S1642
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
radiotherapy." 2011 2nd International Conference on Advancements in Nuclear Instrumentation, Measurement Methods and their Applications. IEEE, 2011.This work was partially funded by European Union – Next Generation EU - PNRR - M4C2, investment 1.1 - PRIN 2022 fund - ALPHA id 2022HHZWRS-LS7-PRIN2022 – CUP B53D23020320006 Keywords: 3D dosimetry Digital Poster 1789 A comparison of surface dose measurements on ring gantry and C-arm linacs R Lee MacDonald 1,2 , Logan G Montgomery 1,2 , Patricia AK Oliver 1,2 1 Medical Physics, Nova Scotia Health, Halifax, Canada. 2 Radiation Oncology, Dalhousie University, Halifax, Canada Purpose/Objective: To investigate surface dose on ring gantry linacs compared with C-arm linacs in various scenarios, considering 6 MV photon beams with flattening filter and flattening filter-free (WFF and FFF, respectively). Material/Methods: 10x10 cm2 square, en face fields were delivered to EBT3 radiochromic film on the surfaceand at 5 mm depth in a solid water phantom at 100 cm SSD. Radiochromic film was also used to measure surface dose and dose under bolus (where applicable) on an anthropomorphic phantom for breast and chest wall tangent and VMAT treatment plans (520 cGy per fraction). Tangent plans included medial and lateral oblique fields with flash and,in the case of chest wall, were delivered with and without the presence of 5 mm tissue equivalent bolus. VMAT breast plans were optimized with 1 cm of flash extending into a1.5 cm thickartificial bolus and were delivered without bolus. VMATchest wall plans were optimized with 2.5 mm flashextending into a 5 mm bolus and were delivered with bolus. Results: For the 10x10 cm2en face fields, the relative surface dose enhancement per monitor unit(MU) is 32% for ring gantry FFF versus C-arm WFF, 23% for C-arm FFF versus WFF, and 7% for ring gantry FFF versus C-arm FFF. The corresponding absolute increases of dose relative to dose at dmaxare 7%, 5%, and 2%, respectively. At 5 mm depth, the relative dose enhancement per MU is 10% for ring gantry FFF versus C-arm WFF, 5% for C-arm FFF versus WFF, and 5% for ring gantry FFF versus C-arm FFF. Results for the breast plans are shown in Figures 1A & 1B, and for the chest wall plans in Figures 2A & 2B.According to the Eclipse treatment planning system (TPS) using the Acuros v18 dose calculation algorithm, the mean
dose to a 1.5 mm thick skin structure at approximately the same location as the filmis hotter than the film measurement by ~30-50%. However, dose under bolus agrees with the TPSwithin 2% on average, with a standard deviation of 3%.
Conclusion: Trends in en face measurements do not translate to optimized plans. Film measurements at the surface
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