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Mammary Tumours

Mammary tumours are common in dogs. They are the most common cancer in unspayed female dogs or those that were desexed after their first or second heats.  In dogs, overfeeding young dogs is also now considered to be a risk factor. Mammary tumors are rare in male dogs and cats.

Around 50% of mammary tumours in female dogs are benign, but benign tumours can still cause problems locally when they grow into large lumps, ulcerate or progress along the line of mammary glands. Over 85% of mammary tumours in cats are aggressively malignant and the prognosis in cats is always poor.


Mammary tumours are usually first detected as small, nobbly lumps underneath the skin, located in and around the mammary glands. Sometimes the tumour will ulcerate and become painful and inflamed. The rate of growth and spread varies with the type of tumour.

The extensive tumours pictured here would have been growing for some time. Leaving lumps to grow and not seeking veterinary advice when first noticed - regardless of whether they are benign or not - makes surgery much more challenging and problematic, both for the surgeon and the pet. Recovery takes much longer and the pet will need more intensive post-surgery care and pain relief.






The most effective treatment is early surgical removal combined with desexing if the dog is entire. By early we mean removing the lumps when they are still small. Too often we are presented with dogs that have large lumps many centimetres in diameter. Rather than a simple lumpectomy, these cases require extensive, sometimes radical surgery, with removal of mammary gland tissue and skin often being required for a successful outcome. The pet has a longer recovery, and pain relief and other aftercare is significantly more complicated.

The prognosis is good following surgical resection for most mammary tumors in female dogs. For cats, the surgery must be more aggressive with removal of one or preferably both sets of mammary glands recommended and the prognosis is always guarded.

If the tumours are well-advanced, CVH vets may recommend taking x-rays of the chest, for example, to check for signs of spread before surgery and taking a biopsy to determine if the tumours are benign or malignant.

The role and benefits of chemotherapy and radiation in cats and dogs with malignant mammary tumors is not yet clear, but consultation with a veterinary cancer specialist may be recommended.


In female dogs, it is well documented that desexing early (before the first or second heat) reduces the risk of mammary tumours significantly.

The American College of Veterinary Surgeons state that the risk of a dog developing a mammary tumor is 0.5% if spayed before their first heat (approximately 6 months of age), 8% after their first heat, and 26% after their second heat – that is, more than a quarter of unspayed female dogs will develop a mammary tumour in their lifetime. Cats spayed before 6 months of age have a 7 times reduced risk of developing mammary cancer and spaying at any age reduces the risk of mammary tumors by 40% to 60% in cats.

Examine your pet at regular intervals for any lumps, bumps, or swellings and make sure you get any lumps checked quickly while small.