ESTRO 2020 Abstract book

S794 ESTRO 2020

VMAT prostate (60 Gy, 20#), VMAT prostate with pelvic nodes (74 Gy, 37#) and conformal breast (40.05Gy, 15#) treatment sites have been analysed so far. Each linac is able to deliver a treatment planned for the other linac. ArcCheck results show no significant difference in gamma results for plans delivered on either type of linac signifying no significant difference in delivery performance. Transferring plans from TB to iX resulted in approximately 2% higher dose for both VMAT and conformal plans with corresponding increase in dose to OAR. The limiting factor for breast transfers was the volume receiving 107% of the prescription. The limiting factor for prostate and prostate-nodes treatments was the dose to rectum and non-rectal bowel. Transferring plans from iX to TB resulted in approximately 2% cooler dose for both VMAT and conformal plans. The limiting factor in all cases was the PTV coverage. Conclusion Treatments planned for one type of linac can be safely and effectively delivered on the other type of linac. DVH analysis demonstrates that for all sites studied, patients can be safely transferred to the other type of linac for part of their treatment. Our department has set the limit to 5 transfer fractions for these treatment sites. PO-1405 SBRT/SRS patient-specific QA using GAFchromic EBT3 e FilmQA software G. Stella 1 , N. Cavalli 1 , E. Bonanno 1 , A.M. Gueli 2 , A. Girlando 3 , C. Marino 1 1 HUMANITAS-Centro Catanese di Oncologia, Department of Medical Physics, Catania, Italy ; 2 University of Catania, Department of Physics and Astronomy "E. Majorana", catania, Italy ; 3 HUMANITAS-Centro Catanese di Oncologia, Department of Radiotherapy, Catania, Italy Purpose or Objective SRS/SBRT treatments require typically pre-treatment (patient specific) procedures to verify, for each patient, the agreement between the measured and calculated dose distribution. In this context, film dosimetry with Gafchromic TM is a reference method thanks to high spatial resolution, excellent energy independence and tissue equivalence. The aim of this work is to verify the use of Gafchromic TM EBT3 and FILMQA TM software for SBRT/SRS patient-specific QA for a routine clinical procedure. Material and Methods SBRT/SRS treatment, related to Brain, Lung and Nodes, were calculated using the AcurosXB 13.6.23 algorithm and TPS Eclipse Varian Medical Systems (13.6), VMAT and Flattening Filter Free (FFF) mode (6X-FFF and 10X-FFF beams). Gafchromic TM EBT3 calibration was obtained using 6X-FFF and 10X-FFF photon beams and VARIAN TRUE BEAM 2.5 (0-25 Gy); they were digitized with EPSON EXPRESSION 10000 XL, one scan method, transmission mode and analyzed with FILMQA software using triple channel method. To avoid scan-to-scan variability and uncertainty, EBT3 films, exposed to each VMAT arc, were digitized in a single scan with two reference films: a film exposed to 80% dose max of the calculated dose frequency distribution and an unexposed film. To obtain a linear dose scaling using the calibration curve and a more accurate dose map, each film was associated to a calibration curve created using 0 Gy, 20% and 40% of calculated max dose and finally the reference film (80% of calculated max dose). The agreement between calculated and measured dose distributions were evaluated in terms of ɣ passing rate. Criteria were: 3%3mm, 2%2mm, 2%1.5mm, 1.5%1.5mm, 3%1.5mm. Results obtained were then compared with the pre-treatment verification procedures normally used: EPID and EPIQA software.

Fig. 1: mean values and relative errors of the ɣ passing rate, for each criteria, obtained both with the EPID-EPIQA and GAF-FILMQA method.

Fig. 2: results obtained, by relating the modulation coefficient and the ɣ passing rate (criteria 3%1.5mm), with the two verification methods. Results For GAF/FILMQA method, mean ɣ passing rate greater than 95% with 2%1.5mm criteria were obtained (better than 7% compared to EPID/EPIQA method results) (fig. 1). GAF/FILMQA method shows to be independent from modulation values obtained from planning, unlike the EPID/EPIQA method (fig. 2). Conclusion GAF/FILMQA method fully respects the accuracy and time- consuming criteria for a routine clinical use for SBRT/SRS patient specific QA. PO-1406 Phantom validation for regional nodal irradiation after mastectomy for breast cancer J.H. Joo 1 , J. Park 2 , H. Jeon 1 1 Pusan National University Yangsan Hospital, Radiation Oncology, Yangsan-si, Korea Republic of ; 2 Kyungpook National University Chilgok Hospital, Radiation Oncology, Daegu, Korea Republic of Purpose or Objective To take advantage of survival benefit of regional nodal irradiation including internal mammy nodes (IMNs), precise target dose reaching the lymph nodes (LNs) rather than scatter to the heart is the most important issue. To improve target dose homogeneity and reduce cardiac exposure, volumetric modulated arc therapy (VMAT) is used. Heart, ribs and air (lungs) are located adjacent to IMN area. Thus, movement caused by respiration and heartbeat is quite large. To determine whether a precisely modulated VMAT plans will be implemented in moving patients, we conducted a dosimetric study using a phantom and a dynamic platform. The planned dose delivery to IMN areas were compared between 3- dimensional wide tangent technique and VMAT.