Stereotactic radiotherapy for rib metastases from prostate cancer: A multicenter real-world analysis of local control, pain response, and treatment tolerance (RIBRISK)

Abstract

Background Stereotactic body radiotherapy (SBRT) is increasingly used for patients with oligometastatic prostate cancer, yet the optimal target definition, dose, and toxicity profile for rib metastases remain uncertain. Objectives We assessed local control (LC), toxicity, and the risk of radiation-induced rib fracture (RF) after SBRT for rib metastases. Design Retrospective multicenter cohort study. Methods We analyzed prostate cancer patients with radiologically diagnosed rib metastases treated with SBRT and >=12 months of follow-up. Outcomes were descriptively compared between gross tumor volume (GTV)-only and GTV plus an intraosseous clinical target volume (CTV) margin. Analyses included Kaplan-Meier survival estimates, cumulative incidence functions, and exploratory univariable Cox and logistic regression; no formal equivalence testing or multivariable validation was attempted because event numbers were very low. Results We included 102 patients with a median follow-up of 20 months. The 1- and 2-year LC rates were 97.9% and 96.3%, respectively. At 3 years, local progression was 2.1% in the GTV-only group, absent in patients treated with >=10 mm margins, and numerically highest with 3-5 mm CTV margins (13.5%, p=0.022). Among 13 symptomatic patients, pain control was excellent, with complete or partial response in all patients within 6 months. No grade >=3 toxicities occurred. RFs were rare (n=3, 2.9%), comprising one symptomatic Grade 2 event (0.9%) and two asymptomatic Grade 1 events (2.0%). Baseline symptoms and extraosseous infiltration were associated with RF in exploratory univariable analysis, but confidence intervals were wide. Higher PTV dose exposure, particularly PTV D50% (36.9 vs. 31.1 Gy, p=0.041), was also observed among RF cases. Conclusions SBRT for rib metastases from prostate cancer was associated with high LC, pain relief, and low observed toxicity in this retrospective real-world cohort. GTV-only contouring may be feasible in carefully selected patients staged and planned with PSMA-PET/CT, but the non-randomized design, baseline imbalances, short follow-up, and very low event count preclude drawing conclusions about equivalence or independent validation. RF predictor analyses should be considered exploratory and hypothesis-generating.