A mammary lump in a cat is malignant until histology says otherwise: 85-90% of feline mammary tumours are cancerous, and more than 1 in 4 cats already has tumour cells in the draining lymph node on the day the lump is found. The one lever is hormonal, and it has a deadline. A case-control study from the University of Pennsylvania found a 91% reduction in mammary carcinoma risk in cats spayed before 6 months (odds ratio 0.09), 86% before 12 months (OR 0.14), and 11% before 24 months. Spaying after the second birthday showed no measurable protection.
Not the dog version of the disease
Roughly half of mammary masses in dogs are benign. In cats the malignant-to-benign ratio is at least 4:1, and they move fast. Mammary neoplasia is the third most common tumour in female cats, about 17% of all feline tumours, at 25.4 cases per 100,000 female cats per year. The average patient is 10 to 12 years old, but Siamese and Oriental cats are over-represented — up to 34% of cases — and are diagnosed around 9 years old.
So a mass on an intact queen is a surgical question, not a watch-and-wait question, and the work-up has a fixed order. The node matters as much as the lump: more than 25% of cats have regional metastasis at diagnosis, and 76% have lung metastases at necropsy.
Three age tiers, and what each buys
| Age at spay | Risk reduction | Odds ratio | For a breeder |
|---|---|---|---|
| Under 6 months | 91% | 0.09 (95% CI 0.03-0.24) | Full benefit, no litter |
| 6 to 12 months | 86% | 0.14 (95% CI 0.06-0.34) | One litter before 12 months keeps most of it |
| 12 to 24 months | 11% | about 0.89 | Most of the protection is gone |
| After 24 months | none reported | — | Reproductive control, not cancer prevention |
Most queens reach their first heat between 4 and 10 months, so "before 6 months" mostly means "before the first cycle". Going from 6 to 12 months costs about 5 percentage points; going from 12 to 24 months costs another 75.
What the 2025 review changed
Four studies met the review's question: Dorn 1968, Hayes 1981, Overley 2005 and Graf 2016. All found protection, and their effect sizes disagree. Dorn reported a relative risk of 0.15, Hayes 0.6 (99% CI 0.33-0.94), and Graf, using 23,216 female cats from the Swiss Feline Cancer Registry, an odds ratio of 0.62 for all mammary tumours and 0.69 for malignant ones. Only Overley measured age at spaying, and only Overley produced 91%. The 2025 RCVS Knowledge review graded this body of evidence weak, because all four studies are retrospective; Overley's reproductive history came from questionnaires answered years later, and its controls were cats with tumours in other organs.
The defensible claim: spaying before the first heat cuts risk substantially — somewhere between 40% and 90% depending on the study — not by exactly 91%.
Parity is not the risk factor owners assume
A litter neither raises nor lowers the risk: in Overley's data, parity had no effect once age at spaying was accounted for. Progesterone exposure does, including long-acting progestin injections used to postpone heat. An ovary-sparing spay keeps that exposure intact, which is why that trade is wrong if cancer is the reason. Cats cycle every 2 to 3 weeks through spring and summer, so an intact queen can pass through 8 to 12 heats in a season.
Sixty seconds a month on an intact queen
Cats have 8 mammary glands in 4 pairs, and the front pairs carry most tumours. Once a month, with the queen standing: run both hands flat from axilla to groin, then pinch each gland between thumb and two fingers. Normal tissue is soft, even and slightly lobulated, and all 4 pairs feel alike. You are hunting asymmetry — one gland firmer, one 5 mm nodule, one gland warm or stuck to the skin.
Measure with callipers in millimetres, in 3 dimensions, and log the date with a photo that has a ruler in frame. Cross 1 cm, double within 4 weeks, or feel fixed to the abdominal wall, and it moves from observation to work-up. A gland that is uniformly enlarged 2 to 3 weeks after a heat is more likely hyperplasia.
