Cutaneous inflammatory pseudotumor. Lesions resembling "inflammatory pseudotumors" or "plasma cell granulomas" of extracutaneous sites.

This report describes four cases of a previously undocumented circumscribed idiopathic inflammatory fibrosclerotic lesion of the skin. The lesions occurred in two black women and two white men; they had been apparent from months to a year. The nodules were solitary; two were located on the arm, one on the calf, and one on the posterior neck. The clinical diagnoses included nodules, keratinous cyst, pilar tumor, pilomatricoma, vascular leiomyoma, dermatofibroma, and metastatic carcinoma. The lesions were grossly white, homogeneous, and circumscribed; one was located in the superficial subcutis and three in the reticular dermis. Histologically, they were sharply circumscribed and surrounded by a dense peripheral rim of lymphocytes and plasma cells. Peripheral germinal centers were noted in two cases. All cases had variable amounts of fibrosis centrally, giving them the low-power appearance of lymph nodes; however, no subcapsular or medullary sinuses were found. In the center of the lesions, the mononuclear infiltrate was accompanied by eosinophils and neutrophils. Virtually imperceptible vasculature was noted in portions of three lesions; in the fourth lesion, high endothelial venules were seen in the lymphoid portions, and "targetoid" perivascular sclerosis was seen in the central portions. There appears to be a transition from the early, mostly inflammatory, lesions to the sclerotic ones. The process seems to be reactive in nature. No obvious local or systemic etiology was found in our series, and follow-up of 2 to 5 years has been noncontributory. These mixed-cell proliferations resemble the extracutaneous masses that have been described in numerous viscera--particularly the lung--as "inflammatory pseudotumors" or "plasma cell granulomas." In our opinion, these lesions also bore a superficial resemblance to cutaneous lymphoid hyperplasia secondary to insect bites. Other differential diagnostic considerations include dermatofibroma, nodular fasciitis, Kimura's disease, epithelioid (histiocytoid) hemangioma, and reactive lymph nodes.