Intense basolateral membrane staining indicates HER2 positivity in invasive micropapillary breast carcinoma

Invasive micropapillary carcinoma is characterized by the inside-out growth of tumor clusters and displays incomplete membrane immunostaining of HER2. According to the 2018 American Society of Clinical Oncology and the College of American Pathologists (ASCO/CAP) HER2-testing recommendation, moderate to intense but incomplete staining could be scored as immunohistochemical 2+. Furthermore, the criteria of immunohistochemical 3+ for this staining pattern are not mentioned. One hundred and forty-seven cases of invasive micropapillary carcinoma with moderate-to-intense HER2 immunostaining were enrolled. Invasive micropapillary carcinoma components of all cases were scored as immunohistochemical 2+ based on the 2018 ASCO/CAP recommendation. The invasive micropapillary carcinoma component varied from 10% to 100% (mean, 80%). Invasive micropapillary carcinoma components of all 147 tumors exhibited reversed polarity and incomplete basolateral HER2 membrane staining. One hundred and seventeen of the tumors (80%, 117/147) had moderate staining, and 38 (32%, 38/117) showed HER2 gene amplification by fluorescence in-situ hybridization. HER2 gene was amplified in all the remaining 30 tumors (20%, 30/147) that exhibited intense basolateral membrane staining. Besides, average HER2 signals per cell and ratio of HER2/CEP17 were significantly higher in the intense-staining tumors compared with the moderate-staining tumors (p < 0.0001). Follow-up data were available for 140 patients. None of the patients were died. The follow-up time ranged from 1 month to 99 months (median, 57 months). Thirteen (9%, 13/140) patients exhibited disease progression (recurrence or metastasis). HER2 gene amplification was correlated inversely with estrogen receptor (p = 0.000) and progesterone receptor (p = 0.000) expression, and positively with histological grade (p = 0.003) and disease progression (p = 0.000). Invasive micropapillary carcinoma with intense clear linear basolateral membrane immunostaining indicates HER2 positivity, even if the staining is incomplete. They should be classified as immunohistochemical 3+ rather than immunohistochemical 2+, which would avoid further fluorescence in-situ hybridization-testing procedure and greatly save the related time, labor, and financial costs. Ultimately, ensure all patients with HER2 gene amplification obtain effective targeted therapy in time.