ESTRO 2021 Abstract Book

S781

ESTRO 2021

A 3D PinPoint (PiP) IC type TW31022 was exposed to reference medium energy x-rays (0.5 to 4 mm Cu HVL). Leakage, reproducibility, linearity, direction dependent response and dose rate, as well as energy dependence were investigated. Stability of the chamber’s air kerma calibration factor (Nk) was assessed. Polarity and ion recombination in reference and user’s beams irradiation conditions were evaluated. The chamber’s correction factor (kch,31022) was experimentally determined. The PiP IC placed in small phantom, was used for an end- to-end test in an IGPIP, with a 10 mm x 10 mm field. Measured and TPS calculated were compared. Results In the reference x-ray beams, corrections for ion recombination were smaller than 0.1%. Saturation curve (Figure 1) shows that, independently of dose rate, the chamber responds differently when collecting negative and positive current (charge). The largest polarity correction factor was 1.014±0.008 (4 mm Cu HVL). Table 1 shows experimentally determined kch for a TW31022 as well as those for a 2611 reference chamber. The implementation of the end-to-end test with the delivery of image targeted small field plans, resulted in differences smaller than 3% between measured and TPS calculated doses.

Figure 1: Saturation curve

Table 1 Experimentally determined chamber correction k(ch,31022)

Conclusion The properties and response of the 3D PiP IC in medium energy x-rays demonstrated its suitability as reference detector for measurements in large and small fields in this energy range. Polarity and ion recombination effects were similar at different dose rates. The possibility to ionometrically validate small fields’ TPS calculations in IGPIPs will contribute to harmonization in dose reporting. This approach could lead to improvement of measurement uncertainties in reference and relative dosimetry of small fields delivered by IGPIP, while maintaining the traceability chain to primary standards. [1] Draeger E, et al. Int J Radiat Oncol Biol Phys 2020 [2] Verhaegen F, et al. Radiother Oncol 2018

Poster discussions: Poster discussion 36: Image-guided radiotherapy

PD-0938 impact of an atlas on radiographer inter-observer contour variation in prostate radiotherapy A. Clough 1 , R. Hales 1 , J. Parker 1 , J. McMahon 1 , L. Whiteside 1 , L. McHugh 1 , L. Davies 1 , J. Sanders 1 , R. Benson 1 , C. Nelder 1 , A. Choudhury 1,2 , C. Eccles 1,3 1 The Christie , Radiotherapy, Manchester, United Kingdom; 2 University of Manchester , Division of cancer sciences, Faculty of Biology, Medicine and Health, Manchester, United Kingdom; 3 University of Manchester, Division of Cancer Sciences, Faculty of Biology, Medicine and Health, Manchester, United Kingdom Purpose or Objective The clinical implementation of MR-guided adaptive radiotherapy (MRgART) requires fast, accurate delineation of target volumes and organs at risk (OARs) to mitigate the potential for contouring inaccuracies and therefore treatment errors. To support the introduction of radiographer (RTT)-led real-time MRgART on the MR Linac (MRL), an MR contouring atlas for prostate radiotherapy delineation was developed. The impact of its use on inter-observer variation is reported in this work. Materials and Methods A contouring atlas was developed through the multi-disciplinary consensus of 3 MRL radiographers, 2 clinical oncologists, and 1 diagnostic radiographer for the prostate, seminal vesicles, bladder and rectum. Contours