisfy protocol requirements for 2 nearby lesions. Examples of passing benchmark plans exhibited a large variability in plan conformity. DISCUSSION NRG-BR001 was developed using expert consensus on multiple issues from the dose fractionation regimen to the minimum image guided radiation therapy guidelines. Credentialing was tied to the task rather than the anatomic site to reduce its burden. Every effort was made to include a variety of delivery methods to reflect current SBRT technology. Although some simplifications were adopted, the successful completion of this trial will inform future designs of both national and institutional trials and would allow immediate clinical adoption of SBRT trials for oligometastases.

Insight gained from responses to surveys on reference dosimetry practices

Purpose To present the results and discuss potential insights gained through surveys on reference dosimetry practices. Methods Two surveys were sent to medical physicists to learn about the current state of reference dosimetry practices at radiation oncology clinics worldwide. A short survey designed to maximize response rate was made publicly available and distributed via the AAPM website and a medical physics list server. Another, much more involved survey, was sent to a smaller group of physicists to gain insight on detailed dosimetry practices. The questions were diverse, covering reference dosimetry practices on topics like measurements required for beam quality specification, the actual measurement of absorbed dose and ancillary equipment required like electrometers and environment monitoring measurements. Results There were 190 respondents to the short survey and seven respondents to the detailed survey. The diversity of responses indicates nonuniformity in reference dosimetry practices and differences in interpretation of reference dosimetry protocols. Conclusions The results of these surveys offer insight on clinical reference dosimetry practices and will be useful in identifying current and future needs for reference dosimetry.

SU-F-T-485: Independent Remote Audits for TG51 NonCompliant Photon Beams Performed by the IROC Houston QA Center

PURPOSE IROC_H conducts external audits for output check verification of photon and electron beams. Many of these beams can meet the geometric requirements of the TG 51 calibration protocol. For those photon beams that are non TG 51 compliant like Elekta GammaKnife, Accuray CyberKnife and TomoTherapy, IROC_H has specific audit tools to monitor the reference calibration. METHODS IROC_H used its TLD and OSLD remote monitoring systems to verify the output of machines with TG 51 non compliant beams. Acrylic OSLD miniphantoms are used for the CyberKnife. Special TLD phantoms are used for TomoTherapy and GammaKnife machines to accommodate the specific geometry of each machine. These remote audit tools are sent to institutions to be irradiated and returned to IROC_H for analysis. RESULTS The average IROC_H/institution ratios for 480 GammaKnife, 660 CyberKnife and 907 rotational TomoTherapy beams are 1.000±0.021, 1.008±0.019, 0.974±0.023, respectively. In the particular case of TomoTherapy, the overall ratio is 0.977±0.022 for HD units. The standard deviations of all results are consistent with values determined for TG 51 compliant photon beams. These ratios have shown some changes compared to values presented in 2008. The GammaKnife results were corrected by an experimentally determined scatter factor of 1.025 in 2013. The TomoTherapy helical beam results are now from a rotational beam whereas in 2008 the results were from a static beam. The decision to change modality was based on recommendations from the users. CONCLUSION External audits of beam outputs is a valuable tool to confirm the calibrations of photon beams regardless of whether the machine is TG 51 or TG 51 non compliant. The difference found for TomoTherapy units is under investigation. This investigation was supported by IROC grant CA180803 awarded by the NCI.

Erratum: Report of AAPM TG 135: Quality assurance for robotic radiosurgery (Medical Physics)

We would like to correct a sentence on p. 2932, Sec. III E. General Patient Safety, Treatment Procedure Monitoring. The incorrect sentence reads: “We recommend that in addition to the treating therapist, a second medical professional (therapist, physicist, or physician) should be in the immediate vicinity at all times to assist when necessary. The corrected section follows below. We regret the error and hope the revised text will clarify the task groups recommendations in regards to the role of QMPs in treatment supervision. Glossary: Personal supervision—Physicist is present at the treatment console. Direct supervision—Physicist is in the department, available for immediate response. Corrected text: All treatments must occur under direct supervision of a QMP. In addition, a QMP must provide personal supervision at the first treatment, and as needed for subsequent treatments. The personal supervision should include participation in a timeout checklist, assessment of patient immobilization, assessment of adequate imaging parameters, tracking accuracy and correct Synchrony respiratory modeling (if applicable), consultation on excessive or unusual patient shift requirements during treatment not clearly caused by patient motion on the treatment couch, as well as other patientor plan-specific needs.

Erratum: “Report of AAPM TG 135: Quality assurance for robotic radiosurgery”

We would like to correct a sentence on p. 2932, Sec. III E. General Patient Safety, Treatment Procedure Monitoring. The incorrect sentence reads: “We recommend that in addition to the treating therapist, a second medical professional (therapist, physicist, or physician) should be in the immediate vicinity at all times to assist when necessary. The corrected section follows below. We regret the error and hope the revised text will clarify the task groups recommendations in regards to the role of QMPs in treatment supervision. Glossary: Personal supervision—Physicist is present at the treatment console. Direct supervision—Physicist is in the department, available for immediate response. Corrected text: All treatments must occur under direct supervision of a QMP. In addition, a QMP must provide personal supervision at the first treatment, and as needed for subsequent treatments. The personal supervision should include participation in a timeout checklist, assessment of patient immobilization, assessment of adequate imaging parameters, tracking accuracy and correct Synchrony respiratory modeling (if applicable), consultation on excessive or unusual patient shift requirements during treatment not clearly caused by patient motion on the treatment couch, as well as other patientor plan-specific needs.