OFP-20-006 Frequency and prognostic significance of TMPRSS2-ERG gene fusion in lymph node positive prostate cancers A. Fleischmann * , I. Zlobec, T. Visakorpi, G. Thalmann * Universität Bern, Inst. für Pathologie, Switzerland Objective: TMPRSS2-ERG incidence and prognostic significance in lymph node positive prostate cancer are virtually unknown. Method: A tissue-microarray was constructed from 119 hormone-naïve nodal positive, surgically treated prostate cancers containing samples from all Gleason patterns (GP) present in every primary tumor (PT) and corresponding lymph node metastases (MET). TMPRSS2-ERG status was determined by fluorescence in-situ hybridization and correlated with various histomorphological tumor features (Gleason score, stage, cancer volume, nodal tumor burden) and biochemical recurrence-free, cancer-specific and overall survival. Results: TMPRSS2-ERG fusion was present in 43.5 % (homogeneous 25.2 %, heterogeneous 18.3 %) of the PT and in 29.9 % (homogeneous 28.7 %, heterogeneous 1.2 %) of the MET. Percentage of TMPRSS2-ERG in GP3/4/5 of PT and MET were: 38 %/37 %/24 % and 23 %/30 %/39 %. Concordance in TMPRSS2-ERG status between PT and MET was poor (Kappa 0.39) showing 20.9 % and 8.1 % of cancers with gene fusion solely in the PT and MET, respectively. TMPRSS2-ERG fusion was not correlated with histopathological tumor features and predicted late biochemical recurrence independently (p=0.041) when present in PT. Conclusion: TMPRSS2-ERG fusion in PT is more frequent and its distribution more heterogeneous compared to MET. The gene fusion in primary tumors independently predicts late biochemical recurrence.