Impact of clinical target volume utilization on outcomes in patients with non-spine bone oligometastases treated with stereotactic ablative radiation therapy.

Purpose: To compare local failure, marginal failure, and toxicity in non-spine bone metastases (NSBMs) treated with versus without a CTV for stereotactic ablative radiotherapy (SABR).

Methods: The study included all patients in British Columbia treated with SABR for NSBMs on the SABR-5 trial (November 2016 - July 2020) and on the BC Oligometastases Registry (August 2020- October 2022). NSBMs were stratified based on CTV use for treatment planning.

Results: 148 patients with 183 NSBMs were included. 145 (79 %) NSBMs were treated with a CTV. Most lesions received 35 Gy in 5 fractions (80 %) or 24 Gy in 2 fractions (15 %). Local failure rates did not differ, with a 2-year local failure of 8.6 % (95 % confidence interval [CI] 3.9-13.2) with a CTV and 8.1 % (95 % CI 0-16.8) without a CTV (p = 0.53). Marginal failure did not differ (6.4 % [95 % CI 2.3-10.5] and 2.6 %, [95 % CI 0-7.7], respectively [p = 0.23]). 2-year cumulative incidence of grade ≥ 2 toxicity did not differ (15.8 %, 95 % CI 9.7-21.9 and 16.2 %, 95 % CI 4.2-28.2 respectively; p = 1.00). On multivariable regression, use of a CTV was not associated with the risk of local-marginal failure (hazard ratio [HR] 1.81, 95 % CI 0.62-5.31, p = 0.28). Extraosseous extension (HR 2.59, 95 % CI 1.2-5.7, p = 0.02) and lack of receipt of systemic therapy (HR 0.27, 95 % CI 0.1-0.5, p = 0.0002) were associated with higher risk.

Conclusions: Use of a CTV was not associated with local or marginal failure or toxicity. Extraosseous extension and lack of receipt of systemic therapy were associated with higher risk of local-marginal failure.