ESTRO 2023 - Abstract Book

S771

Monday 15 May 2023

ESTRO 2023

Conclusion Interim analysis from the ongoing multicentric randomised trial PRIME showed low rate of grade 3 acute toxicities for MHRT and SBRT, with similar toxicities and QOL for these two hypofractionated schedules of radiotherapy in high risk and node positive prostate cancer. OC-0925 2-fraction prostate SABR+/-dominant intraprostatic lesions boost: results from two Phase II trials W.L. Ong 1 , P. Cheung 1 , H. Chung 1 , W. Chu 1 , J. Detsky 1 , S. Liu 1 , G. Morton 1 , E. Szumacher 1 , C. Tseng 1 , D. Vesprini 1 , M. Davidson 1 , A. Ravi 1 , M. McGuffin 1 , L. Zhang 1 , A. Mamedov 1 , A. Deabreu 1 , M. Poon 1 , A. Loblaw 1 1 Odette Cancer Centre, Sunnybrook Health Sciences Centre, Department of Radiation Oncology, Toronto, Canada Purpose or Objective Focal boost to the dominant intra-prostatic lesion (DIL) is an approach for dose escalation in prostate cancer, and it has been shown to be associated with biochemical progression free survival benefit in the FLAME trial. It is unclear whether this benefit applies in the context of ultra-hypofractionation. We aim to report the outcomes of two-fraction prostate stereotactic ablative radiotherapy (SABR) with or without simultaneous DIL boost. Materials and Methods We included two Phase II trials of two-fraction prostate SABR. In 2STAR (NCT02031328), 26Gy (EQD2 110Gy) was delivered to the prostate. In 2SMART (NCT03588819), 26Gy was delivered to the prostate with up to 32Gy boost to the MRI-defined DIL (EQD2 164Gy). Biochemical failure (BF) was defined based on Phoenix criteria, and 4-year PSA response rate (4yrPSARR) was defined as PSA <0.4ng/mL. Acute ( ≤ 3 months) and late ( ≥ 6 months) toxicities outcomes were assessed based on CTCAEv4, while quality of life (QOL) outcomes were assessed using the EPIC-26 questionnaire. Minimal clinically important changes (MCIC) in QOL was defined as decline in EPIC-26 domain score of >0.5 standard deviation of the baseline score. Results 60 men with low-intermediate risk prostate cancer were included in this study (30 men in each trial). In 2SMART, a median DIL D99% of 32.3Gy was delivered. The median follow-up was longer in 2STAR (73 months; range: 62-78 months) compared to 2SMART (44 months; range: 39-49 months). The median PSA nadir was 0.16ng/mL in 2STAR and 0.25ng/mL in 2SMART (P=0.16), and the median time to PSA nadir was 57.5 months in 2STAR and 36.9 months in 2SMART (P<0.001). The 4yrPSARR was 57% (17/30) in 2STAR and 63% (15/24) in 2SMART. The 4-year cumulative BF was 0% in 2STAR and 8.3% in 2SMART (P=0.1). The 6-year BF in 2STAR was 3.5%. For genitourinary toxicities, there were differences in Grade ≥ 1 urinary urgency in the acute (0% vs 47%; P<0.001) and late settings (10% vs 67%; P<0.001) favouring 2STAR. For urinary QOL, no difference was observed in the acute setting, but lower proportion had MCIC in the late setting (21% vs 50%; P=0.03) in 2STAR. There were no significant differences in gastrointestinal and sexual toxicities and QOL in both acute and late settings between the two trials. Conclusion This is the first prospective data comparing the outcomes of two-fraction prostate SABR with or without DIL boost. The addition of DIL boost resulted in similar 4yrPSARR and 4-year BF, but at a cost of slight increase in urinary toxicities and impact on urinary QOL. Longer-term follow-up is required to determine if addition of DIL boost translates into BF benefit as observed in the FLAME trial. Novel approaches of prostate SABR may be required (such as dose de-escalation of the non-