Evaluation of the adjuvant radiation treatment-effect heterogeneity using genomic signature for locoregional relapse and long-term outcome.

1031 Background: In a retrospective analysis of two similar randomized radiation therapy (RT) trials (i.e. British Columbia (BC) and DBCG 82b), we reported significant survival benefits for post-mastectomy RT in Luminal A. Here we examined the predictive value of additional genomic profiles in the BC trial for loco-regional recurrences (LRR) and breast cancer survival (BCSS) in node-positive, pre-menopausal breast cancer patients randomized to adjuvant chemoradiation or chemotherapy. Methods: In the BC trial, 318 patients received adjuvant cyclophosphamide, methotrexate, fluorouracil and were randomized to with or without postmastectomy RT. From 145 formalin fixed paraffin embedded tissues available, expression profiles of 66 genes were done with the Nanostring nCounter. Treatment effects on LRR and BCSS events were examined by subpopulation treatment effect pattern plots. The research-based PAM50 proliferation score, Risk of Recurrence score (ROR-T and ROR-PT), and genes related to basal-like (ie. 13-gen...