ESTRO 2021 Abstract Book

S1341

ESTRO 2021

Conclusion In this study a good agreement between measurements using gafchromic films and Monte Carlo calculation were achieved at whole range of material densities. The main issue was to add a valid extra calibration point. The significant difference was observed only for buildup regions.

PO-1619 Bladder PSD in vivo dosimetry in brachytherapy A. Herreros 1 , L. Moutinho 2 , J. Melo 2 1 Hospital Clínic, Radiation Oncology, Barcelona, Spain; 2 NU-RISE, Lda, Aveiro, Portugal

Purpose or Objective The steep dose gradients in brachytherapy represent a big challenge for in vivo dosimetry (IVD), but the usual optimization techniques and dose constraints make IVD necessary to verify the dose delivery to the target volume and also to the organs at risk (OAR). Placing a detector in the OAR directly allows a measurement that takes into account its relative motion with respect to the source. The purpose of this study was to evaluate the performance of a plastic scintillator detector (PSD) in bladder dosimetry in post-operative endometrial brachytherapy in terms of dwell time accuracy. In addition, the actual signals measured with the PSD were also compared with those calculated in the treatment planning system (TPS). Materials and Methods The PRODOSE in vivo dosimetry system (NU-RISE, Portugal) uses an organic scintillator BCF-12 (Saint Gobain Crystals, France) with a length of 2 mm attached to a PMMA optic fiber with a diameter of 0.5mm. The detection technology consists of a silicon photomultiplier (SiPMs). The system works with an automatic cooling system and a temperature correction for the photodetector. The measurement frequencies used were between 6 Hz and 10 Hz, allowing integration of the signal across 150 and 100 ms. A dosimetry bladder catheter with a Foley balloon with three channels from Rüsch (Teleflex, Ireland) was used to maintain constant bladder filling and also to insert the PSD probe. In Figure 1, a sagittal reconstruction shows the PSD probe with a gold marker, clearly visible inside the bladder catheter.