ESTRO 2021 Abstract Book

S1305

ESTRO 2021

which can be corrected by a dose renormalization using the planned dose or an external reference measurement to perform absolute dosimetry [2]. In this study, we investigate if absolute PG dosimetry may be performed without dose renormalization by including this fractionation effect also in the dose calibration measurement. [1] De Deene and Baldock 2004, doi:10.1088/1742-6596/3/1/E01 [2] Karlsson et al. 2007, doi:10.1088/0031-9155/52/15/021 Materials and Methods A set of 13 Barex TM containers (VELOX GmbH, Germany) was filled with a single batch of PAGAT (PolyAcrylamide Gelatin gel fabricated at ATmospheric conditions) gel. Three gel containers were irradiated with 2, 4 and 6 Gy under reference conditions at a clinical MR-Linac (Viewray, USA) and applying three different fractionation schemes: (A) a single fraction (standard method), (B) five equal fractions, and (C) a fixed fraction size of 1 Gy leading to a varying number of fractions for the three dose levels. One additional gel container was partly irradiated in 5 equal fractions of 1 Gy within a water-filled cylindrical phantom. The time between two consecutive fractions for both calibration and gel container was 10 minutes. PG evaluation was performed at a diagnostic 3T MRI scanner (Prisma fit , Siemens Healthineers, Germany) using a multi-spin echo sequence with 32 equidistant echoes. Using the three irradiation series under reference conditions with fractionation schemes A-C, three separate dose-to-R 2 calibration curves were generated and accordingly three 3D dose maps were created for the partly irradiated gel container. Results The three calibration curves with fractionation schemes A-C are shown in figure 1. A higher gel response (2% increase of R 2 at 6 Gy) was found for fractionation schemes B and C as compared to the standard method where no fractionation was applied (A). Dose profiles (figure 2a) through the half-sided gel irradiation (figure 2b) show a better agreement of measured and planned dose when using calibration B and C while the dose is overestimated when calibration A is applied.

Conclusion In this study, a good agreement of absolute measured and planned dose was found for a fractionated PG irradiation without a dose renormalization if a similar fractionation scheme was applied for dose calibration and 3D gel irradiation. Future experiments will investigate how this effect can be transferred to more complex dose distributions that apply multiple beams from different directions and several segments per beam. PO-1581 a multi-detector comparison of output factors for small circular electron beams S. Bettarini 1 , M. Esposito 2 , A. Ghirelli 2 , S. Pini 2 , S. Russo 2 1 University of Florence, Health Physics Specialization School, Florence, Italy; 2 AUSL Toscana Centro, Santa Maria Annunziata Hospital, Florence, Italy Purpose or Objective The dosimetric performances of the novel PTW microSilicon detector under small radiation therapy electron beams collimated by tubular applicators were investigated by comparison with standard available dosimeters. Materials and Methods High-energy electron beams generated by linear accelerators, typically in the 4-20 MeV range, are used for radiotherapy of superficial tumors. Small electron beams can be shaped by using stainless steel circular applicators fastened to the accelerator head with 2, 3, 4 and 5 cm aperture diameter at 100 cm distance from