Mammary mixed tumours are the most frequent neoplasias in female dogs. In humans, mixed tumours are frequently found in the salivary glands and are known as pleomorphic adenomas. In addition to their histomorphologic similarities, mixed tumours and pleomorphic adenomas have the potential to become malignant and give rise to carcinomas in mixed tumours and carcinomas ex-pleomorphic adenoma, respectively. The factors associated with malignant transformation are still poorly known in the case of canine mixed tumours. However, this form of neoplasia tends to be associated with a better prognosis than other malignant histological types. This paper discusses the main features associated with female canine mammary mixed tumours. 1. Introduction Mammary tumours are the most frequent neoplasia in female dogs; therefore, these tumours represent a serious problem in veterinary medicine [1]. Mixed tumours are one of the most common tumour types in the female canine mammary glands. These tumours exhibit a complex histological pattern because they comprise elements from the epithelium and the mesenchyme and have the capacity to undergo malignant transformation, thereby giving rise mainly to carcinomas and less frequently carcinosarcomas and sarcomas in mixed tumours [2, 3]. Defining the origin of the several cellular elements involved in mixed tumours, as well as the factors contributing to malignant transformation is important in understanding the behaviour and evolution of this type of neoplasia. However, these components of mixed tumours still remain to be elucidate. This paper discusses the main features associated with the clinical-epidemiological characteristics, histogenesis, malignant transformation, and comparative aspects of female canine mammary mixed tumours. 2. Definition/Morphology Benign mixed tumours are characterised by the presence of benign epithelial elements (ductal and/or acinar and myoepithelial cells) and mesenchymal cells with cartilage and/or bone formation eventually combined with myxoid fibrous tissue [2] (Figure 1(a)). Figure 1: (a) Benign mixed tumor in canine mammary gland presenting chondroid and myeloid metaplasia. HE, 10x. (b) Ductal in situ carcinoma in benign mixed tumor in canine mammary gland presenting myoepithelial cells producing myxoid matrix. HE, 40x. (c) Carcinoma in benign mixed tumor in canine mammary gland presenting in situ carcinomatous areas and myoepithelial cell proliferation producing myxoid matrix. HE, 20x. (d) Carcinoma in benign mixed tumor in canine mammary gland presenting invasive areas in the adjacent stroma
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