Cutaneous pseudolymphoma: a case report with an immunohistochemical study.

Cutaneous pseudolymphoma (C-PSL) is defined as reactive polyclonal benign lymphoproliferative process predominantly composed of either B-cells or T-cells, localized or disseminated. It heals spontaneously after cessation of the causative factor (e.g. drugs) or after non-aggressive treatment. The author herein presents a case of C-PSL with an immunohistochemical study. A 78-year-old man consulted our hospital because of slightly itching skin swelling on the arm. He denied insect bite and traumatic injury. His usual intake drugs were drugs of hypertension, hyperlipidemia, diabetes mellitus, and emotional disorders. Physical examination showed mildly erosive swelling of the am. The lesion measured 1 x 1 x 0.2 cm. Biopsy of the lesion was taken, and it revealed excessive proliferation of small lymphoid cells. The lymphoid cells lacked apparent atypical features and appeared matures. Lymphoblastic cells with nucleoli were scattered. Nodular structures were also seen in the lower dermis. Immunohistochemically, the lymphoid cells were positive for vimentin, CD3, CD4, CD5, CD8, CD10, CD15, CD20, CD23, CD30, CD43, CD38, CD138, CD45RO, CD79α, bcl-2, bcl-6, κ-chain, λ-chain, and Ki-67 (labeling index=7%). No light chain restriction is seen. The lymphoblastic cells were positively labeled for CD15 and CD30. Plasma cells positive for CD38, CK79α and CD138 were seen in a significant amounts. They were negative for cytokeratin (CK) CAM5.2, CKAE1/3, CK34BE12, CK5/6, CK7, CK8, CK18, CK19, CK20, EMA, CEA, CD56, CD57, p53, KIT, PDGFRA, and cyclin D1. Because the constituent cells were both B-cells including plasm cells and T-cells, no light-chain restriction was seen, and no histological atypia was seen, a diagnosis of cutaneous pseudolymphoma was made. The low Ki-67 labeling and negative p53 also suggested the diagnosis. The lesion slightly reduced in size (from 1 cm to 0.7 cm), the causative agent was still unknown 11 months after the biopsy.