Original Article DOI: 10.1111/j.1476-5829.2009.00184.x Canine mammary gland tumours; a histological continuum from benign to malignant; clinical and histopathological evidence* K. U. Sorenmo 1,2,3 , V. M. Kristiansen 2 , M. A. Cofone 2,4 , F. S. Shofer 1 , A.-M. Breen 2 , M. Langeland 2 , C. M. Mongil 5 , A. M. Grondahl 2 , J. Teige 2 and M. H. Goldschmidt 1,3 1 School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, PA, USA 2 Norwegian School of Veterinary Science, Oslo, Norway 3 Mari Lowe Center for Comparative Oncology, University of Pennsylvania, PA, USA 4 Veterinary Specialty Center of Delaware, Wilmington, DE, USA 5 Centro de Especialistas Veterinarios de Puerto Rico, Puerto Rico Abstract This study describes the clinical and histopathological findings in dogs with mammary gland tumours, and compares the histopathological and clinical evidence consistent with progression from benign to malignant to human breast cancer epidemiology. Clinical and histopathological data on 90 female dogs with 236 tumours was included. Dogs with malignant tumours were significantly older than dogs with benign tumours (9.5 versus 8.5 years), P = 0.009. Malignant tumours were significantly larger than benign tumours (4.7 versus 2.1 cm), P = 0.0002. Sixty-six percent had more than one tumour, and evidence of histological progression was noted with increasing tumour size. Dogs with malignant tumours were significantly more likely to develop new primary tumours than dogs with benign tumours, P = 0.015. These findings suggest that canine mammary tumours progress from benign to malignant; malignant tumours may be the end stage of a histological continuum with clinical and histopathological similarities to human breast carcinogenesis. Keywords breast cancer, comparative oncology, dog, malignant transformation, tumour Introduction Spontaneous tumours in companion animals represent a valuable and underutilized resource in cancer research. Companion dogs develop tumours spontaneously, are relatively out-bred, immunolog- ically competent and share their living environment with humans. The complex interaction between the *This study was performed at the Norwegian School of Veterinary Sciences, Ullevalsveien 72 Postboks 8146 Dep, 0033 Oslo, Norway and Centro de Especialistas Veterinarios de Puerto Rico, RR16 #3250 San Juan, Puerto Rico 00926. tumour and the host microenvironment associated with tumour development and tumour progression may therefore be effectively captured in these natu- ral models and as such provide a unique insight into cancer biology. The range of cancers diagnosed in dogs varies as much as those in humans and share many similar features in terms of histopathol- ogy, biological behaviour, hormone dependence, risk factors and genetic alterations as their human counterparts. 1 Furthermore, the therapeutic chal- lenges and the obstacles to a cure are also similar; and metastasis and drug resistance are major cause of treatment failure in both dogs and humans. Thus, Correspondence address: K. U. Sorenmo Department of Clinical studies Veterinary Hospital of the University of Pennsylvania 3900 Delancey Street Philadelphia PA 19104 e-mail: karins@vet.upenn.edu 162  c 2009 Blackwell Publishing Ltd