Background In comparative pathology, canine mammary tumours have special interest because of their similarities with human breast cancer. Mixed tumours are uncommon lesions in the human breast, but they are found most frequently in the mammary gland of the female dogs and in the human salivary glands. The aim of the study was to compare clinical, morphological and immunohistochemical features of human salivary and canine mammary gland mixed tumours, in order to evaluate the latter as an experimental model for salivary gland tumours. Methods Ten examples of each mixed tumour type (human pleomorphic adenoma and carcinomas ex-pleomorphic adenomas and canine mixed tumour and metaplastic carcinoma) were evaluated. First, clinical and morphologic aspects of benign and malignant variants were compared between the species. Then, streptavidin-biotin-peroxidase immunohistochemistry was performed to detect the expression of cytokeratins, vimentin, p63 protein, estrogen receptor, β-catenin, and E-cadherin. Results After standardization, similar age and site distributions were observed in human and canine tumours. Histological similarities were identified in the comparison of the benign lesions as well. Metaplastic carcinomas also resembled general aspects of carcinomas ex-pleomorphic adenomas in morphological evaluation. Additionally, immunohistochemical staining further presented similar antigenic expression between lesions. Conclusion There are many similar features between human salivary and canine mammary gland mixed tumours. This observation is of great relevance for those interested in the study and management of salivary gland tumours, since canine lesions may constitute useful comparative models for their investigations.

β-catenin plays a central role in cadherin/catenin cell-cell adhesion complex and is involved in cell signaling pathway. Change in β-catenin distribution has been associated with several human cancers including salivary gland tumors. We studied the immunolocalization of β-catenin in a series of pleomorphic adenomas (PA) and carcinomas ex-pleomorphic adenomas (Ca ex-PA). Ten samples of PA and ten of Ca ex-PA were evaluated by immunohistochemistry using streptavidin-biotin-peroxidase technique and a monoclonal antibody against β-catenin (E-5). Cell membrane/cytoplasmic staining of β-catenin was observed in normal gland parenchyma, PA, and in well-differentiated Ca ex-PA. Cytoplasmic/nuclear β-catenin staining was observed in poorly differentiated carcinomas and, interestingly, in one case of PA. Our data showed decreased cell membrane β-catenin expression in higher-grade tumors suggesting that β-catenin may play an important role in histologic differentiation and transition to malignant phenotype of Ca ex-PA.