Reticulohistiocytoma involving the skin, subcutaneous tissue and a regional lymph node

A 30-year-old female complained of a surface-eroded solitary nodule on the right thigh. Histologically, the dermal lesion consisted of uniform-sized polygonal cells with eosinophilic, 'ground glass' cytoplasm. Mitoses were infrequent. Under the histopathologic diagnosis of amelanotic melanoma, wide resection of the skin and dissection of the inguinal lymph nodes were performed. The subcutaneous tissue and a lymph node showed nodular proliferation of histiocytoid cells, in association with hemosiderin-laden multinucleated giant cells. The mononuclear cells were immunoreactive for factor XIIIa, while the multinucleated cells were positive for CD68, lysozyme and HLA-DR. In the lymph node tissue, a considerable number of mononuclear cells positive for CD68 were noted. CD34, alpha-smooth muscle actin, desmin and HMB45 were negative. Ultrastructurally, the mononuclear cells were rich in 100 nm vesicles and 180-350 nm lysosome-like granules. Interdigitation of the plasma membranes was seen in the multinucleated cells. The patient did not complain of joint symptoms, and has been disease-free for 5 years. The histologic and immunohistochemical features are consistent with so-called 'reticulohistiocytoma', though the site of histiocytic growth was unusual.