Loss of Cytokeratin 7 (CK7) Expression Can be Seen in Significant Proportion of Solid Papillary Carcinoma of Breast and Associated Invasive Component.

Abstract: Cytokeratin 7 (CK7) is a marker of epithelial differentiation and is positive in >90% of breast carcinomas. In the right clinical setting, coexpression of CK7 and GATA3 is considered as strong evidence of breast origin, which can be further confirmed by positive expression of estrogen and progesterone receptors. Pathologists use CK7 in the breast to confirm epithelial differentiation and highlight foci of invasion, and outside the breast it is often used as a marker to support breast origin of metastatic tumors. Few studies have reported loss of CK7 in invasive breast carcinoma, however, association of CK7 loss with any histopathologic subtype has not been well documented in the literature. We stained a series of solid papillary carcinoma of the breast (SPC) and associated invasive ductal carcinoma (IDC) with CK7 to document that CK7 loss can occur in both SPC and associated IDC. Archived breast tumor tissue specimens with a diagnosis of SPC with IDC were identified from our Department of Pathology database from January 2019 to January 2023. Blocks with tumors were initially evaluated on Hematoxylin and Eosin (H&E) slides, and subsequently with CK7, synaptophysin, and chromogranin immunostains. The results of these immunohistochemical markers were tabulated in an Excel sheet. Expression of CK7 was entered as negative, marked loss of CK7 (weak positive staining in <1% of tumor), and positive (diffuse expression >1% to 100% of tumor cells). Out of 26 tumors of SPC, 15 showed negative to a marked loss of expression for CK7 in both in-situ and invasive components (57.7%), while 11 were diffusely positive (42.3%). Neuroendocrine differentiation was present in 57.7% of tumors of which 60% were CK7 negative. However, there was no significant association between neuroendocrine differentiation in SPC and associated IDC with CK7 loss (P=1). In addition, histologic parameters, biomarkers expression, Ki-67 expression, and clinical follow-up were studied in detail.

Conclusion: A significant proportion of solid papillary carcinoma of the breast and the associated invasive component can show loss of CK7.