Reports of testicular leiomyosarcoma are extremely rare in the veterinary literature. Here we report a case of testicular leiomyosarcoma occurring in a 32-weeks-old male senescence marker protein 30 (SMP30) knockout mouse. Grossly, a testicular mass completely replaced the right testis. The mass had a diameter of 1.6×0.9cm, and was well-encapsulated and firm. The cut surface showed heterogeneous white and yellow-brownish components with multifocal hemorrhages. Microscopically, the mass was composed of interlacing bundles of neoplastic cells characterized by a spindle-shaped appearance with elongated blunt-end nuclei and frequently observed mitotic figures. Immunohistochemically, the neoplastic cells showed strong positivity for vimentin and α-smooth muscle actin. Finally, present case was diagnosed as testicular leiomyosarcoma in an SMP30 knockout mouse.

Key words: Mouse, SMP30, Testicular leiomyosarcoma, Vimentin, α-SMA