Clinical And Genomic Factors Associated With Outcome Following Ablative Radiotherapy For Oligometastatic And Oligoprogressive Liver Tumors

515 Background: There is increasing use of ablative radiotherapy (RT) for oligometastatic and/or oligoprogressive cancer, but the population who may benefit from this more aggressive treatment remains poorly defined. We aimed to identify factors associated with improved outcomes following ablative RT for oligometastatic/oligoprogressive liver tumors. Methods: We retrospectively analyzed 106 patients who had tumor genomic profiling and received a 5, 6, or 15-fraction course of ablative RT for liver metastases from 2008-2019. The interval off systemic therapy post-RT was calculated for patients who did not continue treatment through RT. Overall survival (OS) was estimated using the Kaplan-Meier method. The association between clinical and genomic variables and OS were assessed using uni- and multivariable Cox regression. Results: Median follow-up was 12.6 months. Median age was 61.3 years and 57% were male. The most common primary site was colorectum (42%), followed by pancreas (25%) and non-small cell lung cancer (10%). 42% had colorectal adenocarcinoma, 46% had other adenocarcinoma, and 12% had other histology. A BRAF/RAS family mutation (KRAS, NRAS, and/or BRAF) was present in 41%, 69% had > 1 metastasis, and 38% had extra-hepatic disease. Median biological effective dose (α/β = 10) (BED) was 92 Gy. The RT field encompassed all liver metastases in 91%, and 11% received radiosensitizing chemotherapy. Median time off systemic agents was 5 months. Patients with 1 vs > 1 metastasis had a longer interval off systemic therapy (9 vs 4 months, p = 0.026). Median OS was 12.6 months. The table shows the multivariable Cox model for OS. Conclusions: Presence of a BRAF/RAS family mutation, extra-hepatic metastases, exclusion of liver metastases from RT fields, lower BED, and concurrent radiosensitizing chemotherapy were associated with worse OS. This may inform patient selection and RT delivery for aggressive local therapy for liver metastases. [Table: see text]