Papular mucinosis: is the inflammatory cell infiltrate neoplastic? The presence of a monotypic plasma cell population demonstrated by in situ hybridization

Summary Papular mucinosis is a condition reported to be associated with abnormal serum paraproteins and plasma cell dyscrasias. We report a patient with papular mucinosis, without a serum paraprotein or bone marrow plasmacytosis, in whom the affected skin contained a prominent perivascular plasma cell infiltrate. Using in situ hybridization, for k and λ light chain mRNA, these plasma cells were demonstrably monotypic for λ light chain and, therefore, presumably monoclonal and putatively neoplastic. We suggest that the absence of a serum paraprotein and marrow plasmacytosis does not exclude the existence of a plasma cell neoplasm in patients with papular mucinosis. Such plasma cell populations may exist in the affected skin, although their true nature and behaviour remains to be determined.