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&E sections. RFS and interactions with T were explored with Kaplan-Meier and Cox regression analyses. Results: The median FU time was 62 months. 935 (92.6%) and 687 (68.1%) tumors underwent LI and mt analysis, respectively. Correlation of LI assessment between 2 pathologists was 0.78 (p<0.001). 54 mts detected in 13 genes were evaluated. PIK3CA mts (exons 1,2,9,13,18,20) was present in 25.3...