Cutaneous melanoma with heterologous rhabdomyosarcomatous component in a dog.

Melanocyte-derived tumours are frequent in humans and animals and the clinicopathological progression of malignant presentations in dogs is comparable with humans. However, melanocytic tumours are a heterogeneous group of neoplasms with certain morphopathological patterns that may appear completely distant from the expected origin. Biphasic neoplasms with heterologous components representing transdifferentiation are uncommon and represent a challenge in diagnosis. Primary cutaneous melanomas with rhabdomyoblastic elements occur in humans but not in animals. We now describe a spontaneous case of cutaneous melanoma with rhabdomyosarcomatous component in a dog. A striated muscle heterologous component and dispersed cells with scarce melanin were interspersed in many regions of the neoplasm. The proportion of discrete melanocytic cells synthesizing scarce melanin was very low. The heterologous rhabdomyosarcomatous component contained well-differentiated striated muscle fibres, indicating that it was not rhabdoid. The muscle fibres were strongly immunoreactive for desmin, whereas the melanocytic component was diffusely, moderately immunopositive for Melan A, HMB45 and S100. To the author's knowledge, this is the first reported case of cutaneous melanoma with rhabdomyosarcomatous heterologous component in a dog.