Primary intra-axial leiomyosarcoma with obstructive hydrocephalus in a young dog.

A female Chihuahua dog aged 2 months had a 3-week history of progressive neurological signs (abnormal behaviour, disorientation, left-sided circling, and incessant whimpering). Necropsy revealed a primary leiomyosarcoma in the midline of the posterior thalamus with secondary obstructive hydrocephalus of the lateral ventricles. Grossly, an infiltrative, grey-white tumour had partly invaded the third ventricle. Histologically, the tumour formed hypercellular, interlacing bundles of neoplastic spindle cells with blunt-ended nuclei. Neoplastic cells were strongly immunoreactive for vimentin and both muscle-specific and alpha-smooth muscle actin; MIB-1 immunoreactivity indicated a proliferative index of up to 5%. Leiomyosarcoma should be included in the differential diagnosis for primary brain tumours and hydrocephalus in young dogs.