Oral malignant melanoma with rhabdoid cell differentiation and myogenic features in a dog.

Malignant melanoma (MM) is difficult to diagnose because of its pleomorphic morphology and diverse protein profile. We describe a case of canine oral MM that exhibited rhabdoid cell differentiation. The tumor contained two cell types: typical MM cells and aggregated rhabdoid cells. Immunohistochemically, both types of neoplastic cells were positive for vimentin and desmin. Typical MM cells are positive for melanocytic markers; a subset of rhabdoid cells is negative for melanocytic markers and positive for myogenin. Consequently, the tumor was diagnosed as an MM with rhabdoid cell differentiation. This case highlights a potential diagnostic pitfall, as the morphological and immunohistochemical features may closely mimic those of collision tumors and myogenic tumors.