Do we need a CTV? local failure rates and patterns after stereotactic body radiotherapy (SBRT) for non-spinal, non-sacral bone metastases.

Abstract

The optimal use of a clinical target volume (CTV) in stereotactic body radiotherapy (SBRT) for non-spinal, non-sacral bone metastases (NSBM) remains uncertain. This study assessed local failure (LF) rates and patterns according to CTV margins. Patients treated with SBRT for NSBM between 2018 and 2023 were retrospectively reviewed. LF was defined as disease progression within the treated bone that would have been encompassed by a 20 mm concentric CTV around the gross tumour volume (GTV). LF patterns were classified as in-field, marginal, or out-of-field. Marginal or out-of-field LF were considered potentially preventable if > 95% of recurrence volume would have been covered by hypothetical CTVs of 5, 10, or 20 mm. 193 patients with 261 NSBM were included. A CTV margin was applied in 30.4% of cases, mostly 5 mm. Median follow-up was 26.6 months (interquartile range [IQR] 18.1-43.0). The 2-year cumulative LF incidence was 22.8% (95% confidence interval [CI] 17.7-28.3%). CTV use was not significantly associated with reduced LF (p = 0.34), including in multivariable analysis (p = 0.15). Recurrence patterns were not significantly different by CTV use (p = 0.76). 28% of LF were potentially preventable with a hypothetical 20 mm CTV margin, but 90% of these occurred alongside distant progression, half in a polymetastatic setting. CTV use had no significant impact on LF rates or patterns. The potential benefit of larger margins remains difficult to evaluate, as some LF may represent new metastases rather than true local relapses. Prospective studies are needed to refine CTV strategies.