Primary Cutaneous Immunocytoma Presenting with Diffuse Subclinical Involvement and Demonstrating Kappa and Lambda Light Chain Restrictions

A 37 year‐old man with primary cutaneous immunocytoma with lambda light chain restriction involving the left shoulder was treated with Rituxan with clinical resolution. Four years later, he presented with a six‐month history of diffuse asymptomatic erythema. Physical examination revealed three grouped papules on the right upper arm and an irregular sclerotic patch with slight erythema superiorly on the left shoulder at the site of the initial tumor. There was no diffuse erythema. Biopsies were obtained from a papule on the right upper arm, the sclerotic patch on the left shoulder, and clinically uninvolved skin on the right anterior thigh. All three specimens showed a variably dense, predominantly periadnexal and perivascular dermal infiltrate of plasma cells and lymphocytes. The specimen from the right arm demonstrated kappa light chain restriction. The specimen from the right thigh showed lambda light chain restriction. Physical examination six weeks later demonstrated mottled erythema on the anterior thighs. Two biopsies were obtained from the right thigh. One showed immunocytoma. The other was unremarkable. The patient's primary cutaneous immunocytoma demonstrated two unusual findings: (1) histologic presence of tumor in clinically uninvolved and minimally involved skin; and (2) the presence of two distinct monoclonal populations.