Plasmacytoma-like Syphilis

Summary: Dense cutaneous plasma-cell rich infiltrates can be seen in a variety of dermatologic entities, including hematologic disorders (e.g. plasmacytoma, marginal zone lymphoma, Rosai Dorfman disease, and IgG4 related disease) and reactive infiltrates such as cutaneous plasmacytosis. We present a case of a nodular plasma cell infiltrate that highlights the histopathologic overlap of these entities with syphilis. A 60 year-old man with HIV and history of syphilis presented with a 1-2 month history of non-itchy, non-painful skin lesions. On exam, there were numerous erythematous well-demarcated nodules on his back, chest, and forehead, and scaly macules on his palms. A large nodule on the back was shave biopsied, with a differential diagnoses of syphilis, Kaposi sarcoma, and Sweet syndrome. The biopsy demonstrated a dense dermal infiltrate of plasma cells that were polytypic with kappa/lambda in situ hybridization. There were mild admixed T and B-lymphocytes. The infiltrate abutted an atrophic epidermis, which showed scale-crust and mild lymphocyte exocytosis at the periphery. Steiner and a spirochetal immunohistochemical stain were positive for spirochetes. RPR at >1:1024 confirmed a diagnosis of syphilis. This case of a plasmacytoma-like nodule highlights a unique presentation of syphilis, about which one should be aware to avoid misdiagnosing a pseudolymphoma.