The American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) 2013 guidelines for HER2 assessment have increased the number of HER2 equivocal
breast carcinomas following in situ hybridization reflex testing, that is, HER2 "double equivocal" (equivocal
protein expression and equivocal gene copy number). Forty-five double-equivocal
carcinomas were subjected to Prosigna analysis. Twenty-seven cases were investigated for the expression of genes found to be differentially expressed between
estrogen receptor (ER)-positive/HER2-positive (N=22) and ER-positive/HER2-negative (N=22) control cases. Twenty-nine of the 45 cases were also analyzed by targeted sequencing using a panel of 14 genes. We then explored the
pathologic complete response rates in an independent series of double-equivocal
carcinoma patients treated with
trastuzumab-containing
chemotherapy. All cases were ER-positive, with a mean Ki67 of 28%. Double-equivocal
carcinomas were predominantly
luminal B (76%); 9 cases (20%) were
luminal A, and 2 cases (4%) HER2-enriched. The majority (73%) showed a high risk of recurrence by Prosigna, even when the
carcinomas were small (<2 cm), node-negative/micrometastatic, and/or grade 2. Double-equivocal
carcinomas showed TP53 (6/29, 20%), PIK3CA (3/29, 10%), HER2 (1/29, 3%), and MAP2K4 (1/29, 3%) mutations. Compared with grade-matched ER-positive/HER2-negative
breast carcinomas from METABRIC, double-equivocal
carcinomas harbored more frequently TP53 mutations and less frequently PIK3CA mutations (P<0.05). No significant differences were observed with grade-matched ER-positive/HER2-positive
carcinomas. Lower
pathologic complete response rates were observed in double-equivocal compared with HER2-positive patients (10% vs. 60%, P=0.009). Double-equivocal
carcinomas are preferentially
luminal B and show a high risk of recurrence. A subset of these
tumors can be labeled as HER2-enriched by transcriptomic analysis. HER2 mutations can be identified in HER2 double-equivocal cases.