The Impact of 2013 Updated ASCO/CAP HER2 Guidelines on the Diagnosis and Management of Invasive Breast Cancer: A Single-Center Study of 1739 Cases.

Background: The purpose of this study was to determine the impact of revised ASCO/CAP 2013 HER2 guidelines on the clinical practice of pathologists and oncologists.

Materials And Methods: Retrospective analysis of 1739 patients with invasive breast carcinoma who underwent reflex HER2 (fluorescence in situ hybridization [FISH]) testing, using both 2007 and 2013 guidelines (2007-2014).

Results: Using 2013 guidelines, 255 (15%; 95% confidence interval [CI], 13%-16%) cases were classified as HER2 as opposed to 186 (11%; 95% CI, 9%-12%) by 2007 guidelines (odds ratio [OR] 1.4; 95% CI, 1.2-1.8; P = .0005). Sixty-nine cases equivocal by 2007 guidelines (12% of all equivocal cases) were converted to HER2 by 2013 guidelines. Sixty-two of these 69 cases shifted from HER2 equivocal to positive due to change in FISH ratio cutoff from 2.2 to 2.0. Six cases had FISH ratio < 2.0 but immunohistochemistry (IHC) score 3+ in 10% to 30% of tumor cells. One case had FISH ratio of 2.0 and IHC score 3+ in 10% to 30% of tumor cells. FISH and IHC test results were discordant in 5% (95% CI, 4%-6%) of cases using 2013 guidelines. No increase in HER2 FISH equivocal cases was observed. Reflex FISH testing of all IHC 1+ cases at our institution additionally detected 58 patients (5%; 95% CI, 4%-6%) with HER2 amplification.

Conclusions: The 2013 guidelines increase the detection of HER2 cases, without introducing significant difference in discordance rate of the IHC and FISH assays. Inclusion of HER2 copy number criterion does not increase the number of FISH equivocal cases in our cohort. We recommend IHC 1+ cases should be offered reflex FISH testing because failure to test them will miss a small number (5%) of potentially treatable cases.