ESTRO 2021 Abstract Book

S392

ESTRO 2021

39 centers enrolled 569 patients with prostate adenocarcinoma: 284 with low-risk and 285 with intermediate- risk disease. 101 patients had unfavorable intermediate-risk tumors (Gleason 4+3, two unfavorable risk factors, and/or ≥50% biopsy cores positive). All were treated with a non-coplanar robotic stereotactic platform using real-time tracking of implanted fiducials. Two dose regimens were used: 40Gy in 5 fractions of 8Gy, and 38Gy in 4 fractions of 9.5Gy. Adjuvant androgen deprivation therapy (ADT) was not allowed. Early (within 3 months of treatment) toxicity and 5-year survival outcomes have been previously described; we report outcomes in patients consenting to study participation beyond 5 years. Toxicities were assessed using Common Terminology Criteria for Adverse Events (CTCAE) version 3. All reported rates are actuarial, using Kaplan-Meier method. For relapse free survival (RFS), failure included initiating salvage or systemic therapy, local/regional/distant failure, or biochemical relapse using the “nadir + 2” definition. Results Median follow up was 8.1 years, with 109 patients followed for 10 years. There were no grade 4-5 toxicities. Actuarial 10-year grade 2 and grade 3 late GU toxicity rates were 14.9% and 2.2%, respectively. The 10-year late grade 2 GI toxicity rate was 3.6%; there were no grade 3 GI toxicities. For the entire group, 10-year overall survival rate was 88.1%, local failure rate was 2.5%, and RFS rate was 92.0%. 10-year RFS for low- and intermediate-risk groups were 98.5% and 85.8%, respectively. 10-year RFS for favorable and unfavorable intermediate-risk patients were 91.5% and 75.3%. No statistically significant differences in rates of toxicity, survival, local failure, nor RFS were observed between the two dose regimens.

Conclusion 10 years following treatment of organ-confined prostate cancer with ultrahypofractionated robotic stereotactic radiotherapy, toxicity rates continue to be minimal, with few additional events observed beyond 5 years. Overall survival, local control, and RFS rates remain favorable at 10 years, confirming stereotactic radiotherapy as a suitable option for low- and intermediate-risk prostate cancer. OC-0510 SABR +/- pelvic nodal irradiation for higher risk prostate cancer. A randomized feasibility study C. Fairmichael 1 , K.M. Redmond 2 , S.O. Osman 2 , C. Lyons 3 , D.M. Mitchell 4 , J.M. O'Sullivan 1 , A.J. Cole 1 , V. Giacometti 1 , G.S. Lundy 1 , D.M. Irvine 5 , C.K. McGarry 5 , A.R. Hounsell 5 , S. Jain 6 1 Queen's University Belfast, Patrick G Johnston Centre for Cancer Research, Belfast, United Kingdom; 2 Queen's University Belfast, Patrick G johnston Centre for Cancer Research, Belfast, United Kingdom; 3 Cork University Hospital, Oncology, Cork, Ireland; 4 Northern Ireland Cancer Centre, Oncology, Belfast, United Kingdom; 5 Northern Ireland Cancer Centre, Medical Physics, Belfast, United Kingdom; 6 Queen's University Belfast, Patrick G Johnston Centre for Cancer Research , Belfast, United Kingdom Purpose or Objective Stereotactic Ablative Radiotherapy (SABR) is increasingly used to treat low/intermediate risk localised prostate cancer (PCa). However, data on SABR +/- pelvic nodal irradiation (PNI) in high-risk disease is limited to single institution, single-arm studies. The S tereotactic P r o state R adio t herapy (SPORT) trial examined the feasibility of delivering a randomised trial of SABR ± PNI. The composite primary endpoint to determine feasibility comprised: recruitment rate, technical feasibility, acute toxicity (CTCAE v4.0) and quality of life (QoL)(EPIC). Secondary and exploratory endpoints including late toxicity (RTOG) and QoL and potential biomarkers of toxicity (plasma citrulline and lymphocyte DNA damage) are also presented. Materials and Methods Men with localised PCa demonstrating one of T3a, Gleason score ≥4+3 or PSA>20 were eligible. T3b and T4 disease were excluded. 30 participants were randomised 1:1 to prostate SABR (P-SABR) or prostate and pelvis SABR (PPN-SABR). The prostate and proximal 1-2cm seminal vesicles received 36.25 Gy in 5 fractions over 29 days (prostate CTV 40Gy). The PPN-SABR arm also received 25 Gy in 5 fractions to the pelvic nodes. All patients had fiducial markers and a rectal spacer gel in-situ. As initial toxicity rates were low, the final 10 men received an additional integrated boost to the dominant intraprostatic lesion of up to 50 Gy, irrespective of randomisation. Results Feasibility was demonstrated. The target recruitment of 30 men was met (83% high risk). Treatment, including an integrated boost plus PNI, was achievable using strict normal tissue constraints. Acute toxicity and QoL