Outcomes in Stereotactic Body Radiotherapy for Non-Spine Bone Metastases: A Single Institution Review

Stereotactic body radiotherapy (SBRT) is increasingly being used in the setting of oligometastatic (OM) disease and in re-irradiation, with potential benefit over conventional palliative radiation in terms of durable control, pain relief and possibly survival outcomes. Non-spine bone SBRT poses a therapeutic challenge owing to its variable treatment sites, the absence of consensus treatment guidelines, and limited data regarding predictors of fracture or bone necrosis. We report outcomes from a large institutional experience. One hundred and twelve patients with 151 lesions treated with non-spine bone SBRT were reviewed. OM disease was defined as less than/equal to 5 sites of disease, with the majority having metachronous OM disease (62%), while 16% had synchronous OM and 25.5% had oligoprogressive (OP) disease. Forty-two per cent were symptomatic at baseline and 35 lesions (23.3%) were previously irradiated. Treatment fractionation schemes included 16-20Gy in 1 fraction, 27-30Gy in 3 fractions and 25-30Gy in 5 fractions. For statistical analysis, histology was divided into prostate (53.6%), breast (7.1%) and other (39.3%). Outcomes were measured on a per patient and per lesion basis using Cox regression to evaluate predictors in overall survival (OS), progression-free survival (PFS) and local control (LC) and Fisher's exact test and Wilcoxon rank sum test to evaluate predictors of pain response at a per lesion basis. Toxicity was graded according to CTCAEv5.0. Median follow up was 18 months. OS, PFS and LC at 2 years was 78.2%, 37.6% and 86.2% respectively. Male gender, prostate histology, less than/equal to 3 lesions, no prior metastasis-directed therapy, no prior palliative systemic therapy and an ECOG PS of 0-1 were all predictive of improved OS and PFS. Metachronous OM disease was most predictive for OS (p<0.001) while synchronous OM disease was most predictive for PFS (p<0.0001). Twelve local failures occurred at a median time of 12.5 month. Target size (both GTV and PTV) predicted for local failure (p = <.0001 and <.0001 respectively). There was a trend towards significance for local failure in lytic and soft tissue lesions vs blastic lesions (p = 0.06, HR 3.69). A complete or partial pain response was recorded in 79% of patients who had pain outcomes recorded (n = 56). No variable (including lesion type, size, re-irradiation and size) was predictive of pain response. Eight patients experienced acute toxicity greater than/equal to grade 2, with 7 patients experiencing a pain flare and 1 with grade 2 nausea. There were 5 fractures in irradiated sites in total (1 in rib and 4 in pelvis), 2 of which were grade 2 requiring modified activity and 1 grade 3 requiring surgery. SBRT to non-spine bone lesions is an effective treatment for both local control and symptom control with low rates of toxicity. Patients with OM disease have significantly improved outcomes compared with OP disease and should be considered for SBRT.