A new lump near a cat’s nipple should be checked promptly by a veterinarian. Many feline mammary tumors are malignant, but a lump’s appearance or feel cannot confirm cancer: benign masses and other conditions can look similar, and tissue testing is needed for a diagnosis.
What breast cancer in cats is
“Breast cancer” in cats is more precisely called feline mammary cancer or a mammary tumor. A mass may be benign or malignant, and some non-cancerous conditions can also cause mammary swelling. Only veterinary evaluation, including examination of tissue, can determine what a particular lump is.
The likelihood of malignancy is high enough to take any new mammary lump seriously. Cornell University College of Veterinary Medicine estimates that approximately 85% of feline mammary cancers are malignant adenocarcinomas (publication year not stated in the available source). NC State Veterinary Hospital states that 85% of feline mammary tumors are malignant (publication year not stated in the available source). These are source-specific estimates and should not be read as the same measure from the same population.
Symptoms and when to contact a veterinarian
Common signs
A mammary tumor may first appear as a small, firm nodule under or beside a nipple along the cat’s belly or chest. You might notice it while petting or grooming your cat. Early tumors may not hurt or cause any other obvious signs of illness. Some masses ulcerate or bleed, and a cat may lick the area.
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Signs that disease may be advanced
If cancer has spread, a cat may develop changes in appetite or weight, or difficulty breathing. Do not wait for these signs: contact a veterinarian about any new lump along the chest or belly, even if your cat otherwise seems well. A growing, bleeding, or ulcerated lump also warrants prompt attention.
Not every swelling in this area is cancer. UC Davis School of Veterinary Medicine identifies mastitis and fibroepithelial hyperplasia among other possible causes of mammary enlargement. That is why appearance alone cannot provide a reliable diagnosis.
Causes and risk factors
What is known about cause
The precise cause of feline mammary gland cancer is unknown. Cornell’s Margaret McEntee, DVM, professor of oncology at Cornell University College of Veterinary Medicine, states: “The underlying causes of feline mammary gland cancer are unknown.” Cornell also reports that a genetic influence has not been found to play a role.
Hormone exposure and spaying
Exposure to reproductive hormones, including estrogen and progesterone, is associated with risk. Cornell notes that giving these hormones to unspayed cats as contraceptives or for behavior modification may increase risk.
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The American Animal Hospital Association’s 2026 oncology guidelines report a 91% reduced risk — American Animal Hospital Association, 2026 for cats spayed before six months of age. This is a risk reduction associated with timing of spay, not a guarantee that an individual cat will never develop a mammary tumor.
How veterinarians diagnose and stage mammary tumors
The veterinarian will examine the cat and the mass, then recommend tests based on the findings and the cat’s health. A fine-needle sample may help distinguish a mammary mass from some other kinds of tumors, but biopsy followed by histopathology is the definitive method described by Cornell and NC State for diagnosing the tissue and determining whether it is benign or malignant.
Staging checks whether disease may have spread and helps guide treatment and prognosis. Merck Veterinary Manual’s review, last reviewed in September 2024, emphasizes completing staging before treatment because the stage substantially affects both.
Tests that may be part of the work-up
- Biopsy and histopathology: evaluate tissue to establish tumor type and whether it is benign or malignant.
- Lymph-node assessment: sampling regional lymph nodes can help determine whether cancer has spread locally.
- Chest radiographs: evaluate the lungs for possible spread.
- Abdominal ultrasound: assess internal organs and abdominal lymph nodes.
- Blood tests and urinalysis: help assess overall health and readiness for treatment.
- CT or MRI: may be used in selected cases or to help plan surgery.
Treatment options
Treatment depends on pathology and staging, whether the disease appears localized or has spread, the number and location of masses, lymph-node findings, the cat’s overall health, and the expected burden of treatment. The veterinary team can explain likely benefits and limitations in the context of the individual cat and the owner’s goals.
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Surgery for localized disease
Surgery is a principal option when a tumor is localized and the cat is an appropriate candidate. The operation may remove the mass, one mammary chain, or both chains. NC State discusses removal of the affected mammary chain and staged bilateral radical mastectomy; UC Davis describes a range of surgical extents and notes that evidence has not proven that more extensive procedures prolong survival compared with simpler ones. The appropriate extent is therefore a case-specific discussion, not a single operation that is right for every cat.
Systemic treatment
Systemic treatment, including chemotherapy, may be considered according to the pathology and extent of disease. Cornell describes chemotherapy when cancer has spread throughout the body. UC Davis characterizes chemotherapy’s efficacy in cats as limited in its discussion, and the 2018 consensus review describes prognosis as guarded to poor. These sources do not establish a guaranteed benefit for an individual cat; an oncology team weighs the possible benefit against disease extent, health, treatment burden, and goals of care.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Prognosis and survival figures
Tumor size and spread are important prognostic factors, alongside tumor type, growth rate, and lymph-node involvement. UC Davis reports the following group-level median survival figures by tumor size:
| Tumor size in the UC Davis report | Reported median survival | Attribution and qualification |
|---|---|---|
| Greater than 3 cm | Six months | UC Davis School of Veterinary Medicine; publication year not stated in the available source. Group estimate, not an individual prediction. |
| Less than 2 cm | More than four years | UC Davis School of Veterinary Medicine; publication year not stated in the available source. Group estimate, not an individual prediction. |
These figures describe groups, not a timeline for a particular cat; individual circumstances and treatment can differ. NC State also notes that cats with a single small tumor under 2 cm may survive several years after aggressive surgery. A veterinarian can interpret prognosis using the cat’s pathology, tumor size, lymph-node results, imaging, and health.
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