Signet ring lymphoma of the skin: ultrastructural and immunohistochemical features.

Signet ring lymphoma is a recently described morphologic variant of B cell lymphoma. We are reporting the first case of this subtype of follicular lymphoma that was initially confined to the skin and remained so. Light microscopy showed the tumor to be composed of monomorphous plump epithelioid cells with abundant eosinophilic cytoplasm involving the dermis and subcutaneous fat. Cytoplasmic vacuoles were present within some of the tumor cells, suggesting the diagnosis of adenocarcinoma with a signet ring appearance. Some of the vacuoles were positive on periodic acid-Schiff with diastase staining, and no mucin was identified. Immunostaining showed the presence of intracytoplasmic IgG heavy chains and kappa light chains. Characteristic intracytoplasmic inclusions, which were formed by saccules and microvesicular bodies, were seen by electron microscopy. The diagnosis of signet ring lymphoma was made. No systemic involvement was found. Over a period of 7 years, multiple new skin lesions occurred, all of which responded to radiation therapy. This rare variant of follicular center cell lymphoma, which can occur in the skin, should not be confused with metastatic mucinous adenocarcinoma and malignant melanoma.