Canine Spindle Cell Mammary Tumours : a Multicentre Study

Introduction: Canine spindle cell mammary tumours (CSCMTs) are rare mammary tumours that are not well documented. The aim of this study was to collect a reliable number of CSCMTs from three different institutions for evaluation. Materials and Methods: Cases of dogs with mammary tissues submitted for histopathology from 1998–2013 (16 years) were selected retrospectively from the databases when diagnosis was compatible with CSCMT. Selected cases were reviewed histologically and graded, using a grading system for sarcomas (s-grade) and when applicable, a grading system for canine mammary carcinomas (c-grade). Immunohistochemistry (IHC) for Ki67, CKAE1/AE3, CK14, vimentin, calponin, p63 and CD31, and histochemistry (PAS/Masson's trichrome) was performed in a subset of malignant tumours (n = 35). Results: After reviewing a total of 9,936 mammary tumours, excluding dysplasias, a total of 103 CSCMTs of the mammary region from 99 animals were selected. The prevalence was similar in the three institutions: 1.09%, 1.13% and 0.86%, respectively. The origin was mammary in 62.1% of the cases (n = 64) and diagnostic distribution after microscopical review was: malignant myoepithelioma (18.8%), PWT/PNST (17.2%), fibrosarcoma (15.6%), carcinoma and malignant myoepithelioma (15.6%), not determined/undifferentiated sarcoma (12.5%), spindle cell carcinoma (7.8%), hemangiosarcoma (7.8%), benign myoepithelioma (3.1%), and angioma (1.6%). Most neoplasms were s-grade III (55.0%) and c-grade II (63.0%). Conclusions: IHC is necessary for an accurate diagnosis. The diagnosis was changed after IHC in 34.1% of cases, becoming malignant myoepithelioma (48.8%) the predominant type. Histochemical stains represent a useful diagnostic tool, the presence of PAS+ membranes and collagen (Masson's trichrome) allowed the identification of myoepitheliomas and fibrosarcomas, respectively.