20P Qrt-Pcr Based Determination of Her2 Status of Invasive Breast Carcinomas Equivocal by Both Immunohistochemistry (Ihc) and Fluorescence in Situ Hybridization (Fish) Testing

Abstract

ABSTRACT Background Determination of HER2 status of invasive breast carcinoma is typically performed by either immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH). Equivocal results by either method are resolved by reflex testing using an alternative approved testing method; most commonly, IHC equivocal results are resolved by reflex FISH testing. Cases showing equivocal results by both IHC and FISH {double equivocal cases (DEC)} present a management challenge. Approach and methods Of 2145 breast cancers tested by primary FISH testing, 103 (4.8%) were equivocal (HER2/CEP17 ratio 1.8 - 2.2). Reflex IHC resolved all but 35 cases (1.6% DEC). HER2 expression in macrodissected tumor tissue was assessed for these cases by qRT-PCR. Trastuzumab therapy was tracked for all patients in the DEC population. Results qRT-PCR of macrodissected breast cancer tissue definitely correlated with known HER2 amplified/over-expression status in a validation set of 111 cases, and successfully stratified the DEC cases. Of the 7 patients identified as positive for HER2 over-expression by qRT-PCR, 1 case (14.3%) received Trastuzumab in addition to standard therapy. In the 19 cases identified as not over-expressed for HER2 by qRT-PCR, 6 cases (31.6%) received Trastuzumab. In this double equivocal cohort, 6 patients developed metastasis (2 that received Trastuzumab and 4 that did not receive Trastuzumab). The follow up time post Trastuzumab therapy ranged from 1 to 36 months with an average of 15.7 months. 7/26 DEC cases were classified as HER2 over-expressed by qRT-PCR and would have been eligible for Trastuzumab therapy. In actual practice, without a definitive test for HER2 determination, only 1 patient received Trastuzumab therapy. Therefore 6 of 7 patients did not receive potentially beneficial therapy. Conclusions qRT-PCR represents a quantitative alternative approach to resolution of breast cancers demonstrating equivocal results by both IHC and FISH. Limited follow up and small number of patients receiving Trastuzumab, we could not directly assess clinical utility. This will be an important question to be addressed in future studies. Disclosure All authors have declared no conflicts of interest.