Mammary Tumors in Cats: What Every Cat Owner Should Know

Mammary tumors in cats are usually cancer. Between 80% and 90% are malignant, which is very different from dogs, where about half are benign (Lopes, 2024). They show up as one or more firm lumps along the belly or chest, under or around the nipples, and they're most common in older female cats (Lopes, 2024).

Size at diagnosis matters more than almost anything else. Median survival is about 3 years when a tumor is smaller than 2 cm, about 2 years at 2 to 3 cm, and only about 6 months once it's larger than 3 cm (Lopes, 2024). That's why any lump in a cat's mammary area should be checked right away, not watched.

The main treatment is surgery to remove the mammary chain. In a study of 107 cats, removing both chains was associated with longer survival without progression (542 days) than removing one chain (289 days) (Gemignani et al., 2018). Prevention is powerful too: spaying before 6 months of age reduces the risk of mammary carcinoma by 91% (Overley et al., 2005).

Key points

• 80% to 90% of mammary tumors in cats are malignant (Lopes, 2024).
• Tumors larger than 3 cm carry a much worse prognosis than smaller ones (Lopes, 2024).
• Spread to the lymph nodes is a significant negative factor, and almost half of cats with distant spread show no signs of illness (Lopes, 2024).
• Bilateral mastectomy was associated with longer progression-free survival than unilateral mastectomy, but with more complications (Gemignani et al., 2018).
• The evidence for chemotherapy after surgery is weak and mixed (Gonzalez-Ormerod, 2024).
• Spaying before 6 months cuts risk by 91%, before 1 year by 86%, and after age 2 gives no protection (Lopes, 2024; Overley et al., 2005).
• Young cats can develop a benign mammary swelling tied to progesterone, which also needs a biopsy to rule out cancer (Scully, 2026).

Detailed explanation

What mammary tumors are in cats

Cats have mammary glands in two chains, one on each side of the belly, running from the chest to the groin. A mammary tumor is a growth in one of these glands. In cats, most are carcinomas (adenocarcinomas), a cancer of glandular tissue. Older cats and purebred cats are overrepresented, and purebred cats generally have a worse prognosis than domestic shorthairs (Lopes, 2024).

This is one of the biggest differences between cats and dogs. In dogs, about half of mammary tumors are benign. In cats, 80% to 90% are malignant (Lopes, 2024). If you've read about mammary tumors in dogs, know that the outlook for cats is more serious.

Signs to watch for

The first sign is usually a lump you can feel under the skin of the belly or chest, near a nipple. There may be one lump or several, in one or both chains. Larger tumors can ulcerate, and ulceration is one of the factors associated with the disease progressing after surgery (Gemignani et al., 2018).

Because almost half of cats with distant spread show no signs of illness (Lopes, 2024), a cat that seems perfectly well can still have advanced disease. The practical takeaway is to run your hands along both sides of your cat's belly regularly, from armpit to groin. Any lump the size of a pea should be sampled by a veterinarian (Ettinger, 2015), and with mammary lumps in cats, waiting a month isn't advisable.

A benign look-alike in young cats

Not every mammary swelling in a cat is cancer. Mammary hypertrophy, also called fibroadenoma complex or fibroadenomatosis, is a benign, progesterone-associated overgrowth of mammary tissue. It's seen most in young, intact females after a heat cycle, in pregnant cats, and in cats of either sex, including neutered males, receiving long-term progestin medication. The glands can enlarge quickly and look red and swollen, and the condition can be mistaken for mastitis (Scully, 2026).

It may resolve on its own, and treatment focuses on removing the source of progesterone, through spaying or stopping the medication, or with antiprogestin drugs. Even so, a biopsy is recommended, because 80% of mammary tumors in cats are malignant (Scully, 2026).

How the diagnosis is made

Your veterinarian will examine both mammary chains and the nearby lymph nodes, and will usually recommend a needle sample or biopsy. Before surgery, staging tests look for spread. Lymph node status matters a great deal: lymph node spread is a significant adverse prognostic factor, and cats with stage IV disease, meaning distant spread, have a median survival of 44 days (Lopes, 2024). In a 2021 study of 137 cats, those with stage III disease had 2.4 times the risk of recurrence and 3.1 times the risk of death compared with stage I (Petrucci et al., 2021, as cited in Gonzalez-Ormerod, 2024).

Surgery

Surgery is the main treatment. Surgery for mammary carcinoma in cats usually means removing an entire mammary chain (radical mastectomy), not just the lump. The question is whether to remove one chain or both.

