Gastrointestinal spindle cell tumours of the dog: histological and immunohistochemical study.

To assess the relevance of spindle cell tumours in the canine gastrointestinal (GI) tract and to classify them, a retrospective study was carried out on haematoxylin and eosin-stained sections from formalin-fixed paraffin wax-embedded samples of 105 primary GI tumours. Seventeen out of 105 (16%) GI tumours were mesenchymal, 48% were epithelial and 36% were round cell tumours. Spindle cell tumours were stained by Masson trichrome, Orcein-Van Gieson and labelled immunohistochemically (vimentin, desmin, smooth muscle actin, protein S100, glial fibrillar acid protein, CD117 and MIB-1) and the histological grade, mitotic index, nuclear size and cellular density were also assessed. The 17 gastrointestinal mesenchymal tumours were classified as 10 leiomyomas (10/10 positive for desmin and smooth muscle actin; 6/10 positive for vimentin) 2 leiomyosarcomas (2/2 positive for desmin, smooth muscle actin and vimentin) and 5 gastrointestinal stromal tumours (GISTs) (5/5 positive for CD117 and vimentin; 3/5 positive for smooth muscle actin). Canine GISTs appeared as densely packed spindle cell tumours, with a diffuse, strong, cytoplasmic immunopositivity for c-kit protein (CD117). GISTs, defined as CD117-positive spindle cell or epithelioid or pleomorphic neoplasms that presumably derive from interstitial cells of Cajal, are reported in recent medical studies as the most common mesenchymal tumours of the GI tract. Our data suggest that GISTs represent a significant portion of canine GI spindle cell tumours, which can be definitely distinguished from leiomyosarcomas only by their expression of CD117.