Low Rate of Local Salvage Therapy Following Radiation Therapy for Non-Spine Bone Metastases ― An Indication for Appropriate Patient Selection

Purpose/Objective(s) Local therapy for patients with non-spine bone metastases is evolving, with recent data suggesting that stereotactic body radiotherapy (SBRT) may improve pain response in the palliative setting. However, the rate of local salvage therapy (LST) after SBRT compared to conventional RT is unknown. We therefore characterized rates and risk factors for LST in patients with non-spine bone metastases. Materials/Methods We reviewed records of all courses of RT for non-spine bone metastases at our institution from 1/1/2016 to 12/31/2018. LST was defined as the first occurrence of either RT or surgery for progression to a bone metastasis following initial RT. Functions of the cumulative incidence of re-treatment were generated for each RT technique. We identified associations with various predictors using the Fine and Gray method for competing risk regression. Results Of the 1,754 patients included, 2,648 bone metastases were treated with RT and analyzed with a median follow up of 15 months (IQR = 6.4 – 25.0 months). 51% of episodes were multi-fraction EBRT, 10% were single fraction EBRT, and 39% were SBRT. Of the treated metastases, 120 (4.5%) episodes required LST (median time to LST = 6.2 months, range = 0.3-35.6 months), with a cumulative incidence at 6 and 12 months of 2.4% and 3.9%, respectively. Cumulative incidence of re-treatment at 6 months was 2.6% for SBRT, 3.5% for single-fraction conventional, and 2.1% for multi-fraction conventional regimens (P = 0.76). On multivariable analysis, younger age, higher Karnofsky performance status (KPS), and location of the metastasis in the pelvis had higher hazard of re-treatment (Table 1). There was no significant difference in hazard of LST among RT techniques (P = 0.44), nor was there a significant interaction between RT technique and radioresistant histology (P = 0.33). Conclusion In our large institutional cohort, the rate of LST was low, with no difference between RT techniques. Our results suggest that judicious selection of patients at high risk for eventual treatment failure (i.e., those with longer expected survival) for SBRT may reduce the rate of retreatment rate in the population. When patients are appropriately selected for single fraction conventional RT, such as those who are older or have poor KPS, rates of LST remain low. Further studies to validate our findings are warranted to help clinicians to select the appropriate treatment technique for patients with bone metastases.