Tumor PIK3CA mutations, lymphocyte infiltration, and recurrence-free survival (RFS) in early breast cancer (BC): Results from the FinHER trial.

507 Background: Tumor PIK3CA mutations (mts) and lymphocyte infiltration (LI) are prognostic in BC, but their importance is unknown in HER2-positive (HER2+) BC treated with adjuvant trastuzumab (T). Methods: The FinHER trial randomized 1010 patients (pts) with pN+ or high-risk pN- BC to 3 cycles of docetaxel (D) or vinorelbine, followed by 3 cycles of FEC. Pts with HER2+ BC were further randomized to 9 weeks of T or no T (n=232). Pts treated with D and T had superior outcome in prior analyses. BC samples were subjected to somatic hotspot mt profiling (Sequenom) and quantification of percentage tumor LI using full-face H 2) LI is a strong prognostic factor in ER-/HER2- BC and 3) for the first time that extensive LI predicts benefit from adjuvant T.