Amelanotic malignant melanoma presenting as malignant schwannoma

A 78-year-old woman presented with a 14-month history of a nodule on the sole of her left foot. It had been increasing in size, and had become ulcerated. Histological, immunochemical and ultrastructural studies of the primary tumour revealed melanocytic and Schwannian characteristics, and posed diagnostic difficulties. The final diagnosis of a malignant melanoma with Schwannian differentiation was established on the basis of the clinical course, with the development of metastases in the subcutis, lymph nodes, liver and brain, as well as a shift in differentiation of the metastases towards cells containing giant melanosomes, typical of melanoma.