A 50-year-old man presented with an asymptomatic, 1.5 × 1.5 cm, dark-brown noduloplaque with a rubbery consistency (Fig. 1) on the lateral aspect of the left lower leg of uncertain duration. His general condition was healthy, and he did not recall any trauma or insect bite at this site. No similar skin lesions were found elsewhere and no lymphadenopathy was observed. The lesion revealed a nonencapsulated, but well-circumscribed, deep dermal nodule with several lymphoid aggregates and germinal center-like structures within the tumor and also at the periphery, when examined microscopically at scanning power (Fig. 2a). The epidermis showed no remarkable changes, except for basal hyperpigmentation. At higher power, a mixed inflammatory infiltrate composed of histiocytes, foamy histiocytes (Fig. 2b), lymphocytes, and abundant plasma cells (Fig. 2c) with Russell bodies was revealed. The stroma contained mainly hyalinized and sclerotic collagen fibers (Fig. 2d). Prominent venules were noted, especially in the sclerotic areas, and some were surrounded by dense collagen fibers. No vasculitis or emperipolesis was found. No foreign materials were observed by polarization microscopy, and no organisms could be identified by periodic acid–Schiff (PAS), Grocott methenamine silver (GMS), Giemsa, Gram, acid-fast, or fite stains. The results of testing for infection by Epstein–Barr virus (EBV) (latent membrane protein 1, LMP-1) were negative. No spindle cells were found in the lesion.
Immunohistochemical studies demonstrated mature plasma cells stained with CD138, and polyclonality was confirmed by the expression of both kappa and lambda light chains. The germinal center-like lymphoid aggregates were found to be B cells, which reacted positively with CD20. Scarce S100-positive cells and even rarer CD1a-positive cells were detected. Test results for smooth muscle actin (SMA) and anaplastic lymphoma kinase (ALK) were negative. Abundant CD68+ macrophages were observed within the lesion (Fig. 3a), and about 50–75% of the inflammatory cells were found to express cyclooxygenase-2 (COX-2) (Fig. 3b). The patient's condition was diagnosed as cutaneous plasma cell granuloma (CPCG). One year after excision, no evidence of recurrence was observed.