ESTRO 2023 - Abstract Book

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ESTRO 2023

Table. Measured values of rectum probe (diodes R1, R2, R3, R4, R5) readings for all temperature stop-points sequence during experiment.

Conclusion 1. Measured values for temperature dependence of calibration factors of diodes are ~0.1%/K. As results, we need to correct minus 1-2% of INVIVO diodes readings in treatment procedure, if we want to be precise. However, up-to-now in our clinic we do not do this on our practice, because there are many others factors of uncertainties. 2. Found temperature dependence of calibration factors ~0.1%/K is in disagreement with results (~ 0.5%/K) of authors [1, page 160]. Good point for dispute. [1] - Fröhlich et al., „In vivo dosimetry of the rectum...“, Reports Prac. Oncol&Radiother., Vol.24, p. 158, 2019

PO-2166 Commissioning of new rectal applicator using electronic brachytherapy source

N. Tomic 1 , L. Liang 1 , A. Esmaelbeigi 2 , J. Kalinowski 2 , S. Enger 2 , T. Vuong 1 , S. Devic 3

1 Jewish General Hospital, McGill University, Radiation Oncology, Montreal, Canada; 2 McGill University, Medical Physics Unit, Montreal, Canada; 3 Jewish GeneralHospital, McGill University, Radiation Oncology, Montreal, Canada Purpose or Objective The Xoft Axxent electronic brachytherapy source with 50 kVp miniature X-ray tube is currently used to treat patients with rectal carcinoma by mimicking Papillon approach with radiation emitted through the rectoscope-like applicator (“end-on” geometry), which is uncomfortable for patients and might result in patient movement and in treatment miss. We developed a Xoft New Rectal Applicator (NRA) with side-on collimator (Fig. 1a), which allows for supine treatment position and live tumor monitoring with optical cable. We present results of measured NRA dose rate and dose distribution uniformity. Materials and Methods We determined beam quality of the NRA with Xoft 50 kVp source by measuring HVL with parallel-plate ion chamber (PPIC) under narrow beam geometry. We first performed relative dose measurement using EBT3 model GAFCHROMICTM film for NRA with single source position. Then, we simulated dose distribution for multiple source positions to find dwell positions and times with best achievable dose uniformity across the NRA collimator. We used air Kerma in air based reference EBT3 model film dosimetry system to estimate 2D dose distribution and the output. We performed the reference air Kerma in air measurements (Fig.1b), using calibrated Exradin A20 PPIC at reference point (AAPM TG-61). At the same point, film pieces were irradiated (Fig.1c) to create calibration curve, air Kerma in air as a function of film response ((PV0/PV) -1), which (using green color channel) was found to be linear. In-phantom treatment plan measurement setup schematic is presented in Fig.2a. The dose distribution was measured using wrapped film pieces around NRA (Fig.2b). The in-phantom NRA output measurements were performed using PPIC (Fig.2c). Dose to water, for both measurements, was determined (AAPM TG-61) by multiplying measured air kerma in phantom with (µen/ ρ )waterair.