How Serious Are Mammary Tumours in Cats?
Mammary tumours are the third most common tumour type in cats, after lymphoma and skin tumours, and they represent one of the most significant oncological challenges in feline medicine. Unlike dogs, where approximately half of mammary tumours are benign, the picture in cats is dramatically more concerning: approximately 85 to 90 per cent of feline mammary tumours are malignant. This means that when a cat owner discovers a lump in their cat's mammary chain, the assumption must be that it is cancer until proven otherwise.
The majority of malignant feline mammary tumours are adenocarcinomas — cancers arising from the glandular epithelium of the mammary tissue. They have a strong tendency to spread to regional lymph nodes and to the lungs, and in many cats the disease has already metastasised at the time of diagnosis. This underscores the critical importance of early detection and rapid action.
Who Is at Risk?
Feline mammary tumours occur almost exclusively in intact (unspayed) queens or in cats that were spayed after sexual maturity. Entire queens are seven times more likely to develop mammary tumours than spayed cats. The disease most commonly affects middle-aged to older cats, with a peak incidence between ten and twelve years of age, though tumours can occur in cats as young as five or six.
Among breeds, Siamese cats are particularly predisposed to developing mammary tumours. Studies have found that Siamese cats develop the disease at a younger age than other breeds and may experience a more aggressive disease course. This breed predisposition is thought to reflect an underlying genetic susceptibility, though the precise mechanisms are not yet fully understood. Domestic shorthairs and other breeds can equally be affected, and no entire queen should be considered immune.
There is some evidence that prolonged use of progestogen-containing medications — sometimes used historically to suppress oestrus or treat skin conditions — increases the risk of mammary tumour development, likely because of the role of progesterone receptors in mammary tissue growth.
The Protective Effect of Early Spaying
The most powerful tool available for preventing feline mammary tumours is early neutering. The data on this is compelling and consistent. Queens spayed before six months of age — before their first oestrous cycle — have an approximately 91 per cent reduction in the risk of developing mammary tumours compared to intact queens. This is one of the strongest preventive effects of neutering observed in veterinary medicine.
Spaying between six and twelve months of age still provides significant but reduced protection, with a risk reduction in the region of 86 per cent. After two years of age, neutering appears to provide little or no protective benefit against mammary tumour development, as the mammary tissue has already been exposed to cumulative hormonal stimulation during multiple oestrous cycles.
These figures make a strong argument for early neutering of queens not intended for breeding, and this is now the recommended approach in most veterinary guidelines.
Finding a Lump: What to Do
Cats have four pairs of mammary glands arranged in two chains running from the axilla (armpit) to the inguinal (groin) region on the underside of the body. Owners can check these glands periodically by gently palpating the skin on the cat's underside. Any firm, discrete nodule, thickening, or mass warrants a prompt veterinary assessment.
At the vet, the mammary mass will be assessed alongside the regional lymph nodes — particularly the axillary and inguinal nodes — for enlargement. Blood tests and urinalysis provide baseline health information. The most important staging investigation for mammary tumours in cats is thoracic radiography — chest X-rays to evaluate the lungs for pulmonary metastases.
Staging and the Importance of Chest X-Rays
Staging classifies the extent of the tumour spread and guides treatment decisions and prognosis. For feline mammary tumours, the key staging investigations are measurement of the primary tumour (size is strongly prognostic), palpation and possibly ultrasound or fine needle aspiration of regional lymph nodes, and three-view thoracic radiographs to assess for lung metastases.
Tumour size is one of the most powerful prognostic indicators in feline mammary cancer. Tumours smaller than two centimetres at diagnosis carry a significantly better prognosis than larger tumours. Cats with tumours over three centimetres have median survival times considerably shorter than those with smaller tumours, all else being equal. This is why early detection matters so much — monthly checking by owners gives the best chance of finding tumours at a small, more surgically manageable stage.
Chest X-rays are non-negotiable before surgery. Cats with pulmonary metastases at the time of diagnosis have a much shorter expected survival, and knowledge of metastatic status appropriately informs the owner's decision about the extent and aggressiveness of treatment they wish to pursue. It would be inappropriate to put a cat through a radical surgical procedure if widespread lung metastases are already present.
Surgery: Radical Mastectomy Over Lumpectomy
Surgical excision is the primary treatment for non-metastatic feline mammary tumours, and the extent of surgery matters significantly. Multiple studies in cats have demonstrated that radical or modified radical mastectomy — removing the entire affected mammary chain — results in significantly longer disease-free intervals and overall survival times compared to conservative excision or lumpectomy.
This is in contrast to the situation in some other species, where breast-conserving surgery is standard. In cats, the aggressive biological behaviour of these tumours and the high rate of local recurrence after conservative surgery make a more extensive approach preferable in most cases. If tumours are present on both chains, bilateral mastectomy can be performed either in one surgery or staged over two procedures, depending on the cat's overall condition and the surgeon's assessment.
Clean surgical margins are a key prognostic factor — tumours removed with inadequate margins have a substantially higher risk of local recurrence. The excised tissue should be submitted for histopathological examination to confirm the diagnosis, assess tumour grade, evaluate margins, and identify features associated with more aggressive behaviour such as lymphovascular invasion.
Chemotherapy
Adjuvant chemotherapy — given after surgery — may be recommended for cats with high-grade tumours, incomplete excision margins, or lymph node involvement. Doxorubicin-based protocols are the most studied in feline mammary cancer, though the evidence base for chemotherapy improving survival in cats is less robust than in human oncology. Cyclophosphamide is sometimes combined with doxorubicin. The decision to pursue chemotherapy should be made with a veterinary oncologist, weighing the potential benefit against the tolerability of treatment in the individual cat.
Prognosis
The prognosis for feline mammary carcinoma is generally guarded. Median survival times for cats with malignant mammary tumours treated with surgery are commonly reported in the range of six to twelve months, though this figure varies substantially according to tumour size, grade, lymph node involvement, and completeness of surgical excision. Cats with small tumours excised with clean margins can live considerably longer, sometimes several years.
- Spay before six months of age to achieve maximum protective benefit — 91% reduction in risk.
- Check your cat's mammary glands monthly for any new lumps or thickening.
- Any mammary mass should be assessed by a vet promptly — do not adopt a wait-and-see approach.
- Chest X-rays are essential before surgery to assess for lung spread.
- Radical mastectomy of the affected chain gives better outcomes than lumpectomy.
- Siamese cat owners should be particularly vigilant given their breed's higher predisposition.
Feline mammary tumours are serious, but with early detection, appropriate staging, and aggressive surgery when indicated, meaningful survival times with good quality of life are achievable for many cats.