An unusual finding of schwannoma in the mammary gland of a dog.

Peripheral nerve sheath tumours (PNSTs) are rare in the mammary glands of dogs. Here, we report a case of a schwannoma, composed of two parts, of the mammary gland of a dog. The first part consists of clusters of uncircumscribed, alternating, more concentrated and looser regions. In the more concentrated parts, typical fascicularly arranged schwannoma intermingle in plexiform arrangement, more subtle in appearance than in neuronal tumour areas. The second part undergoes regression consisting predominantly of residual shorter rosettes of cells with the presence of a peculiar variably sized swirling of target-like formations consisting of compact, thicker, pinkish lamellae also with occasional adjacent cells. Immunohistochemically, the tumour cells are positive for evidence of vimentin and neuro-specific enolase. They exhibit the variable expression of the S-100 protein, show mild CD56 positivity, and focally mildly accentuated proliferative activity as assessed by Ki-67. The tumour elements are negative for evidence of cytokeratin 7, cytokeratin 20, and oestrogen receptors. Hybrid tumours may change their morphology in combination with atypical localisation and may be underdiagnosed in veterinary biopsy practice. They differ from epithelial tumours prognostically, as well as in their development and behaviour, therefore it is essential to clearly differentiate them.