Stereotactic total ablative radiotherapy (STAR) with MR-LINAC for synchronous oligometastatic prostate cancer

Darren Poon^1*Jing Yuan²Oi Lei Wong²Bin Yang³Sin Ting Chiu⁴Kin Yin Cheung³George Chiu⁴Siu Ki Yu³

• ¹Comprehensive Oncology Centre, Hong Kong Sanatorium and Hospital, Hong Kong, Hong Kong, SAR China
• ²Research Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong, SAR China
• ³Medical Physics Department, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong, SAR China
• ⁴Department of Radiotherapy, Hong Kong Sanatorium & Hospital, Happy Valley, Hong Kong, SAR China

Objectives: To prospectively investigate the feasibility, toxicity, and preliminary clinical outcomes of magnetic resonance (MR)-guided stereotactic total ablative radiotherapy (MRgSTAR) for simultaneous treatment of the prostate and pelvic bone metastasest ic lesions in patients with synchronous oligometastatic prostate cancer (OMPC).This study included patients with histologically confirmed synchronous OMPC, defined as ≤ 5 lymph node or pelvic bone metastases identified via prostate-specific membrane antigen positron emission tomography (PSMA-PET). Real-time adaptive MRgSTAR was delivered using a 1.5T MRintegrated linear accelerator (MR-LINAC) in five fractions, administered twice weekly, targeting the prostate (33.5-40 Gy) and nodal/bone metastases (36.5-40 Gy) simultaneously. Androgendeprivation therapy (ADT) was initiated prior to MRgSTAR, with the addition of androgen receptor pathway inhibitor (ARPI) at the physician's discretion. Adverse events (AEs) were assessed using the Common Terminology Criteria for AEs v5.0, and tumor response was evaluated per the Response Evaluation Criteria in Solid Tumors v1.1. Progression-free survival (PFS) and overall survival (OS) were estimated using the Kaplan-Meier method, with log-rank tests used to explore clinical factors associated with survival outcomes.Results: Forty-three patients underwent MRgSTAR, with a median follow-up of 36.5 months (range: 15.4-57.6 months). ADT combined with ARPI therapy was administered in 22 patients (52%). All patients completed the five-fraction regimen. Biochemical progression occurred in three patients, of whom two had out-of-field metastases and one had local progression as per follow-up PSMA-PET. Field Code Changed The estimated 3-year OS and PFS rates were 100% and 95.2% (95% confidence interval: 89.0%-100%), respectively. No clinical factors, including ARPI use, significantly correlated with survival outcomes. No radiotherapy-related AEs of grade ≥ 3 were observed.MRgSTAR demonstrates promising early survival outcomes and a favorable toxicity profile in synchronous OMPC, warranting further investigation to confirm its therapeutic role.