125 - Impact of Clinical Target Volume Utilization on Outcomes in Patients with Non-Spine Bone Oligometastases Treated with Stereotactic Ablative Radiation Therapy

E. OReilly¹, E. Johal², H. Clark³, B. Mou⁴, R. E. Cereno⁵, M. Liu⁶, D. Schellenberg³, W. Jiang³, T. Berrang⁷, A. S. Alexander⁷, H. Carolan⁸, S. Atrchian⁴, E. M. Dunne⁹, S. Tyldesley⁶, R. A. Olson¹⁰, and S. Baker³; ¹BC Cancer- Vancouver Center, Vancouver, BC, Canada, ²UBC, Prince George, BC, Canada, ³BC Cancer - Surrey, Surrey, BC, Canada, ⁴BC Cancer - Kelowna, Kelowna, BC, Canada, ⁵BC Cancer Kelowna, Kelowna, BC, Canada, ⁶BC Cancer - Vancouver, Vancouver, BC, Canada, ⁷BC Cancer - Victoria, Victoria, BC, Canada, ⁸Division of Radiation Oncology, Department of Surgery, University of British Columbia, Vancouver, BC, Canada, ⁹BC Cancer Vancouver, Vancouver, BC, Canada, ¹⁰BCCA - Prince George, Prince George, BC, Canada

Purpose/Objective(s): Despite advancements in stereotactic ablative radiotherapy (SABR) for non-spine bone metastases (NSBMs), uncertainty remains surrounding target volumes. While expert consensus guidelines recommend a clinical target volume (CTV), patterns of failure analyses are lacking, and larger treatment volumes may be associated with higher toxicity. This study aims to compare local failure, marginal failure, and toxicity in NSBMs treated with versus without a CTV in a population-based cohort. Materials/

Methods: A retrospective review was conducted on all patients in British Columbia treated with SABR for NSBMs on the single-arm phase II SABR-5 trial (Nov. 2016 – Jul. 2020) and on the BC Oligometastases Registry (August 2020 - October 2022). Use of a CTV was optional for both SABR-5 and the Registry. NSBMs were stratified based on CTV use for treatment planning.
Results: A total of 158 patients (113 on SABR-5 and 45 on Registry) with 200 NSBMs were included. 159 (80%) NSBMs were treated with a CTV and 41 (21%) without a CTV. The most common histologies were prostate (60%), breast (17%) and lung cancer (6%), and lesions received 35 Gy in 5 fractions (81%) or 24 Gy in 2 fractions (14%). Rib (34%) and pelvis (47%) were the most common lesion sites. Groups with vs without a CTV did not differ in baseline patient or tumor characteristics. After a median follow-up time of 33.7 months (interquartile range [IQR] 19.5-48.9), local failure rates did not differ, with 2-year local failure 9.3% (95% confidence interval [CI] 4.4 – 14.2) in lesions treated with a CTV and 7.6% (95% CI 0 – 15.8) without a CTV (p=0.39). Marginal failure, defined as disease recurrence outside of the GTV but within 1 cm of the PTV, occurred in 15 (8%) of lesions and 2-year cumulative incidence did not differ between groups (6.2% [95% CI 2.3 – 10.1] and 2.6%, [95% CI 0 – 7.5], respectively [p=0.16]). Overall survival (OS) was also similar (2-year OS 84.6%, 95% CI 78.1 – 91.1, and 90.3%, 95% CI 79.9 – 100, respectively; p=0.64). The most common grade = 2 toxicities were pain (n=18, 9%) and fracture (n=8, 4%). There were no grade 4 or 5 toxicities. The 2-year cumulative incidence of grade = 2 toxicity did not differ between groups (14.9%, 95% CI 3.9-25.9and 15.6%, 95% CI 9.9-21.3 respectively; p=0.78). Due to low number of events, local and marginal failure events were collated for a multivariable regression analysis. On multivariable regression, use of a CTV was not associated with the risk of local-marginal failure (hazard ratio [HR] 2.41, 95% CI 0.81 – 7.18, p=0.11). Extraosseous extension (HR 2.62, 95% CI 1.07-6.38, p=0.035) and lack of receipt of systemic therapy (HR 3.03, 95% CI 1.40-6.67, p=0.005) were associated with higher risk.
Conclusion: 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. This may assist in informing future approaches to treatment planning in this patient population.