Primary cutaneous signet-ring cell melanoma with pseudoglandular features, spindle cells and oncocytoid changes.

The authors report a 69-year-old man with a primary nodular malignant melanoma on the shoulder showing unusual histological features. It mostly consisted of markedly neoplastic melanocytes, which exhibited abundant clear cytoplasm compressing the nuclei to the periphery, resulting in a signet-ring appearance. In other areas of the melanoma, there were focally desmoplastic and pseudoglandular patterns and oncocytoid changes. Immunohistochemical studies showed that the tumor cells were uniformly positive only for S-100 protein and vimentin. Signet-ring cell melanoma is a rare histopathological variant of malignant melanoma with only a few described cases. To the best of our knowledge, this is the first case reporting a combination of the signet-ring cell melanoma with desmoplastic, pseudoglandular, and oncocytoid features.