Four-Year Feasibility and Safety Results of a Phase 1/2 Single-Arm Prospective Clinical Trial of Stereotactic Magnetic Resonance-Guided Adaptive Radiation Therapy for Metachronous Oligometastatic Abdominopelvic Lymph Node and Soft Tissue Metastases.

[PURPOSE] Metachronous oligometastases may represent a favorable disease state for local therapy after prior curative treatment. Stereotactic Magnetic Resonance-Guided Adaptive Radiation Therapy (SMART) provides precise targeting of nodal and soft tissue metastases. The primary objective was to assess the feasibility and safety of SMART for abdominopelvic metachronous oligometastases. Secondary objectives included assessing rates of toxicities and evaluating local control (LC).

[METHODS AND MATERIALS] Ten patients were enrolled with solid tumor metachronous abdominopelvic nodal or soft tissue metastases, ≤7 cm in maximal diameter, and ≤3 sites of active disease. All patients received 40 Gy in 5 fractions. Acute toxicities were graded per Common Terminology Criteria for Adverse Events v5 per-protocol follow-up over 1 year. Late toxicities and clinical outcomes were elucidated by chart review. LC, distant progression-free survival, and overall survival were analyzed using the Kaplan-Meier Method.

[RESULTS] Eight patients with prostate cancer and 2 with renal cell carcinoma were enrolled in the study. All patients were successfully treated with SMART per-protocol without complications. The median follow-up after SMART was 4.22 years. Three patients experienced acute grade 1 toxicities; there were no higher grade or late toxicities. Among these 10 patients, 4-year LC and overall survival were both 90%, and 4-year distant progression-free survival was 20%. Two patients (1 prostate cancer, 1 renal cell carcinoma) remain with no evidence of disease, each at over 4 years following SMART and without receiving further systemic or local therapies.

[CONCLUSIONS] With 4 years median follow-up, this small prospective trial reports low toxicity, supporting the feasibility of SMART metastasis-directed therapy for metachronous oligometastases with minimal risk of acute or late toxicity.