Adult-onset reticulohistiocytoma presenting as a solitary asymptomatic red knee nodule: report and review of clinical presentations and immunohistochemistry staining features of reticulohistiocytosis.

Reticulohistiocytomas are benign dermal tumors that usually present as either solitary or multiple, cutaneous nodules. Reticulohistiocytosis can present as solitary or generalized skin tumors or cutaneous lesions with systemic involvement and are potentially associated with internal malignancy. A woman with a solitary red nodule on her knee is described in whom the clinical differential diagnosis included dermatofibroma and amelanotic malignant melanoma. Hematoxylin and eosin staining and immunoperoxidase studies of the biopsy specimen established the diagnosis of adult-onset reticulohistiocytoma (solitary epithelioid histiocytoma). Reticulohistiocytoma is characterized by mononuclear, and occasionally multinuclear, histiocytes with eosinophilic "glassy" cytoplasm. The immunohistochemical profile of a reticulohistiocytoma demonstrates consistent positive expression for CD68 (a marker that is expressed by histiocytes but can also show positive staining in melanomas and carcinomas), CD163 (a very specific marker for histiocytes), and vimentin. Reticulohistiocytomas show variable positive expression for MITF (microphthalmia transcription factor) and S100 protein, both of which are more commonly used as markers for melanocytes. Recurrence of a reticulohistiocytoma is rare, even for patients with an incompletely removed lesion. However, our patient elected to have her residual tumor completely excised.