ESTRO 2021 Abstract Book

S464

ESTRO 2021

Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark; 3 Medical Physics, Department of Oncology, Aarhus University Hospital, 8200 Aarhus N, Denmark

Purpose or Objective The accuracy of dose calculations in spot-scanning proton therapy of breast cancer is challenged by the curved patient surface as well as lung, bone and soft tissue interfaces. Patient-specific dose verification is typically performed field-by-field, on a flat detector, neglecting the anatomical complexities. Silicone-based 3D radiochromic dosimeters can be cast into anthropomorphic shapes. We have previously shown that such a system can be used to obtain accurate dose measurements if linear energy transfer (LET) dependent detector quenching is taken properly into account. The aim of this study was therefore to verify the 3D dose distribution in a clinically realistic breast geometry using radiochromic silicone-based dosimetry including quenching correction. Materials and Methods The radiochromic silicone-based phantom used in this study was cast in a spherical cap mould (12 cm base diameter, 5 cm radius) based on an average breast size. The phantom consisted of silicone elastomer, curing agent, leucomalachite green and chloroform, according to an established protocol (Fig. 1). Using clinically established planning procedures a spot-scanning proton plan delivering 12 Gy (to the breast ‘target’) was created in the Eclipse planning system (Varian Medical Systems). For pre-and post-irradiation read-out, the phantom was scanned using an optical CT scanner (Modus Medical). To take detector quenching into account, the measurements were corrected with a previously established calibration model for this system using Monte Carlo calculated LET distributions. Both the uncorrected and the quenching corrected measurements were compared with the TPS calculated dose distributions using a gamma comparison (including all regions receiving at least 10% of the calculated dose).

Fig.1: The radiochromic silicone-based phantom.

Results Overall, the measured dose captured the specific patterns of the planned dose distribution. The signal quenching for higher LET was visible in the measured distribution at larger depths. Following the quenching correction, gamma pass-rates increased from 80% to 89% (3%/3mm criteria) (Fig 2).

Fig. 2: Measured dose distribution, quenching corrected measured doses and TPS calculated doses for a single field in the breast dosimeter. The LET-calibrated measurements were compared to the TPS dose calculations using 3%/3mm criteria, showing an 89% pass-rate. Conclusion A radiochromic silicone-based 3D dosimetry system was found to be applicable for dose verification of spot- scanning proton therapy in a clinically realistic breast geometry. Using a previously established LET calibration model led to a considerably better agreement between measured and calculated doses. In ongoing studies, we are exploring the use of a dose-rate inclusive LET quenching correction model to improve upon these results.

PH-0600 3D in-vivo dosimetry for detecting interplay effect: a phantom study F. Biltekin 1 , G. Ozyigit 1 1 Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey