Two-fraction stereotactic MRI-guided ablative radiotherapy with simultaneous boost to dominant intraprostatic lesion: Results from the 2SMART phase 2 trial.

349 Background: Prostate stereotactic ablative radiotherapy (SABR) for localised prostate cancer is commonly delivered over 5 fractions. Focal boost to the dominant intraprostatic lesion (DIL) seen on multiparametric magnetic resonance imaging (mpMRI) is an approach for dose-escalation in prostate SABR. This is the first report of the outcomes of the 2SMART trial, a phase 2 single-arm study using 2-fraction prostate SABR with DIL boost. Methods: Men with low to intermediate risk prostate cancer were enrolled in the study. Three gold fiducial markers were inserted for image guidance. The clinical target volume (CTV) included the prostate gland, and the planning target volume (PTV) was a 2mm expansion antero-posterior and laterally, and 2.5mm supero-inferiorly. The DIL was contoured on fused mpMRI. The prescribed dose was 26Gy in 2 fractions (EQD2 110Gy, α/β of 1.4) to the CTV, and up to 32Gy in 2 fractions (EQD2 164Gy) to the DIL as long as the dose constraints for the organs at risks were not exceeded. Each fraction was delivered 1 week apart. Daily image guidance with cone-beam computed tomography was used pre- and post-treatment. The primary endpoint was acute (≤3 months) changes in quality of life (QOL), assessed using the EPIC questionnaire. Minimal clinically important change (MCIC) in QOL was defined as an EPIC score decrease of >0.5 standard deviation of the baseline EPIC score for each domain. Secondary endpoints were acute and late toxicities (assessed using CTCAEv4), and biochemical failure (based on Phoenix criteria). Results: 30 men were enrolled in the study, of which 2 (7%) had low risk and 28 (93%) had intermediate risk prostate cancer. The median follow-up was 44 months (range: 39-49 months). The median PSA nadir was 0.2ng/mL, with median time to nadir of 37 months. One patient (3%) had biochemical failure at 44 months post-treatment. 1 (3%) and 17 (57%) had acute Grade 2 GU and GI toxicities, while 3 (10%) and 15 (50%) had late (>6 months) Grade 2 GU and GI toxicities. No acute or late Grade ≥3 GU or GI was reported. 10 (33%), 6 (20%), and 3 (10%) men had acute MCIC in urinary, bowel and sexual domains respectively. 15 (50%), 9 (30%) and 13 (43%) had late MCIC in urinary, bowel and sexual domain respectively. Conclusions: Two-fraction prostate SABR with DIL boost is a safe approach for dose-escalation for localised prostate cancer, with minimal impact on acute QOL, and no grade 3-4 toxicities. Clinical trial information: NCT03588819 .