Serious severity

Mammary Tumours

Lumps in the mammary (breast) tissue that in cats are malignant in the great majority of cases. Because feline mammary cancer spreads readily, finding and removing lumps early makes an enormous difference — and early spaying dramatically lowers the risk in the first place.

⚠️ Common symptoms

One or more firm lumps or nodules along the two rows of mammary glands on the bellyNipples that are swollen, discoloured, or leaking fluidSkin over a lump that becomes ulcerated, reddened or infectedLumps that grow noticeably over weeksReduced appetite, weight loss or lethargy if the disease has spreadDifficulty breathing in advanced cases where the cancer has reached the lungs

Mammary tumours are one of the more sobering lumps a cat owner can find, because unlike in dogs — where roughly half of such tumours are benign — in cats the overwhelming majority are malignant. They arise in the mammary tissue that runs in two chains down the belly, and they most often appear in older, unspayed female cats. The first sign is usually a small, firm nodule felt beneath the skin during stroking or grooming; over time lumps may enlarge, multiply, and eventually ulcerate through the skin.

The biology behind them is closely tied to reproductive hormones, which is what makes this cancer so preventable. Every heat cycle exposes the mammary tissue to oestrogen and progesterone, and it is this cumulative hormonal exposure that drives tumour development. That single fact underpins the most powerful piece of feline preventive medicine there is: spaying a cat while she is still young. A female spayed before her first heat has a fraction of the mammary-cancer risk of a cat left entire, and the benefit shrinks steadily the longer the operation is delayed.

Because feline mammary cancer tends to spread — to the local lymph nodes and then to the lungs — the guiding principle of treatment is to act early and act decisively. Vets generally remove the whole affected mammary chain rather than picking out a single lump, and they stage the disease with a biopsy, lymph-node assessment and chest X-rays. The size of the tumour when it is removed is one of the strongest predictors of how a cat will do afterwards, so a lump caught at a centimetre carries a far better outlook than one left to grow.

The practical takeaway for owners is twofold. First, prevention is real and powerful: spaying early is the best protection you can give a female cat. Second, vigilance saves lives: make a habit of gently feeling along the two rows of mammary glands, and treat any new lump as a reason to see the vet without delay rather than something to monitor at home.

🔍 Causes

Feline mammary tumours are strongly hormone-driven. Exposure to the reproductive hormones oestrogen and progesterone over a cat's lifetime promotes their development, which is why unspayed females and cats given progestin-type hormone drugs are at much higher risk. Age is a major factor — most affected cats are older, typically around ten to twelve years — and certain breeds, notably Siamese, appear more prone. Male cats can develop mammary tumours too, but it is rare.

🛡 Prevention

This is one of the great success stories of preventive surgery: spaying a female cat early sharply reduces her lifetime risk of mammary cancer. Cats spayed before their first heat (around six months of age) have a dramatically lower risk than those spayed later or left entire, and the protective benefit fades the longer spaying is delayed. Avoiding progestin-based hormone medications also helps. For every cat, and especially older or unspayed females, running a hand gently along the two rows of mammary glands during regular cuddles helps catch new lumps while they are small and most treatable.

💊 Treatment

Any mammary lump in a cat should be treated as potentially cancerous until proven otherwise. Treatment usually centres on surgery, and because feline mammary tumours are so often malignant, vets typically remove the entire affected mammary chain (a radical mastectomy) rather than just the lump, which lowers the chance of the cancer returning locally. The removed tissue is sent for biopsy to confirm the type and grade, and chest X-rays and lymph-node checks are used to see whether it has spread. Chemotherapy may be recommended after surgery for larger or higher-grade tumours. Outcome depends heavily on tumour size at the time of surgery — which is exactly why prompt action on a small lump matters so much.

When to see a vet: Book a veterinary appointment as soon as you feel any lump, swelling or thickening along your cat's mammary chain, however small — do not adopt a wait-and-see approach, because in cats the odds that a mammary mass is malignant are high and tumour size at surgery is one of the biggest factors in survival. Also see your vet promptly for any nipple discharge, ulcerated skin over a lump, or unexplained weight loss and lethargy in an older cat. Early removal of a small tumour offers by far the best outlook.

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⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

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