Parotid involvement by desmoplastic melanoma

Desmoplastic malignant melanoma often arises in sun damaged skin of the head and neck and shows frequent neurotropism. Although metastatic melanoma frequently involve the parotid, direct spread to the parotid has been rarely reported. We evaluated five cases of desmoplastic malignant melanoma involving the parotid gland with clinical and pathological evidence of precursor cutaneous lesions in four of the five cases. The parotid involvement in four cases was tumoural, and three of these were not clinically suspected to be melanoma. The histological appearance in all five cases was that of a sarcomatoid tumour. Immunohistochemistry and electronmicroscopy performed on three of the cases showed only evidence of schwannian differentiation: the tumour cells were positive for S-100 protein and vimentin, and negative for cytokeratin and HMB-45. Electronmicroscopy showed no evidence of melanogenesis. All five tumours showed histological evidence of prominent neurotropism with one case demonstrating extension from overlying skin along cutaneous nerves to the superficial parotid. Thus, desmoplastic malignant melanoma may involve the parotid by neurotropic spread and can be pathologically indistinguishable from malignant schwannoma, a diagnosis which may be made erroneously in the absence of clinical information.