Mammary Tumours and your pet

Whether you are a wife, mother, sister, or daughter there is a sinking feeling when you discover a nodule in your breast. Breast Cancer is something we are all very aware of.

* According to the World Health Organisation

Awareness among pet parents is severely lacking Fortunately we have a simple and effective solution…spaying at an early age goes a long way to preventing the disease.

But before we go into further detail, the terminology surrounding lumps, tumours and cancer can be a little daunting sometimes, and is often a source of confusion. One thing to keep in mind is that the word tumour does not always mean cancer. Cancer implies malignancy, or a predilection to grow rapidly and spread to other parts of the body: it’s a process known as metastasis, in which a tumour says, in essence, "I’m outta here" and decides to infiltrate other areas of the body. A tumour can be benign or malignant, but cancer is always malignant.

A mammary tumour is a mass that appears on or along the mammary (milk producing) chain in dogs and cats. It is important to be somewhat familiar with the normal mammary anatomy. There are five pairs of mammary glands, although some individuals may only have nine (It is not unusual for asymmetry of mammary glands to be found.) The normal glands should be soft and pliant, especially towards the rear legs. There should be no firm lumps. Most tumours occur in the glands nearest the rear legs. These abnormal growths that form in the mammary glands of dogs and cats are often overlooked.

If your dog is unspayed, was known to have had puppies, or was spayed in adulthood (particularly animals adopted as adults from rescue organisations), she fits into the high-risk group for mammary cancer development. 

There are two significant risk factors associated with mammary tumour development:

Hormonal fluctuations, notably during heat cycles, significantly influence mammary tumour development in dogs and cats. Elevated oestrogen levels during these cycles can stimulate mammary tissue growth, increasing tumour risk.

Obesity is also significant risk factor for mammary tumours in pets, particularly obesity that develops at a young age. Excess body weight can disrupt hormonal balance, fostering tumour growth. Responsible weight management through balanced diet and exercise is pivotal, reducing the likelihood of tumour development and promoting overall health.

Swift action is vital as early detection substantially improves treatment efficacy. Regular veterinary check-up’s, especially in aging pets, can unveil potential issues. Timely intervention can often result in better outcomes for our cherished companions. Perhaps the most significant prognostic factor is the size of the tumour at presentation with tumours < 3cm in diameter having significantly better long term prognosis. Invasion and ulceration of the overlying skin carries a poor prognosis. Surprisingly the tumour location and number of tumours has little bearing on prognosis.

In dogs, mammary tumours are known as the 50% tumours – 50% are malignant and 50% will have already spread at the time of diagnosis, with the most common sites of metastasis being the lungs and regional lymph nodes. 

Sadly, there is no easy way. A veterinarian will perform a fine needle aspirate, or FNA, if you are hip to the lingo, on most lumps. When it comes to mammary masses though, FNA’s are notoriously unreliable as far as predicting malignancy goes. It does, however, help us make sure that the lump is from mammary tissue and not something else that decided to set up shop in the same neighbourhood. 

Confirming the diagnosis requires a biopsy and sending a piece of the tumour to a pathologist for histopathology. This may either be an excisional biopsy where the whole tumour is excised and forms part of the treatment plan; or an incisional biopsy where just a small piece is taken. The latter is more likely when there is concern that the disease is not isolated to the mammary gland and may already have spread. It is then part of what we call staging of the disease – determining the degree of aggressiveness and how far the cancer has spread.

Once a diagnosis is confirmed treatment is primarily surgical, with adjuvant chemotherapy (in the case of malignant tumours). Surgery can be quite extensive and may involve a complete mammary strip - removing all the glands on one side (the equivalent of a mastectomy), and if both sides are involved, may have to be done in two separate surgeries.

I know this sounds complex, intimidating and potentially invasive – not to mention expensive. 

The take-home on this is simple: spay your pet at a young age*, unless you have a rock-solid breeding plan for your purebred pet, and have your veterinarian check out any suspicious lumps. Even if your bitch fits in the high risk category, spaying prior to the onset of mammary tumours can still have a significant impact on the prognosis should they still develop. That’s about it.

*I have to add a foot note here – in light of the data we have available on mammary tumour prevention, the majority of the literature will recommend spaying before the first heat. However, there is a rising concern in large breed dogs particularly, that spaying before they have achieved physical maturity may predispose to the development of a number of orthopaedic conditions and some other tumours. Fortunately, the incidence of mammary tumour is still very low in animals spayed between their first and second heat cycles. There are numerous other considerations in determining the best age to spay your female dog and is a conversation best had with your veterinarian. 

Article written by Dr Richard Smith