Mammary cancer in cats — why it is almost always malignant
Feline mammary tumours, unlike canine ones, are almost always malignant — 85% to 95% in most series. Small tumours caught early have excellent prognosis with chain mastectomy; large tumours or metastatic disease have very poor prognosis. The single most consequential variable is tumour size at the time of surgery, which means time-to-vet matters more for cats than for dogs.
When to euthanise a cat with cancer
The companion essay; oral SCC, mammary, lymphoma compared.
The short answer
A lump near a nipple in a cat — particularly an unspayed or late-spayed cat — should be evaluated within days, not weeks. Most are malignant. Small ones are often curable with appropriate surgery; large ones are usually not. Time-to-vet is the variable that makes the most difference.
Why so often malignant
Feline mammary tissue is unusually responsive to hormonal stimulation across many heat cycles. By the time a cat is 8–10 years old, the cumulative oestrogen exposure has produced cellular changes that, when they progress to neoplasia, almost always do so toward an aggressive phenotype.
Compare to dogs, where the malignancy rate of mammary tumours is around 50%. The biology of feline mammary tissue makes the cancer different, and the urgency higher.
The early-spay protective effect
One of the strongest risk-reduction effects in veterinary oncology:
- Spayed before first heat (~6 months): ~90% reduction in lifetime mammary cancer risk.
- Spayed after first heat: ~85% reduction.
- Spayed after second heat: ~10% reduction.
- Spayed after age 2: minimal protective effect.
This is the single best argument for spaying female cats early. It is also why feline mammary cancer is now uncommon in regions with established spay-neuter programmes and remains common in regions without.
How it presents
A firm, often nodular, sometimes ulcerated mass associated with one or more mammary glands. The lumps are often painless and discovered by the owner during grooming or stroking. Some cats present with a single small lump; others with multiple lumps spanning a chain.
By the time some lumps reach 2–3 cm, lymph node involvement may already be present. By 4–5 cm, pulmonary metastases are common.
Surgery — chain mastectomy
Standard treatment is unilateral chain mastectomy (removing all four mammary glands on the affected side) or bilateral chain mastectomy if multiple glands are involved. The local recurrence rate after single-lump excision is unacceptably high; the larger surgery is justified.
A typical surgery takes 60–90 minutes, with 7–10 days of post-operative recovery. Most cats tolerate it well. Pain control is multimodal — local block, NSAID, opioid taper.
Survival numbers
- Tumour <2 cm, complete excision: Median survival >3 years; many cats cured.
- Tumour 2–3 cm, complete excision: Median 1–2 years.
- Tumour >3 cm: Median <1 year.
- With nodal involvement: Median 6–12 months.
- With distant metastases (lung): Median 2–3 months.
When it has metastasised
Once lung metastases are visible on radiographs, the trajectory is usually 2–4 months. Chemotherapy (doxorubicin) provides modest extension; quality of life is the main driver of decision-making.
Symptoms of advanced disease: laboured breathing, coughing, weight loss, lethargy. Welfare-driven euthanasia is often the right call once the dyspnoea sets in.
Common questions
My cat has a small lump near a nipple. How urgent is this?
Does spaying help even after a tumour appears?
Why chain mastectomy and not just excision of the lump?
Is this related to feline kidney disease or hyperthyroidism?
Editorial reference, not veterinary advice. — Dr. NRS, last reviewed 28 April 2026.