BMC Cancer 2007, 7:218 http://www.biomedcentral.com/1471-2407/7/218 Background Animal models have been widely used to investigate sev- eral forms of human neoplasias. Because of centuries of coexistence with humans in the same environment, dogs are of particular interest as they provide important evolu- tionary information. In addition, both species show great genotypic similarities [1]. Thus, spontaneous tumours of canine mammary glands have been proposed as compar- ative models for the study of human breast cancer, since these lesions share epidemiological, clinical, behavioural and antigenic features [2-5]. There is also a well-known relationship between the inci- dence of human mammary and salivary glands tumours [6-9]. Morphological similarities have been described between certain tumours of salivary glands and breast neoplasias such as those existing between polymorphous low-grade adenocarcinoma and invasive lobular carci- noma [10], between acinic cell carcinoma and invasive secretory carcinoma [11], and between epithelial-myoep- ithelial carcinoma and adenomyoepithelioma [12]. Duc- tal carcinomas [13,14], adenoid cystic carcinomas and mixed tumours with similar patterns may be found in both organs [15,16]. Mixed tumours are unusual lesions in the human breast [17], but they are frequent in both human salivary and canine mammary glands [18-20]. In a comparative evalu- ation of the available literature, pleomorphic adenoma (PA) and its malignant counterpart, the carcinomas ex- pleomorphic adenomas (Ca ex-PA) have several interest- ing similarities to benign mixed tumours (MT) and to metaplastic carcinomas (MC) of canine mammary glands. First, all of them are derived from exocrine glands, which depict similar tissue architecture. Next, with few varia- tions, both are microscopically characterized by a mixture of ductal and myoepithelial elements intermingling an apparently mesenchymal stroma of variable constitution [18-20]. In addition, malignant transformation is acknowledged for both for human PA and canine MT, par- ticularly in lesions with long evolution and frequent recurrences [20-25]. In spite of these similar aspects, to the best of our knowledge no specific comparative investiga- tion between human salivary and canine mammary glands tumours is available. Thus, the present work aimed to perform objective mor- phological microscopic comparison between mixed tumours derived from human salivary and canine mam- mary glands, as well as to evaluate the immunohisto- chemical expression of some relevant antigens in order to characterize these two types of neoplastic alterations. Methods Samples Ten samples of PA and 10 of Ca ex-PA were obtained from the Department of Pathology of School of Medicine, Fed- eral University of Minas Gerais (UFMG, Belo Horizonte, Minas Gerais, Brazil), A. C. Camargo Cancer Hospital (São Paulo, São Paulo, Brazil), and the National Cancer Institute (Rio de Janeiro, Rio de Janeiro, Brazil). Ten sam- ples of MT and 10 of MC of mammary glands of dogs without defined breed were obtained from the records of the Laboratory of Comparative Pathology, Biological Sci- ences Institute, UFMG. Ca ex-PA diagnosis was restricted to cases with clinical features (such us a previous benign tumour excised from a site in which recurrent malignant tumour), and/or histological evidence of arising in or from a benign lesion (identification of at least a focus benign tumour) [18]. The clinical analyses of the tumours studied are summarized in Tables 1, 2, 3, 4. The malignant components of Ca ex-PA were further identified and sub- typed according to World Health Organization (WHO) and Armed Forces Institute of Pathology (AFIP) criteria [18,26] and are summarized in Table 2. All samples were formalin-fixed, paraffin-embedded, and new histological sections were independently reviewed by two experienced observers to confirm the diagnosis. Clin- ical and demographic data (age, gender, affected salivary or mammary gland) from affected individuals were retrieved from medical and veterinary charts. Immunohistochemistry To further compare tumours, immunohistochemical assays were carried out to detect antigens related to cellu- lar differentiation (cytokeratins, vimentin, and p63), adhesion (E-cadherin and β-catenin), and hormonal sta- tus (estrogen receptor), which have been shown to be rel- evant for the study of human salivary and breast cancer [27-36]. Streptavidin-biotin-peroxidase technique was used, employing the antibodies described in Table 5. Briefly, 3 μm thick histological sections were deparaffin- Table 1: Clinical characterization of human salivary glands pleomorphic adenomas. (M – male; F – female) Case Age Gender 1 2 3 4 5 6 7 8 9 10 30 35 42 53 29 51 32 39 58 27 M F M F F F F F M M Gland minor (lip) parotid parotid parotid parotid parotid parotid submandibular parotid parotid Page 2 of 9 (page number not for citation purposes)