Answer: Langerhans cell histiocytosis

Histology: The lesion is characterized by a dense dermal infiltrate, which is fairly small and symmetrical and lacks epidermal involvement. The infiltrate is composed of larger cells with irregular nuclei, some of which show nuclear grooves. Despite the large size of the nuclei and scattered mitotic figures, nucleoli are not prominent. There is also a second population of small benign appearing mature lymphocytes. Other cell types, such as eosinophils and multinucleated histiocytes, are present though not numerous. Immunohistochemical stains for CD1A were strongly positive in the large cells consistent with Langerhans’ cell histiocytosis.

Discussion: On purely morphologic grounds, it may be difficult to exclude a lymphoma and one would need marker studies to characterize the large cells more completely. Arguing against lymphoma would be the difficulty in its classification. The large cells would suggest a diagnosis of large cell lymphoma. However, the presence of a population of small round mature lymphocytes would be somewhat unusual for this diagnosis, as would the lack of more prominent nucleoli. Against the diagnosis of malignant melanoma are the lack of epidermal involvement, the lack of prominent nucleoli, and the presence of scattered benign appearing multinucleated giant cells. The relatively small size, circumscription, and symmetry of the lesion also argue against melanoma. Lymphocytoma cutis consists of a mixture of benign appearing histiocytes and small normal appearing lymphocytes. The larger cells in this case do not have the appearance of normal looking histiocytes. Rather, many if them are cytologically atypical with irregular nuclear contours and occasional folds.