3551 - Stereotactic Body Radiotherapy for Osseous Low Alpha-Beta Resistant Metastases for Pain Relief - SOLAR-P

Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) is increasingly being used in palliative scenarios as it has been shown to potentially provide prolonged pain relief in patients with symptomatic bone metastases. Tumors with low a/ß ratios have inherent radioresistance to standard palliative fractionation and may benefit from extreme hypofractionation to yield more potent biological efficacy. We assessed the use of single fraction SBRT to non-spine bone metastases in patients with low a/ß ratio primary malignancies.

Materials/Methods: This was an open label, phase II single arm trial whereby patients who had primary malignancies including: prostate cancer, breast cancer, renal cell carcinoma (RCC) or melanoma, with painful bone metastases were enrolled to receive a single fraction of SBRT ranging from 15-20Gy. Pain was assessed using the Brief Pain Inventory (BPI) and converting daily analgesic use to oral morphine equivalents (OME). Assessments were completed at baseline, 1 month, 3 months and 6 months after SBRT completion. The primary outcome was the proportion of patients with complete (CR) or partial response (PR) using BPI and/or reduction in OME =25% at 3 months. Our hypothesis was that 80% of patients would have a CR/PR; this was compared to the null value of 60% using a one sample binomial test. The estimated sample size was 40 patients. Secondary endpoints included toxicity (CTCAE v5.0), local control, and quality of life.

Results: From December 2020 to May 2024, 26 patients with 28 metastases were accrued, the trial was stopped due to slow accrual. Median age was 69.3 years (35.3-82.9), patients had breast (n=10), RCC (9), prostate (5), and melanoma (2) primaries respectively. Main sites treated were ribs (n=9), pelvis (9), hip/femur (6) and humerus/shoulder (4). Median SBRT dose was 16 Gy. At 3 months, when assessing by lesion (n=19 lesions in 20 patients), 42.1% demonstrated a CR and 26.3% PR. The overall response was 68.4% (95% CI 43.5-86.4%, p=0.26435). Pain flare occurred in 6/26 patients (23%) with only 1 grade 3 pain flare observed which resolved prior to 1 month post SBRT; this patient had a PR to treatment. No other grade 2 or higher toxicity was observed. Mean global quality of life scores using the EORTC Q15-PAL were 46.3 at baseline, 52.0 at 1 month, and 64.7 at 3 months. Two patients had local progression prior to the 6 month assessment, both had initial PR at 1 month, but worsening pain at 3 and 6 months. No patient required surgical salvage or developed a pathological fracture following treatment.

Conclusion: Single fraction SBRT did not result in an improved rate of overall pain response compared to historical controls for low a/ß malignancies, however CR did occur in 42% of lesions treated at 3 months. SBRT was associated with minimal severe toxicity and pain flare rates comparable to those seen with conventional fractionation. The promising CR rate should prompt further investigation into the benefits of SBRT in this patient population, especially those with a predicted favorable prognosis.