The 1 cm rule: aspirate, do not watch
Cytology from a fine-needle aspirate — 25 to 27 gauge, 3 to 5 passes, usually no sedation — returns in 24 to 48 hours. In cats, unlike dogs, it is reasonably reliable at confirming carcinoma, because most of these tumours are malignant, and much weaker at clearing a mass as benign. A benign report on a growing 1 cm mass does not cancel surgery. If cytology suggests carcinoma, staging comes first: 3-view chest radiographs, abdominal ultrasound, and aspiration of the draining node. Then excision with margins, then histology with type, grade I to III, mitotic count and the closest margin in millimetres.
| Step | US USD | UK GBP | China CNY |
|---|---|---|---|
| Fine-needle aspirate with cytology | 50-150 | 40-120 | 100-300 |
| Staging: bloods, 3-view chest, abdominal ultrasound, node aspirates | 350-1,100 | 280-950 | 1,400-4,800 |
| Unilateral chain mastectomy | 900-1,800 | 750-1,600 | 4,000-9,000 |
| Histopathology with margins | 120-320 | 95-270 | 400-1,100 |
Bilateral surgery and second operations: cat mastectomy costs.
What size buys in survival time
Size is the easiest thing an owner can measure. Tumours over 3 cm have a reported median survival of about 6 months; tumours under 2 cm, over 3 years. Stage tracks the same curve: in one early series, median survival was 29 months for stage I, 12.5 for stage II, 9 for stage III and 1 month for stage IV. Grade III carcinomas do markedly worse. Chemotherapy has not been shown in controlled studies to extend survival meaningfully, so surgery and margins carry most of the weight.
If you want one litter first
Spay at 12 months instead of 6 and you keep roughly 86% of the effect; the cost is one heat cycle and a 6-month delay. Spay after a litter at 2.5 years and the protection is gone. The compromise that keeps most of the benefit: mate on the second or third heat, wean at 8 weeks, spay 3 to 4 weeks after weaning, when the mammary tissue has largely involuted. Two or three litters a year until age 6 is the arrangement that gives these numbers their worst-case reading, and the highest rates of mastitis, eclampsia and pyometra. Breeding queens are usually retired between 6 and 8 years, and a queen spayed before any litter faces a routine operation gated on weight and bloodwork.
The decision, in five lines
- Not breeding her: spay before the first heat, which for most queens means 4 to 6 months.
- Breeding one litter: mate on the second or third heat, wean at 8 weeks, spay 3 to 4 weeks later, accept 86%-band protection.
- Keeping her entire past 2 years: stop counting the spay as cancer prevention.
- Finding a lump: callipers, millimetres, a log, and an aspirate at 1 cm.
- Choosing an ovary-sparing spay: no mammary protection, because ovarian tissue keeps producing progesterone.
This article does not replace a veterinary diagnosis. A mammary mass in a cat needs a physical examination, and usually cytology or histology, before any decision about surgery. If a lump is growing, ulcerated, or the cat is losing weight or breathing faster, book an appointment this week.
Terms mentioned in this article
Frequently asked
If I spay my queen after one litter, how much cancer protection do I lose?
Little, if the litter happens before 12 months. In the Pennsylvania case-control data, spaying before 6 months cut mammary carcinoma risk by 91% and before 12 months by 86% — a 5-point difference. The figure falls to 11% for a spay between 12 and 24 months and to nothing after 2 years. One litter costs a little; two years of litters costs nearly the whole benefit.
Can a spayed cat still get mammary cancer?
Yes. Spaying lowers risk, it does not remove it: cats spayed before 6 months still develop mammary tumours, and the risk keeps rising with age. A tumour that began before surgery is not affected either. Any new lump on a spayed cat gets the same work-up — measure it, aspirate at 1 cm, and stage before excision if cytology suggests carcinoma.
Is a 1 cm lump too small to test?
No. One centimetre is the size at which testing starts. A fine-needle aspirate takes 3 to 5 passes, usually needs no sedation, costs about USD 50-150 and returns in 24 to 48 hours. Because most feline mammary tumours are malignant, cytology is good at confirming carcinoma and poor at clearing a mass, so a benign report on a growing 1 cm mass does not cancel surgery.