Cytokeratin 5/6 immunohistochemistry assists the differential diagnosis ofatypical proliferations of the breast

Aims: This study was performed to determine the diagnostic value of keratin 5/6 (CK 5/6) immunophenotyping on routinely processed breast tissues. Methods and results: Six hundred and ninety-nine breast lesions, including normal tissues as well as benign and malignant lesions in 321 formalin-fixe d, paraffin-embedded samples from 158 different patients were investigated immunohistochemically, following wet autoclave pre-treatment for antigen re trieval. In normal breast tissues, both myoepithelial and luminal epithelia l cells expressed CK 5/6 in varying amounts. While myoepithelial immunoreac tivity was most pronounced in the duct system, luminal epithelial immunorea ctivity was strongest in the terminal duct lobular units. In ductal hyperpl asias (DH), luminal epithelial cells predominantly revealed CK 5/6 immunore action. In contrast, neoplastic epithelial cells in atypical ductal and lob ular hyperplasias (ADH and ALH) lacked such an expression, whereas in ducta l in-situ carcinomas (DCIS) and in infiltrating ductal carcinomas 3.7% and 7.7%, of the cases respectively, showed positive immunostaining for CK 5/6. Conclusions: Immunophenotyping of keratin 5/6 expression can be helpful in the diagnosis of atypical hyperplasias and in-situ carcinomas of the breast . It is particularly valuable in the differential diagnosis between benign and atypical proliferative lesions.