Cutaneous Plasmacytosis: A Rare Dermatological Condition Mimicking Systemic Plasma Cell Disorders.

Cutaneous plasmacytosis (CP) is a rare condition characterized by benign proliferation of mature plasma cells in the skin. It presents as reddish-brown macules, papules, or plaques, typically located on the neck, face, and trunk. The etiology remains unknown, though it is believed to be reactive rather than malignant. CP can occasionally become systemic, affecting two or more extracutaneous organs. This report describes a 54-year-old Caucasian male patient with erythematous, scaly plaques on his lower back and arms. Histopathology revealed a dense polyclonal plasma cell infiltrate with a predominance of IgG4+ cells. The patient was treated with topical corticosteroids and referred for further systemic workup. This case emphasizes the importance of distinguishing CP from systemic plasmacytosis, which can mimic multiple myeloma, plasmacytoma, or Waldenstrom's macroglobulinemia, each requiring distinct management approaches.