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Amelanotic vulvar melanoma is a rare type of malignant melanoma. This paper describes a case of an asymptomatic ulcerated nodule 20 mm in size. The tumor cells from the nodular lesion showed positive staining immunohistochemically for Melan-A, but negative staining with HMB-45. The cells showed negative reactivity to S-100 except in one region. The melanoma cells in the epidermis were detected in one of the specimens from the excised tumor nodule. The cells in the epidermis showed positive staining for Melan-A and S-100 and partial staining with HMB-45. The tumor was diagnosed as malignant melanoma of the vulva and immunohistochemically shown to have intratumor histological heterogeneity. This case suggests the importance of viewing non-pigmented nodules on the vulva of elderly females as potentially malignant melanoma, and that a combination of immunohistochemical stains may be useful for recognizing the stage of the melanosomes in the melanoma cells.

A case of amelanotic melanoma arising in the upper molar region, which was difficult to diagnose histologically, is reported. The patient was a 79-year-old woman, who complained of a painful swelling in the gingiva of the left upper molar region. Routine histological examination showed that the lesion was composed of diffusely scattered atypical cells with round, spindle-shaped and irregular nuclei and scanty fibrous connective tissue. A fascicular arrangement was often found in the lesion, and no cancer nests were observed. Immunohistochemical study demonstrated positive staining for S-100 protein in both the nuclei and cytoplasm of the tumor cells. Electron microscopic examination revealed that cell organelles were abundant, and an interrupted basal lamina was often found along the cell membrane. The preliminary diagnosis was a non-epithelial malignant tumor. After surgery, histological examination of metastases in lymph nodes from the submandibular region revealed that the tumor cells contained melanin pigment in the cytoplasm, as confirmed by Masson's melanin stain. The final pathological diagnosis was therefore amelanotic melanoma. Immunohistochemical staining for S-100 protein may be useful for differential diagnosis of amelanotic melanoma in conjunction with electron microscopic examination.