Invasive micropapillary carcinoma (IMPC) of the mammary gland, despite its rare occurrence in humans and dogs, is an important neoplasm due to its aggressive behaviour. The aim of this study was to evaluate the clinicopathological and immunophenotypical characteristics of IMPC and to determine the overall survival of dogs with this tumour. Of the selected cases, the majority had >3 cm neoplasms (15/19, 78.95%) and lymph node metastases (16/16, 100%), but only two cases (2/9, 22.2%) had distant metastases. The IMPCs were classified as either pure (15/22, 68.18%) or mixed (7/22, 31.82%) types. There was a predominance of moderate histological grade tumours (16 grade II) and the average overall survival was 120 days. Positive immunohistochemical staining for epithelial membrane antigen and negative staining for CD-31, p63 and cytokeratin (CK) AE1AE3 in cystic formations confirmed the micropapillary nature of these neoplasms. A proportion of cases exhibited positive epithelial staining for p63 (4/20, 20%) and CK34βE12 (20/22, 90.9%). Most cases were positive for oestrogen (19/20, 95%) and progesterone (19/20, 95%) receptors, but lacked HER-2 (16/22, 72.72%) and epidermal growth factor receptor (15/22, 68.18%) over-expression. The mean proliferation index was 14.8%. The findings demonstrate that, similar to humans, canine IMPCs behave aggressively with high rates of metastasis to regional lymph nodes and short overall survival times.
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