Differential diagnosis of serous papillary carcinoma of the gynaecological tract and basal breast carcinoma: an immunohistochemical approach.

Aims: There is a high frequency of serous gynaecological and basal breast cancers in patients with germline BRCA1 mutations. These patients often undergo prophylactic salpingo-oophorectomies, where small tumours may be identified in which morphological distinction between primary serous and metastatic basal breast cancer may be problematic. These two cancer types share similar molecular genetics and have immunophenotypic overlap. We aimed to develop an immunohistochemical panel which could differentiate between these two tumour types.

Methods: Immunohistochemistry was performed on a training set of 20 serous gynaecological and 20 basal breast cancers, from which a small differential panel was developed. This differential panel was tested in an additional validation set of serous and basal cancers.

Results: WT1, ER and CK5/6 expression successfully differentiated serous gynaecological and basal breast cancers. WT1 and ER expression was significantly more common in serous cancers and CK5/6 expression more common in basal cancers.

Conclusion: There is considerable genetic, morphological and immunophenotypic overlap between serous gynaecological and basal breast cancers. A limited panel of WT1, ER and CK5/6 was found to optimally differentiate these tumour types. Differences in the expression of these proteins appear to reflect the expression pattern of their cell of origin, rather than identify aberrant oncogenic pathways.