ESTRO 2021 Abstract Book

S1404

ESTRO 2021

Conclusion An offline reconstruction method was developed that successfully emulated the image quality of the ProBeam system. Low dose CBCT images were effectively simulated and will enable the investigation of optimal image acquisition parameters using existing scans. Visual assessment of the generated images showed that bone and soft tissue contrast remain even in the lowest exposures simulated. Future work will use these methods to analyse registration uncertainty and quantify the dosimetric benefits of using low exposure 3D rather than 2D imaging for set up in IGPT. PO-1679 Semiautomatic contouring of dominant intraprostatic lesions in prostate using diffusion weighted MRI V. Giacometti 1 , A.R. Hounsell 2 , S. Jain 1 , C.K. McGarry 2 1 Queen's University Belfast, School of Medicine, Dentistry and Biomedical Sciences, Patrick G Johnston Centre for Cancer Research, Belfast, United Kingdom; 2 Northern Ireland Cancer Centre, Radiotherapy Physics, Belfast, United Kingdom Purpose or Objective Advances in high-precision radiation therapy in prostate cancer (PCa) radiotherapy currently allow escalation of the dose to the dominant intraprostatic lesion (DIL), thus improving the therapeutic ratio. The aim of this study was to investigate the possibility of providing guidance for the clinicians to contour the DIL, using thresholding and image registration methodologies. Materials and Methods Five patients from the Stereotactic PrOstate RadioTherapy (SPORT) trial ( https://www.clinicaltrials.gov/ct2/show/NCT03253978 ) were imported into Varian Eclipse (v13.5) (Varian Medical Systems, Palo Alto) treatment planning system (TPS). Clinical DILs for these patients were contoured based on biopsy, imaging, prostate specific antigen level, history and treated with a boost of 45-50Gy. The patients were imaged with T1, T2 diffusion-weighted (DW) magnetic resonance (MRI), and CT scan before the implantation of a hydrogel rectal spacer [1], and with T2 MRI and CT scan (planning CT) after the spacer was implanted. The prostates were manually contoured in each set of images and a threshold was applied to the DW MRI for contouring the DILs. Thresholded and clinical DILs were compared by calculating the Dice coefficients, measuring the volume and centre of mass distance. To ensure that pre and post spacer images were correctly aligned, they were rigidly registered by aligning the anterior of the contoured prostates. Results The prostate contours for the five selected patients are consistent among different image modalities with volume differences oscillating from 0% to 26%. Dice coefficients above 0.8 were calculated for the all the prostate contours while coefficients between 0.3-0.6 were measured for the thresholded DIL volumes (Fig. 1). The clinical and thresholded DIL volumes were between 0.8-2.1 cm 3 and 0.3-3.8 cm 3 , respectively. An example