Abstract Book

S939

ESTRO 37

Purpose or Objective The parameters that characterize the initial electron beam in the simulation of a clinical linac greatly influence on the simulated dose distributions. The users of a Monte Carlo code for the simulation of radiation transport must match the simulated dose distributions to measurements and this is achieved usually in a lengthy iterative process, by tuning the simulation parameters and comparing the simulated with the measured dose distributions. To reduce the time needed for this tuning task, the PRIMO Monte Carlo software for the simulation of clinical linacs [Strahlenther. Onkol. 189 , 881–886 (2013)] proposes default values of the initial beam parameters for each nominal energy of the included linac models. This work investigates the suitability of the PRIMO default beam parameters for the 6 MV photon beam from Varian Clinac 2100 linacs with a Millennium 120 MLC, by comparing dosimetric data obtained from PRIMO simulations with a published dataset based on measurements on a large series of linacs of the same model. Material and Methods We used PRIMO v. 0.3.1.1363 to obtain a phase-space file (PSF) with 450 million histories of a Clinac 2100 with a 6 MV photon beam (E=5.4 MeV by default in PRIMO). We used this PSF to estimate dosimetric parameters in a water phantom for some setups of interest (static fields). Point measurement distributions provided by the Imaging and Radiation Oncology Core-Houston (IROC-H) Quality Assurance Center were used as benchmark data [Med. Phys. 43 (5), 2374–2386 (2016)]. The accuracy of PRIMO simulation results was assessed for percentage depth doses, jaw output factors, MLC small-field output factors, and off-axis ratios. Results All evaluated dosimetric parameters obtained from the simulations with PRIMO agreed within 2% with the experimental data from IROC-H, except the SBRT-style output factors, which agreed within 3%. Although a fine- tuning is possible with PRIMO to closely match simulation results with a particular linac, the results obtained with the default beam parameters are consistent with the typical values found for 6 MV photon beams from Varian Clinac 2100 linacs. Conclusion The PRIMO default initial beam parameters for 6 MV photon beams from Varian Clinac 2100 linacs allows obtaining dose distributions in a water phantom which agree within 3% with a database of experimental dosimetric data from a large series of linacs. The findings of this work represent a first step in the validation of the Monte Carlo software PRIMO for independent verification of radiotherapy plans computed by a commercial treatment planning system. EP-1752 15 years of independent peer review results of beam output at more than 2000 Institutions Worldwide R. Howell 1 , S. Smith 1 , J. Palmer 1 1 ut Md Anderson Cancer Center, Radiation Physics Outreach, Houston, Usa Purpose or Objective Independent peer review is an important tool for improving quality of the clinical physics program. While there have been many reports of results of such programs, they have largely focused on institutions that participate in clinical trials. The purpose of this study was to report independent peer review results from a broad spectrum of institutions, including both academic and non-academic centers. Material and Methods We analyzed the results from independent peer review of beam output for different types of radiation therapy

beams, i.e. Photons (2 – 25 MV), electrons (2 – 20 MeV), and orthovoltage (1.9 mm AL – 3 mm Cu). Specifically, calculated summary statistics for the ratio between dose measured by independent peer review and dose reported by the institution. The analysis included data from over 2000 institutions in the United States and more than 150 from other countries. All beams monitored over the past 15 years (2001 – 2016) were included in the analysis. In total data for 155, 237 instances of individual beam output checks were analyzed. Figure 1: Map showing the distribution of institutions where beam output was monitored. Results The mean ratio between measured and stated doses for all beams, photon, electron, and orthovoltage beams were 0.999±0.018, 1.000±0.016, 0.999±0.019, and 0.995±0.033, respectively. While the mean values for each beam type were very close to one, > 5% of the beams monitored were more than more than ± 3% from a 1.000. Often discrepancies were found to indicative of incorrectly calibrated beams, misinterpreting the irradiation instructions, or errors in completing the irradiation form. In many instances communication with individual institutions led to identifying and correcting specific issues; specific examples will be included in the presentation. Figure 2: Results of independent peer review of external beam radiation therapy beam output for more than 155, 237 instances (Figure 1) Conclusion For a large sample of academic and non-academic institutions located throughout the world, the majority of beams monitored were found to be well within ± 3 of the stated dose. However, there were many instances where we identified serious calibration related issues that were subsequently corrected. Dissemination of such data can help prevent future similar errors from occurring. EP-1753 Sensing ability of EPID-based in vivo dosimetry for VMAT M. Tanooka 1 , K. Tarutani 2 , H. Doi 2 , H. Suzuki 2 , Y. Takada 2 , M. Fujiwara 2 , Y. Toda 3 , H. Fujimoto 3 , M. Miyashita 3 , A. Okumura 3 , K. Kagawa 3 , N. Kamikonya 2 , K. Yamakado 2 1 Takarazuka City Hospital, Department of Radiotherapy, Takarazuka, Japan 2 Hyogo College of Medicine, Department of Radiology, Nishinomiya, Japan 3 Japan Organization of Occupational Health and Safty Kansai Rousai Hospital, Department of Radiotherapy, Nishinomiya, Japan Purpose or Objective Electronic Portal Imaging Device (EPID) is used to detect X-rays that are emitted from patient during treatment. In a commercialized system, the first image is used as a reference to be compared with the second and subsequent images which are acquired in the treatment course. Since the verification method uses integrated data, the detectability of errors decreases as the amount of the integrated data increases. The aim of this study was to investigate sensing ability of EPID-based in vivo dosimetry by dividing and evaluating treatment plan. Material and Methods We used TrueBeam (Varian Medical Systems, Palo Alto, CA) as a treatment machine and aSi-1200 as an EPID. An ExacTrac (BrainLAB AG, Feldkirchen, Germany) pelvis verification phantom was also used. One arc prostate treatment plan was used and divided into 12 sections every 30 degrees. Then, we placed the phantom on the couch and acquired images using X-rays that passed through the phantom. Furthermore, we generated four different patterns of errors in one section of them. One was generated by changing the opening of the MLC into 3 mm, 2 mm and 1 mm. The other three were generated by