ESTRO 2021 Abstract Book

S1137

ESTRO 2021

Results Median age was 74 years (range 63-82). Intermediate and high risk prostate cancer accounted for 70% and 30% respectively. Median International Prostate Symptom Score (IPSS) score at baseline was 8 (range 2-14). Median PTV volume was 81.2 cc (range 48.9-128.5). Average total treatment time lasted 9.3 minutes (range 6-14), 5.8 minutes (range 3-9) for setup and 3.5 minutes (range 2-5) for beam delivery. In 41% of the monitored fractions, a new CBCT was mandated. The prostate was found within 1 mm from its initial position in 78.7% of the beam-on time, between 1 and 2 mm in 19.1%, and exceeds 2 mm only in 2.2%. All patients completed the treatment in the expected time and their compliance to the procedure was excellent. No clinically significant acute Grade 2 or higher GI (rectal) and GU toxicity was observed. Only one patient experienced acute Grade 1 GI toxicity, while acute Grade 1 GU toxicity occurred in five (50%) patients. Conclusion Linac-based SBRT by means of VMAT-FFF technique coupled with daily image guidance including real-time EM tracking allowed dose-escalated treatment with negligible early side effects. The procedure was implemented rapidly and resulted well tolerated and less invasive than the surgically implanted transmitter. PO-1387 Diffusion MRI as an early response marker in management of high-risk non-metastatic prostate cancer M. Stancliffe 1 , R. Davda 1 , H. Payne 1 , A. Mitra 1 , U. McGovern 1 , D. Pendse 2 1 University College London Hospital, Oncology, London, United Kingdom; 2 University College London Hospital, Radiology, London, United Kingdom Purpose or Objective To evaluate kinetics of apparent diffusion coefficient (ADC) of tumour and normal prostate in men with high- risk non-metastatic prostate cancer undergoing neo-adjuvant treatment with Androgen Deprivation Therapy (ADT) with or without docetaxel prior to radiotherapy (RT). Materials and Methods Retrospective analysis of multiparametric prostate MRI in 37 men with high-risk non-metastatic prostate cancer defined as ≥2 of: T3/T4 disease, PSA ≥40ng/ml or Gleason Score ≥8. MRI was analysed at baseline and after neo-adjuvant treatment (ADT +/- docetaxel chemotherapy), prior to radiotherapy. MRI images were reviewed by a single expert uro-radiologist. Regions of interest (ROI) were drawn, outlining both prostate tumour and normal prostate on the contralateral side. ROI position on the post-treatment study was manually matched to the location of tumour ROI on the pre-treatment MRI. The mean ADC values were generated for the tumour and normal ROIs for both baseline and post treatment MRIs. Statistical analysis was performed using Wilcoxon matched pairs signed rank test. Results The increase in tumour ADC suggests diffusion MRI can be used as an early quantitative biomarker of treatment response. These results warrant further evaluation with long-term follow-up to determine whether it can predict for progression free survival in this group of patients at risk of disease relapse. Evaluation of ADC response may predict clinical outcomes and allow stratification of patients who may benefit from treatment intensification such as focal dose escalation to the tumour or dose escalation using brachytherapy boost.Tumour ADC increased after neo-adjuvant treatment from baseline median ADC 590 mm2/s to post- treatment median ADC 702 mm2/s showing statistical significance in difference for matched pair values (p=0.004, W=377, pairs=37). A significance in difference for matched pair values was seen in sub-group analysis of ADT plus docetaxel (p=0.02, W=122, pairs=20), and there was a trend towards a significance in ADC in the ADT alone group (p=0.07, W=77, pairs=17) (figure 1). Normal tissue ADC decreased from baseline median ADC 1352 mm2/s to post-treatment median 958 mm2/s after neo-adjuvant treatment, showing statistical significance in difference for matched pair values (p=0.009, W=-341, pairs=37) (figure 2).