ESTRO 2022 - Abstract Book

S1370

Abstract book

ESTRO 2022

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Results of the unit-cell measurement

Conclusion In this study, we demonstrated that the photoconductive material has dosimetric characteristics for high energy radiation dosimeter and it’s possible as an alternative of indirect material. Therefore, it is considered to be of great significance as a fundamental study in applying photoconductive materials to high energy radiation dosimeters.

PO-1589 Gating Device L: an external surrogate for respiratory motion tracking on TrueBeam

M.M.B. Nielsen 1 , N.K.G. Jensen 2 , S.M.S. Damkjær 3

1 Zealand University Hospital, Oncology and Palliative Care, Næstved, Denmark; 2 Rigshospitalet, Oncology, Copenhagen, Denmark; 3 Herlev and Gentofte Hospital, Oncology, Herlev, Denmark Purpose or Objective Patients with breast cancer treated in deep inspiration breath hold with external beam radiotherapy require a motion management (MM) system. The gating system for TrueBeam from Varian provides MM with an external surrogate. When an external surrogate is used for MM, placing it closer to the breast increases the precision of the tracked target motion (Skyttä et al. Acta Oncol. 2016;55:970-75). However, placing the surrogate closer to the target may result in the treatment beams going through the surrogate thus causing an unintended bolus effect. It is therefore crucial that the design of a surrogate minimizes the bolus effect to allow for placement near the breast. In this study, we have tested a new surrogate: Gating Device L (GDL) and compared it to Varian’s Truebeam reflector block (RB), as well as a modified version of the reflector block (MRB) (see figure 1), with respect to surface dose, bolus effect and tracking capabilities.

Figure 1: The three surrogates investigated: The Gating Device L (GDL, the Reflector Block (RB) and the Modified Reflector Block (MRB). Materials and Methods Percentage depth dose curves and profiles with and without surrogates were measured with Gafchromic film and performed in-silico in the treatment planning system (TPS) to assess the surface dose and bolus effect in a 6 MV beam. RB and GDL were compared in the TPS to evaluate the dosimetric foot print on dose distributions on a static field directly through the surrogates and by the Volumetric Modulated Arc Therapy (VMAT) technique and compared by calculating the Dice Similarity Coefficient (DSC). The tracking capabilities of RB and GDL by the gating system were investigated with induced pitch, roll and yaw of the surrogates. Results Surface dose was increased up to 295%, 277% and 190% for RB, MRB and GDL respectively compared to no surrogate with GDL having a smaller physical surface area of 11 cm 2 compared to 60 cm 2 of RB and MRB (see figure 2). In a static field