ESTRO 2022 - Abstract Book

S732

Abstract book

ESTRO 2022

93.5% for SRS cases. The mean gamma passing rate of SRS plans compared to TPS becomes 98.2% after the removal of two fields characterized by low dose spread (< 25 cGy) related to the MBM fields that is not correctly estimated by the pencil beam algorithm. Conclusion The myQA ® SRS device was is a suitable tool for PSQA for SBRT and SRS treatments even in the presence of small field conditions. Furthermore the device yields measurements that are nearly equivalent to the golden standard being film measurements.

PD-0813 In vivo dosimetry: Optical fibre characterisation for use in adaptive HDR pelvic brachytherapy

O. Houlihan 1,2 , E. Marlow 2 , S. Esteve 3 , G. Workman 3 , M. Byrne 3 , C. McGarry 3 , M. Grattan 3 , S. O'Keeffe 4 , K. Prise 2 , A. Hounsell 3,5 , S. Jain 1,5 1 Northern Ireland Cancer Centre, Belfast City Hospital, Clinical Oncology, Belfast, United Kingdom; 2 Queen's University Belfast, Patrick G. Johnston Centre for Cancer Research, Belfast, United Kingdom; 3 Northern Ireland Cancer Centre, Belfast City Hospital, Radiotherapy Medical Physics, Belfast, United Kingdom; 4 Optical Fibre Sensors Research Centre, University of Limerick, Department of Electronic and Computer Engineering, Limerick, Ireland; 5 Queen’s University Belfast, Patrick G. Johnston Centre for Cancer Research, Belfast, United Kingdom Purpose or Objective To characterise and calibrate 1 mm core PMMA optical fibre sensors as a first step in the integration of optical fibre sensors, developed as part of the EU funded H2020 project “ORIGIN”, within HDR pelvic brachytherapy for use in adaptive brachytherapy. Materials and Methods An optical fibre connected to a scintillation detector of 0.5 mm radius was used to undertake measurements on an Elekta Flexitron HDR system. Initial measurements were performed by placing the fibre within a HDR prostate Perspex needle phantom capable of holding 20 HDR needles in parallel, placed between two solid water blocks. The fibre was placed within a plastic HDR needle inserted in a channel 2 cm from the Iridium-192 source within the phantom, for a dwell time of 35 seconds (Fig. 1(a) and 1(b)). Measurements were performed three times without removing either the fibre or the source from the setup to assess repeatability. The fibres were then placed in a water tank jig at 1 cm intervals, with positioning supported between the lid of the tank and a support bridge within the water(Fig. 2(a) and 2(b)) The Perspex support bridge was built within the tank to anchor the distal end of the brachytherapy needles containing the fibre and source, ensuring vertical placement within the jig. The distance from the lid of the water tank to the support bridge was approximately 15 cm to simulate full scatter conditions with water. The output of the fibres was assessed using an 192 Ir radioactive source for a dwell time of 15 seconds and compared with estimated outputs from the treatment planning system (TPS). Results The repeatability measurements of the optical fibre within the Perspex needle phantom demonstrated variation of less than 1% (Fig. 1(c)). The output signal, as the source-to-sensor distance is increased in 1 cm increments from 1 cm to 10 cm is shown in Fig. 2(c). As expected, given the inverse square law, a large drop in the optical signal was observed for measurements closest to the source, with a slower taper at greater distances. The relative difference between the output of the optical fibre and that of the TPS increased significantly with increasing distance, due to the fibre sensor’s energy- dependence, and a correction factor must be determined to reduce this deviation (Fig. 2(c)).