Stereotactic body radiotherapy for non-spinal bone oligometastases: a mono-institutional experience

Stereotactic body radiotherapy (SBRT) has entered daily clinical practice in the management of oligometastatic disease. Similarly to the evidence in support of spinal metastases, the use of SBRT has been recently reported also for the treatment of non-spinal bone metastases (NSBM).

This is a single-institutional experience of oligometastatic patients treated with SBRT for NSBM. Oligometastases were defined according to the recent ESTRO/EORTC consensus. Inclusion criteria were as follows: ECOG PS ≤ 2, written informed consent, up to 5 lesions to be treated at the same time, and treatment with radiotherapy schedules applying a minimum of 6 Gy per fraction. The primary endpoint of the study was local control (LC); acute and late toxicity, distant progression-free survival (DPFS), time-to-next systemic treatment (TNST), and overall survival (OS) were secondary endpoints. Toxicity was assessed according to CTCAE criteria v5.0. Survival estimates were performed using the Kaplan-Meier method, uni- and multi-variate analyses were carried out to identify any potentially significant correlation.

A total of 74 bone oligometastases in 52 patients were treated in our institution between February 2020 and December 2024. All patients received SBRT with Helical Tomotherapy for a median total dose of 33.7 Gy (range, 24-35 Gy) delivered in 3-5 fractions. In 51.9% of cases, SBRT was delivered to oligoprogressive lesions, to oligorecurrent lesions in 42.3%, while the remaining were patients with synchronous oligometastases and primary disease. Prostate cancer was the most frequent primary histology in 50% of cases, followed by breast (26.9%), uterus (7.7%), NSCLC (5.8%). Median age was 70 years (range 49-88), Males = 30; Females = 22). In the majority of patients (73%) SBRT was delivered to a single oligometastasis, with up to 4 NSBM treated simultaneously, with the thorax the most frequent site of SBRT (53.8%), followed by the pelvis (46.1%). Concurrent systemic therapy was administered in 55.8% of patients. After a median follow-up of 31.8 months (range, 12-33.4), 1- and 2-year LC rates were respectively 98.1% and 96%, while DPFS rates were 57.7% and 31.5%. 1- and 2-year TNST were respectively 82.7% and 42.9% with worse outcomes for patients with oligoprogressive disease at univariate analysis (p = 0.018). Concerning OS, 1- and 2-year rates were, respectively, 94.3% and 90%, with a statistically significant relation with male gender and prostate histology for improved outcomes (p = 0.003 and p = 0.004); also > 1 metastasis treated was reported to correlate with lower OS rates (p = 0.016). No G ≥ 3 adverse events were observed, with 15.4% of cases developing acute G2 pain after SBRT, fully resolved after oral steroids.

In our experience, SBRT for NSBM was safe and effective with minimal toxicity and excellent results in terms of LC. Prostate histology relates to improved outcomes and better disease control rates.