ESTRO 2021 Abstract Book

S1311

ESTRO 2021

Conclusion We successfully designed a robust phantom that showed suitable for output stability verification. The ETHOS output shows a small drift and a distinct atmospheric dependence. Although MPC is capable of detecting daily output fluctuations of the ETHOS, it has a large tolerance and is not a substitute for periodical independent output verification. PO-1586 Impact of curing time and temperature on dose response for silicone-based 3D radiochromic dosimeters M.B. Jensen 1 , P. Balling 2 , J.B.B. Petersen 3 , S.J. Doran 4 , L.P. Muren 1 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Aarhus University, Department of Physics and Astronomy , Aarhus, Denmark; 3 Aarhus University Hospital, Department of Clinical Medicine, Aarhus, Denmark; 4 The Insitute of Cancer Research , Cancer Research UK Cancer Imaging Centre, London, United Kingdom Purpose or Objective Silicone-based radiochromic dosimeters allow for dose read-out in 3D with high spatial resolution and has over the years attracted growing interest in the field of radiotherapy due to increasing complexity of new treatment delivery techniques. However, material composition and curing conditions must be carefully optimized to maintain dosimetric properties from batch to batch. This study investigated the impact of curing time and temperature on dose response and dose-rate dependency. Materials and Methods Following a previously established protocol for this dosimeter, cuvette sized dosimeters (1 cm x 1 cm x 4.5 cm) were fabricated from silicone elastomer, curing agent, chloroform and leucomalachite green. The dosimeters cured in the cuvettes protected from light, but at different curing temperatures and curing times: 60 cuvettes were cured at 15 degrees Celsius for 2.2 days (Group 1); 36 cuvettes were cured at room temperature for 3.3 days (Group 2) while another 36 also cured at room temperature but for 6.2 days (Group 3). The cuvettes in Group 1 were irradiated to dose levels of 5, 10, 15 and 20 Gy with 5 cuvettes per dose level while cuvettes in Group 2 and 3 were irradiated to dose levels of 5, 12.5 and 20 Gy with 4 cuvettes per dose level. To obtain different dose-rate conditions, the dosimeters were placed in between a 5 cm solid water backscatter slab and 4.5, 9.5 and 14.5 cm solid water build up slabs and irradiated with a TrueBeam linear accelerator (Varian Medical Systems) with a 10 x 10 cm 2 field, flattening filter mode, 6 MV, 600 MU/min and source-to-surface distance of 95 cm. The different build up slabs corresponded to dose rates of 3.5, 4.6 and 6.0 Gy/min. The dosimeters were read-out prior to and after irradiation using a spectrophotometer (Spectroquant Pharo 100) and the optical response, Δα, was calculated as the difference in optical density divided by optical path length (1 cm) and plotted against dose with the slope being the dose response.