Immunohistochemical techniques to compare primary vs. metastatic mucinous carcinoma of the skin.

Background: Primary and metastatic mucinous carcinomas in skin are histologically indistinguishable. Immunohistochemical panels help differentiate primary and metastatic adenocarcinoma of skin, but data regarding mucinous carcinoma is scant. Methods: We stained five primary mucinous carcinomas, two mucinous carcinomas metastatic to skin and five primary breast and colon mucinous carcinomas with p63, CD15, CK5/6, CK7, CK20, calponin and D2-40 to identify patterns that might differentiate primary from metastatic disease. We also searched for myoepithelial cells in all cases. Results: All cases of primary mucinous carcinoma of the skin were positive for CK7, 40% showed rare cells labeled for p63 and 20% of cases labeled focally for CK5/6. The breast mucinous carcinomas metastatic to the skin were negative for all markers except CK7, although 60% of primary breast carcinomas labeled for p63. Colon mucinous carcinoma labeled only for CK20. Conclusions: In a small subset of mucinous carcinomas (20% in this series), positive labeling for CK5/6 indicated primary cutaneous tumor. Staining with p63 also favored primary over metastatic disease. Myoepithelial cell layers were not consistently identified to enable the identification of primary disease. Levy G, Finkelstein A, McNiff JM. Immunohistochemical techniques to compare primary vs. metastatic mucinous carcinoma of the skin