Mammary (Breast) Tumours in Dogs and Cats, and the Neutering Link

Mammary tumours grow in the breast (mammary) tissue that runs in two chains down the underside of the body, from the chest to the groin. Dogs and cats both have several pairs of glands along each chain, so a lump can appear anywhere along that line, and quite often there's more than one. In dogs, multiple tumours involving more than one gland are very common, seen in roughly half to two thirds of cases (MSD Veterinary Manual, 2024).

They're overwhelmingly a disease of entire (un-neutered) and late-neutered females, and of older animals, with the average dog around eight years old at diagnosis (CVS Vets, 2024). They're far rarer in males. The reason is hormonal, and it's the same reason that the prevention story below is such a strong one.

This is the single most important thing to hold on to, so it gets its own section.

In dogs, roughly half of mammary tumours are benign and half are malignant (American College of Veterinary Surgeons, 2024; Cornell Riney Canine Health Center, 2024). A benign one is not cancer, won't spread, and is usually cured by removing it. Even among the malignant half, many are slow and are dealt with by surgery, although it's fair to say that the more aggressive malignant ones can spread, and somewhere between half and three quarters of those will eventually cause real trouble by recurring or reaching the lungs within a year or two if left (Cornell Riney Canine Health Center, 2024). So a canine mammary lump is a "get it checked soon" situation, not usually a "panic tonight" one.

In cats, the picture is sterner. Over 85% of feline mammary tumours are malignant (American College of Veterinary Surgeons, 2024), and they tend to be aggressive, spreading to the lymph nodes and lungs, and they're often advanced by the time they're found (EveryCat Health Foundation, 2024). That isn't said to frighten you. It's said because it changes the timeline: a mammary lump in a cat needs seeing quickly, not watching, because catching it small is the thing that most changes the outcome.

If you take one thing from this page, take that card. The same lump in the same place means two quite different things depending on whether you're stroking a dog or a cat.

This is the part that genuinely puts something in your hands, especially if you have a young female pet, because mammary cancer is one of the most preventable cancers there is.

The mammary tissue is driven by the hormones of the seasons (heat cycles). Each season a female goes through adds to the hormonal exposure that, over years, can tip cells towards a tumour. Spaying removes the ovaries and switches that hormonal driver off, and the earlier it's done, the more dramatic the protection.

The figures that are quoted across the profession, originally from work in the late 1960s, are striking. A dog spayed before her first season has roughly a 0.5% risk of ever developing a mammary tumour. Spayed after her first season, that rises to about 8%. After her second season, to about 26%. By around two and a half years of age, spaying no longer reduces the mammary risk much at all (American College of Veterinary Surgeons, 2024; Cornell Riney Canine Health Center, 2024; originally Schneider et al., 1969). In other words, the protection is real, but it's front-loaded, and it leaks away with each season that passes.

Now the caveat, because you deserve the current state of the evidence and not just the headline. Those classic percentages come from older studies, and careful systematic reviews have pointed out that the quality of that early evidence is weaker than the confident numbers suggest, with the size of the protective effect and the exact timing still genuinely debated (Beauvais et al., 2012; Guirguis & Beggs, 2025). The direction of travel isn't in doubt, early spaying lowers mammary risk, but the precise figures should be treated as a strong guide rather than a guarantee. It's also why this is a conversation to have properly with your own vet, because spaying timing affects more than mammary cancer (joints and a few other cancers come into the balance for some breeds), and the right answer can depend on the individual dog.

In cats the same logic holds and may be even stronger: spaying before six months of age has been found to cut the risk of feline mammary cancer by around 90% (Overley et al., 2005). Given how aggressive feline mammary cancer is, that prevention is well worth having.

Your vet can't tell benign from malignant by feel, any more than you can, so the lump itself needs sampling. That usually starts with a fine needle aspirate, and often goes to a biopsy or to examining the whole lump after it's removed, which is the definitive answer (see the article on aspirates, biopsies and what the results mean).

Because the malignant ones can spread, your vet will usually want to "stage" the cancer before or around surgery, especially in a cat or with a larger or angrier-looking canine tumour. That means chest images (radiographs) to look for spread to the lungs, and checking the nearby lymph nodes, since lungs and regional nodes are the commonest places these tumours travel to (MSD Veterinary Manual, 2024). It feels like a lot of tests, but staging is what tells you and your vet whether surgery alone is likely to do the job (see the article on what grade, stage and prognosis actually mean).

Surgery is the mainstay for both, but how much surgery differs sharply between the two, and it follows directly from the odds above.

In dogs, surgery can afford to be more measured. Because so many canine tumours are benign or low grade, removing the lump itself or just the affected gland is often enough (American College of Veterinary Surgeons, 2024). Your vet will usually also recommend spaying at the same time if your dog is still entire, both to remove the hormonal driver and to prevent other reproductive problems. Chemotherapy isn't routine for most canine mammary tumours and hasn't reliably been shown to extend survival in dogs (CVS Vets, 2024), so it's reserved for the aggressive ones on specialist advice (see the article on cancer surgery and getting clean margins).

In cats, surgery is deliberately more aggressive, because the cancer is. The recommendation is usually to remove the whole affected mammary chain, and often both chains, rather than just the lump (American College of Veterinary Surgeons, 2024). Chemotherapy after surgery is far more commonly considered in cats than in dogs, because it can buy useful extra time against an aggressive disease, and a specialist will help you weigh it (see the article on why pet chemotherapy is not human chemotherapy).

For both species, size at the time of surgery is one of the strongest predictors, which is exactly why early action matters so much.

In dogs, the outlook is generally good when a tumour is caught and removed while it's still small, and benign ones are simply cured (Cornell Riney Canine Health Center, 2024). Larger and higher-grade malignant tumours carry more risk of spread.

In cats, the size threshold is stark and worth knowing. When a feline mammary tumour is removed while it's small, under about 3cm, cats can go on to live a couple of years; once it's over 3cm at surgery, the typical survival drops to somewhere in the region of four to twelve months (American College of Veterinary Surgeons, 2024; MacEwen et al., 1984). The gap between those two numbers is, in effect, the value of finding it early, and it's the whole reason a feline mammary lump should never be sat on.

If you've already found a lump, the powerful move is simple: get it checked soon, and sooner still if it's on a cat. Most canine ones turn out fine, and even the ones that don't are far more manageable small than large. You can measure it, photograph it and keep an eye on any change while you wait for the appointment using the Lump & Bump Tracker, which gives you and your vet a clear record of whether it's growing.

And if you have a young female dog or cat and the lump hasn't happened yet, this is one of the rare cancers you can genuinely tip the odds against. Talk to your vet about the right time to spay her, knowing that for mammary risk earlier is more protective, and that the decision sensibly weighs up everything else about her too. It's not often that prevention is this much within your hands.