Primary cutaneous signet ring cell carcinoma expressing cytokeratin 20 immunoreactivity.

Primary cutaneous signet ring cell carcinoma (PCSRCC) is a very unusual but distinctive clinicopathologic entity that can simulate metastatic adenocarcinomas. It is defined as a diffuse malignant epithelial neoplasia localized in the dermis and subcutis without epidermal involvement, showing variable amounts of signet ring cells, without evidence of visceral adenocarcinoma. We present 2 cases of PCSRCC, which involved eyelids and axilla respectively. Despite thorough systemic workup, primary sources could not be demonstrated in either case. The tumor cells are positive for gross cystic disease fluid protein 15 in addition to a variety of glandular markers. Furthermore, both cases were immunostained with cytokeratin 20 (CK20). In conclusion, we report 2 cases of PCSRCC expressing CK20 immunoreactivity. CK20-positive primary cutaneous tumors should include PCSRCC in addition to Merkel cell carcinoma.