Differential Diagnosis of Canine Gastrointestinal Stromal Tumor

Background: Stromal Grastrointestinal Tumors (GIST) are rare mesenchymal neoplasms that affect the wall of the gastrointestinal tract (GIT). Histologically this tumor shows a spindle, epithelioid or mixed aspect and routine hematoxylin and eosin (HE) staining does not allow adequate differentiation from other mesenquimal neoplasms. Histochemical and imunohistochemical techniques are useful tools in this differentiation. The aim of this study was to report a case of canine GIST, emphasizing the use of histochemical and immunohistochemical techniques to differentiate it from other histomorfologically similar mesenchymal neoplasms. Case: A ten-year-old male Pinscher dog was submitted to surgical excision of a tumor in the pyloric region of stomach. The dog was euthanized 90 days after surgery. Tumor specimens were collected, fixed in 10% neutral buffered formalin solution and embedded in paraffin. Afterwards, 4 mu m histological sections were obtained and stained with HE and Gomori's Tricrome (GT). For immunohistochemical analysis a biotin-peroxidase system was used and secondary antibodies were identified using Advance HRP. Cytokeratin (CK) AE1/AE3, c-KIT, vimentin, Ki-67, S-100 and Smooth Muscle Alfa Actin (SMA) expression were evaluated. For vimentin, CKAE1AE3, SMA, S100 and c-KIT antibodies, a semiquantitative method was used and neoplasms were classified as negative (-), positive with focal or multifocal staining (+) and positive with diffuse staining (++). The proliferative index was calculated by counting nuclei positive for Ki-67 (anti-MIB-1) in a total of 1000 neoplastic cells. Macroscopically, the stomach fragment revealed an intramural nodule covered by mucosa and with a slightly lobulated surface. Microscopically, the neoplastic mass was located in the submucous layer of stomach and composed by a spindle cell proliferation forming intersecting fascicles. Neoplastic cells revealed vacuolated eosinophilic cytoplasm and multiple conspicuous nucleoli. Absence of chromatic affinity for GT was observed in this tumor. Immunohistochemical analysis revealed intense and diffuse cytoplasmic staining for vimentin (++), moderate and diffuse staining for c-KIT (++), moderate and multifocal staining for S-100 (+) and negative staining (-) for SMA and CK AE1/AE3. The proliferation index was 6.8%. Discussion: GISTs are mesenquimal neoplasms that affect the GIT. The spindle aspect of this tumor is similar to other mesenquimal tumors of the GIT. A mesenquimal appearance associated with epithelioid areas was observed in this case report. Mesenquimal histogenesis was confirmed by positivity for vimentin and negativity for cytokeratin. GT evaluation revealed absence of muscular and collagenous components within the neoplasm, discarding the diagnosis of leiomiosarcoma or fibrosarcoma. The absence of smooth muscle tumor cells was confirmed by immunohistochemical negativity for SMA. In addition, multifocal and diffuse cytoplasmic imunostaining was observed for S-100 and c-KIT, respectively. According the literature, human and canine GISTs are positive for c-KIT and negative for S-100. The S-100 expression described in this study, has been reported in humans GISTs and is probably associated with neural differentiation. Therefore, histomorfological and immunohistochemical findings allowed a definitive diagnosis of canine GIST, differentiating the tumour from other mesenchymal neoplasms of the GIT.