Canine spindle cell mammary tumours : diagnosis, grading and clinical behaviour

Introduction: Canine spindle cell mammary tumours (CSCMTs) represent 0.64% of all mammary tumours and their characteristics and clinical behaviour are mostly unknown. The aims of this study were to characterize microscopical features, clinical aspects and bio- logical behaviour in a series of CSCMTs. Materials and Methods: A total of 70 CSCMTs with demonstrated mammary origin were included in the study. Histology and immunohis- tochemical expression (in 58 cases) of CKAE1/AE3, CK14, vimentin, calponin, p63 and CD31 were evaluated. CSCMTs were graded as sar- comas (s-grade) and as mammary tumours (m-grade) when possible. Results: Dogs with CSCMTs were of different breeds, generally entire (25% neutered) and with a mean age of 10.7 years. CSCMTs were large in size (mean 6.8 cm, 53.1% T3) with infrequent lymph node (6.9%) and distant (3.1%) metastases at diagnosis. Lymph node metas- tases were associated with a high mitotic index (P 5 0.07) and m-grade III (P 5 0.011). After immunohistochemistry (IHC), the most com- mon diagnoses were malignant myoepithelioma (MM) (59.3%), carci- noma and MM (16.9%), haemangiosarcoma (8.5%) and PWT/PNST (6.8%). MMs, best marked by calponin, were frequently solitary tu- mours (P 5 0.049). CSCMTs had a low recurrence/metastatic rate (27.6%). Distant metastases were associated with extensive necrosis (O50%) in the primary tumour (P 5 0.025) and the histological type of CSCMT (P 5 0.04). Cause of death was generally the develop- ment of other non-CSCMT malignant mammary tumours (P 5 0.023). Conclusions: CSCMTs generally demonstrated low recurrent/met- astatic potential. Extension of necrosis and identification of the spe- cific histological type with the aid of IHC may assist in the identification of CSCMTs with malignant behaviour.