In a multi-center study of 107 cats with mammary adenocarcinoma, median progression-free survival was 542 days after bilateral mastectomy and 289 days after unilateral mastectomy. Among cats without spread, unilateral mastectomy was a significant risk factor for dying of the disease (Gemignani et al., 2018). The trade-off is complications: they occurred in 19.7% of cats after unilateral surgery, 35.7% after staged bilateral surgery (both chains removed in two operations), and 40.6% after both chains were removed in a single surgery. The authors supported bilateral mastectomy and suggested a staged approach may reduce complications (Gemignani et al., 2018).

Chemotherapy

Whether chemotherapy after surgery helps cats live longer isn't settled. A 2024 evidence review of five studies concluded there isn't enough evidence to say that adding chemotherapy improves survival, and that better-designed trials are needed (Gonzalez-Ormerod, 2024). Results across studies were mixed. One study found chemotherapy associated with a lower risk of dying from the disease (Gemignani et al., 2018), while others found no significant difference (Gonzalez-Ormerod, 2024).

That uncertainty is a reason to have a detailed conversation with a veterinary oncologist, who can weigh your cat's tumor size, stage, and lymph node status.

Prognosis

Because of how aggressive feline mammary carcinoma tends to be and how often it spreads, the prognosis is guarded (Lopes, 2024). The factors that matter most include:

• Tumor size: median survival of about 3 years for tumors under 2 cm, about 2 years for 2 to 3 cm, and about 6 months for tumors over 3 cm (Lopes, 2024)
• Lymph node involvement: a significant adverse factor (Lopes, 2024)
• Type of surgery: bilateral mastectomy associated with longer progression-free survival (Gemignani et al., 2018)
• Ulceration and location: both tied to a higher risk of progression (Gemignani et al., 2018)

Prevention: spaying early

This is one of the few cancers where prevention is well documented. In a study comparing cats with and without mammary carcinoma, spaying before 6 months reduced the risk by 91%, and spaying before 1 year reduced it by 86% (Overley et al., 2005). Spaying after age 2 offers no protective benefit (Lopes, 2024). If you have a young female kitten, ask your veterinarian about timing.

For an overview of other common cancers in cats, see our cat cancer guide. If you're exploring gentler approaches, our article on shrinking a tumor in a cat naturally explains what the evidence does and doesn't support.

When to contact a veterinarian

Book an appointment within days if:

• You feel any lump along your cat's belly or chest, even a small one
• An existing lump is growing or a new one appears
• A young cat's mammary glands suddenly enlarge after a heat cycle or while on a hormone medication

Call the same day if:

• A mammary lump has opened, is bleeding, or looks infected
• Your cat has trouble breathing, stops eating, or seems weak

Questions to ask your veterinarian

• Is this lump likely a tumor, or could it be benign mammary hypertrophy?
• Can we biopsy it, and which staging tests do you recommend first?
• Are the nearby lymph nodes involved?
• Should we remove one mammary chain or both, and in one surgery or two?
• What complications should I watch for after surgery?
• Would chemotherapy help my cat, given the current evidence?
• If my kitten isn't spayed yet, when is the best time?

Sources

Ettinger, S. (2015, June 8). See something, do something: Why wait? Aspirate. dvm360. 

Gemignani, F., Mayhew, P. D., Giuffrida, M. A., Palaigos, J., Runge, J. J., Holt, D. E., Robertson, N. A., Seguin, B., Walker, M., Singh, A., Liptak, J. M., Romanelli, G., Martano, M., Boston, S. E., Lux, C., Busetto, R., Culp, W. T. N., Skorupski, K. A., & Burton, J. H. (2018). Association of surgical approach with complication rate, progression-free survival time, and disease-specific survival time in cats with mammary adenocarcinoma: 107 cases (1991-2014). Journal of the American Veterinary Medical Association, 252(11), 1393-1402. 

Gonzalez-Ormerod, G. (2024). Knowledge summary: In cats with mammary carcinomas undergoing surgical removal, does adjuvant chemotherapy increase survival time? Veterinary Evidence, 9(4). 

Lopes, M. (2024). Mammary tumors in dogs and cats. Merck Veterinary Manual, Professional Version. Retrieved November 6, 2026, from https://www.merckvetmanual.com/reproductive-system/mammary-tumors/mammary-tumors-in-dogs-and-cats

Overley, B., Shofer, F. S., Goldschmidt, M. H., Sherer, D., & Sorenmo, K. U. (2005). Association between ovarihysterectomy and feline mammary carcinoma. Journal of Veterinary Internal Medicine, 19(4), 560-563. 

Scully, C. M. (2026). Mammary hypertrophy in cats. Merck Veterinary Manual, Professional Version. Retrieved November 6, 2026, from https://www.msdvetmanual.com/veterinary/reproductive-system/reproductive-diseases-of-the-female-small-animal/mammary-hypertrophy-in-cats