Histopathological evaluation of canine mammary gland tumours: a study of 92 cases

Neoplasms of the mammary glands are the most common tumours in female dogs, albeit rare cases in male dogs are reported as well. Canine mammary tumours (CMT) are a naturally occurring heterogeneous group of cancers with a high incidence rate of up to 25-50% among females. Despite many years of research, the aetiology and high incidence of these tumours are still poorly understood. There is a need to present epidemiological data on mammary tumour occurrence in different breeds of dogs in different countries to select prognostic risk factors and identify breeds with higher susceptibility to tumour development. The aim of this study was to describe histopathological findings for 92 animals, including 87 female and five male dogs, diagnosed with mammary gland tumours (MGTs) in Poland and to determine such epidemiological characteristics as age, breed and size as well as their relationship with tumour incidence and malignancy. Histopathological analyses showed that 82 (89.1%) of the 92 dogs had malignant tumours, seven dogs (7.6%) had benign tumours, and three dogs (3.3%) had non-neoplastic changes in the form of dysplasias. Thirty (33.7%) of the 89 dogs with mammary gland tumours had more than one neoplasia. The highest number of cases were epithelial tumours (79.8%), followed by mesenchymal (14.6%) and mixed (5.6%) tumours. The vast majority of malignant tumours were tubulopapillary carcinomas (33.7%), complex carcinomas (24.7%) and solid carcinomas (11.2%). It is worth noting a large variation in the degree of tumour malignancy among the tubulopapillary carcinoma cases. The present study showed a higher incidence of mammary gland tumours in female dogs aged 8-13 years. The vast majority of female cases were diagnosed at the age of 12 years. The average year of surgical intervention in the male dogs was 9 years. Among 25 breeds of dogs in which MGTs were noted, the highest number of affected animals were crossbreeds (30.3%), German Shepherds (23.6%) and English Cocker Spaniels (9.0%). The large size of the tumour (≥ 5 cm) was strongly associated with its malignancy.