Correspondence to

mna dog and review of gastrointestinal stromal tumours J. S. REis-FILHO, C. GONCALVEs, F. GARTNER MESENCHYMAL tumours of the digestive tract are unusual neoplasms in dogs; most are either benign or malignant tumours with smooth muscle differentiation features (Head 1990, LaRock and Ginn 1997, ter Haar and others 1998). However, it has been frequently reported that the distinction between leiomyomas, leiomyosarcomas (LMSS) and true gastrointestinal stromal tumours (GISTS) is difficult, or even impossible, based only on morphological features (Miettinen and others 2000, 2001, Rubin and others 2000, Miettinen and Lasota 2001). In dogs, LMSS of the gastrointestinal tract are aggressive, malignant, smooth muscle tumours which usually affect the small intestine and proximal large bowel, mainly the jejunum and the caecum (LaRock and GAinn 1997). Tlo the authors ' knowledge, only a handful of cases of LMss affecting the rectum of dogs have been recorded (Head 1990, LaRock and Ginn 1997, ter Haar and others 1998). Most of these reports lack a comprehensive immunohistochemical analysis that could allow them to be distinguished from other puta-tive differential diagnoses (Head 1990, LaRock and Ginn 1997). This short communication describes a i,ms affecting the rectum of a dog, and gives details of new concepts involving G,isTs and smooth muscle tumours of the gastrointestinal tract. A 12-year-old mixed-breed dog was presented with a slowly growing, ulcerated, solitary nodular lesion, partially protruding through the anal sphincter. The mass measured approximately 3-5 cm in diameter. A marginal surgical resec-tion of the mass was carried out, and the specimen was fixed in 10 per cent buffered formalin and submitted for patho-logical examination. Grossly, the specimen consisted of a tan-to-whitish Coloured, multinodular hard mass, with scattered areas o