An unusual finding ofschwannoma inthe mammary gland ofadog.

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Peripheral nerve sheath tumours (PNSTs) are rare inthe mammary glands ofdogs. Here, wereport acase ofaschwannoma, composed oftwo parts, ofthe mammary gland ofadog. The first part consists ofclusters ofuncircumscribed, alternating, more concentrated and looser regions. Inthe more concentrated parts, typical fascicularly arranged schwannoma intermingle inplexiform arrangement, more subtle inappearance than inneuronal tumour areas. The second part undergoes regression consisting predominantly ofresidual shorter rosettes ofcells with the presence ofapeculiar variably sized swirling oftarget-like formations consisting ofcompact, thicker, pinkish lamellae also with occasional adjacent cells. Immunohistochemically, the tumour cells are positive for evidence ofvimentin and neuro-specific enolase. They exhibit the variable expression ofthe S-100 protein, show mild CD56 positivity, and focally mildly accentuated proliferative activity asassessed byKi-67. The tumourelements are negative for evidence ofcytokeratin7, cytokeratin20, and oestrogen receptors. Hybrid tumours may change their morphology incombination with atypical localisation and may beunderdiagnosed inveterinary biopsy practice. They differ from epithelial tumours prognostically, aswell asintheir development and behaviour, therefore itisessential toclearly differentiate them.