ESTRO 2020 Abstract Book

S745 ESTRO 2020

rest of the teeth, mandible and maxilla received mean doses <50Gy. Map 2: A 61 year old male with pT2N2b left oropharyngeal squamous cell carcinoma (SCC). He was treated adjuvantly with 66Gy to high-risk regions, 60Gy to left levels II-V, and 56Gy to right levels II-V. No tooth or region of maxilla or mandible received a mean dose of >50Gy. Conclusion We believe this to be the first study generating dental radiation dosimetric maps of estimated doses to each tooth and each third of the mandible and the maxilla for common examples of head and neck cancer faced by radiation oncologists. Adoption of these dental maps may help improve clinical work flow efficiency for dental providers and radiation oncologists by providing estimates of dental doses for common primary head and neck cancers, both for patients pending future radiation treatment and for previously irradiated patients when radiation plans are not available. PO-1402 Conventional Linac QC v Automated Linac QC: 3 years of experience of daily Varian MPC PO-1404 Safe number of transfers between Truebeam & Clinac for different treatment sites without replanning T. Newbold 1 1 Poole Hospital NHS foundation trust, Radiotherapy, Poole, United Kingdom Purpose or Objective Our department currently has one TrueBeam (TB) linac and two iX linacs. The two iX linacs are matched but the iX & TB linacs are not matched; the beam models for iX and TB in the Eclipse planning system are similar, though not identical. Having a single TB linac represents a single point of failure for which a contingency, other than a total re-plan for an iX linac, is desirable. Having a similar beam model suggests that it may be possible to treat a number of fractions on the ‘wrong’ machine without leading to detrimental patient dosimetry. The aim of this project is to ascertain whether treatment planned for one type of linac is deliverable on the other, and if so, for each treatment site, the safe number of fractions for which it is safe to transfer patients without re-planning the treatment. Material and Methods Treatment planning is performed in Eclipse. Plans originally produced for one type of linac were recalculated for the other by changing the beam model and fixing the MUs to the original value. Reported MLC errors were ignored. Appropriate DVH parameters were compared between the original plan and the same plan on the alternative machine. A predicted dose to the ArcCheck was calculated in Eclipse from the original plan. The original treatment plan was delivered on both linacs and an ArcCheck measurement taken. Gamma analysis was used to compare to the predicted and measured dose distributions for both deliveries. Results Analysis of the data obtained enabled recommendations for the maximum number of unplanned machine transfer fractions to be suggested. Outcome-based options for each site were given to clinicians for consideration. Table 1 shows the outcome based options for transfer number. Abstract withdrawn PO-1403 FMEA of a Linac QC program based on Varian MPC, DailyQA3, Delta4 and Conventional QC checks. Abstract withdrawn

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