Mammary tumours are extremely common in dogs and cats and can be emotive cases because clients are more aware of breast cancer and may have some personal experience.
Throughout the oestrus cycle there is significant development and regression of the mammary tissue. This can be observed histologically in dogs and cats. For this reason, normal mammary tissue can look very different histologically depending on the stage of oestrus or neutering state.
There is a lot of data regarding risk factors for developing tumours in women and the same processes have been applied to animals. These include increasing age, with an increase in tumour formation beyond about 7 years in the dog and 9 years in the cat; age at neutering; breed susceptibility/genetics; exogenous hormone therapy and diet/obesity. For dogs and cats, pregnancy, parity and pseudopregnancy in the bitch do not appear to affect the risk of developing mammary tumours.
There are classification systems for naming mammary tumours in both dogs and cats. Over 90% of mammary tumours in elderly cats are malignant. Canine mammary tumours are more diverse, with more benign tumours recognised, including the very common mixed and complex adenomas. Roughly 45% of canine tumours are malignant.
Breast cancer in women is graded using the Elston-Ellis (or Nottingham) grading system. We can grade both canine and feline mammary tumours using modifications of this system. Tumours are graded based on tubule formation, degree of nuclear pleomorphism and mitotic count per 10 high-power fields to give a total score. From the total score tumours can be graded as 1: low, 2: intermediate and 3: high grade. Another prognostic indicator is tumour diameter. Tumours greater than 3 cm diameter in dogs and 2 cm in cats tend to have a poorer outlook. Further clinical staging includes evidence of lymphovascular invasion and lymph node spread. Obviously further metastatic spread to the lungs is indicative of a poor prognosis.
Immunohistochemistry has been used to subdivided breast cancers in women and has prognostic value. Common markers include oestrogen and progesterone receptors and human epidermal growth factor receptor 2 (HER2). Recently there have been attempts to classify canine and feline mammary carcinomas by similar molecular subtypes. The outcome has varied depending on the study, probably due to the different panels of antibodies used and, although the subtypes have identified themselves in some studies, the results are contradictory and additional investigations are being done to try to standardise protocols. For the time being we use COX2 in dogs, and COX2 and Ki67 in cats as prognostic markers.
Surgery remains the mainstay of treatment, with oncologists choosing additional chemotherapy and/or radiotherapy as necessary, either as adjuvant treatment, or for palliative care to reduce pain and shrink tumours.
NationWide Laboratories have also hosted a free online webinar on this topic entitled “Lunchtime basic bitesize pathology: small animal mammary pathology” with Sandra Dawson (BSc, BVMS, FRCPath, MRCVS). Watch on demand: https://www.thewebinarvet.com/webinar/lunchtime-basic-bitesize-pathology-small-animal-mammary-pathology