Amelanotic acral melanoma masquerading as fibrous histiocytic tumours. Three case reports

We report 3 cases of amelanotic melanoma developing on the finger, whose histology disclosed dermal invasion of histiocyte-like tumour cells. One of the 3 cases was subungual melanoma and the other 2 were on the volar surface of the finger tip. Because of the characteristic dense infiltration of large histiocyte-like tumour cells, including many multinucleated giant cells, we initially considered histiocytic tumours. However, there were some histiocyte-like cells that displayed inclusion-like intranuclear invagination of cytoplasm, and almost all tumour cells, including the giant cells, were positive for S-100 protein. In addition, ultrastructural demonstration of premelanosomes within the cytoplasm of the tumour cells established the diagnosis of amelanotic melanoma. These features were distinct histologically from other variants of vertical growth phase amelanotic malignant melanoma, including desmoplastic or neurotropic melanoma. Because we encountered cases 2 and 3 within just a year after the first case, we think that the misdiagnosis of amelanotic "histiocytic" melanoma can be avoided through enhanced clinical awareness and subsequent appropriate histopathologic studies.