Sentinel lymph node in breast cancer: Form morphology to molecular examination L. Di Tommaso * , B. Fernandes, B. Fiamengo, C. Navligu, P. Spaggiari, S. Manara, C. A. Garcia Etienne, G. Masci, A. Testori, C. Tinterri, M. Roncalli * IRCCS Istituto Clinico Humanitas, Dept. of Pathology, Rozzano, Italy Objective: Sentinel lymph node (SLN) examination is a standard in breast cancer treatment. It can be performed on formalin-fixed paraffin-embedded material (FFPE) or on frozen sections (FS). FFPE or FS suffer two drawbacks: 1) partial examination; 2) operator's dependence. To avoid these limitations, a molecular technique (OSNA) targeted to quantify a tumoral fingerprint (CK19), has been introduced. Our aim is to compare the performance of FFPE, FS and OSNA. PS-17-026 HER2 assessment in invasive breast cancer using IHC and FISH: Results from 7479 consecutive cases P. Drev * , B. Gazic, J. Contreras * Inst. of Oncology, Dept. of Pathology, Ljubljana, Slovenia Objective: HER2 in invasive breast cancer (IBC) should be assessed according to recommended algorithms employing IHC as screening tool and ISH only in equivocal cases. Therefore there is little data on possible missdiagnosis in discordant cases. Method: HER2 was assessed by both IHC and FISH in 7479 consecutive IBC. Distribution of IHC and FISH scores, incidence of HER2+ IBC, concordance and level of amplification in discordant cases were analysed. Results: IHC distribution: neg(0) 49.6 %, neg(1+) 27.0 %, equivocal(2+) 11.3 %, pos(3+) 12.1 %. FISH distribution: nonamplified 84.6 %, amplified 13.6 %, equivocal 1.8 % of which in 42.1 % ratio was ≥2.0. 15.0 % IBC were HER2+ and 0.7 % were double-equivocal. 14.4 % of equivocal(2+) were amplified. Discordance was infrequent (1.2 %) (p< 0.0001): among neg(0) and neg(1+) 0,3 % and 1,6 % were amplified, while among pos(3+) 4.7 % were nonamplified. 64 IHC negative tumors, representing 5.7 % of all HER2+, were amplified, however in 89.1 % ratio was ≤4.0.