Langerhans cell histiocytosis mimicking malignant melanoma: a diagnostic pitfall.

Langerhans cell histiocytosis (LCH), especially with an involvement limited to the skin, is a rare entity in adults. In formulating a differential diagnosis of a solitary skin lesion, LCH is rarely considered. Morphologically, cells seen in LCH can mimic those seen in a melanocytic tumor; moreover, they both show S-100 protein reactivity with immunoperoxidase staining. A 63-year-old male presented to a dermatology clinic with a solitary hyperpigmented macule on his right calf. A biopsy specimen showed epithelioid cells within the dermis, singly and in small groups, surrounded and infiltrated by collections of histiocytes and lymphocytes. These cells were diffusely positive for S-100 and negative for Melan-A. A diagnosis of malignant melanoma, spitzoid variant, was rendered, and the patient was sent to our melanoma center for surgical treatment. On histologic examination, some of the lesional cells had reniform, vesicular nuclei with central grooves. Additional immunoperoxidase staining showed strong, diffuse positivity for CD1a, supporting the diagnosis of LCH. LCH is morphologically similar to and can be misdiagnosed as malignant melanoma. It is important to be aware of this pitfall and utilize immunohistochemical and ultrastructural analysis to achieve correct diagnosis.