Mammary Tumours in Cats

At a glance

  • About: A mammary tumour is a benign or cancerous tumour originating in the mammary gland. 85-95% of mammary
    tumour in cats are cancerous.
  • Symptoms: Single or multiple firm, nodular masses under the skin which may or may not be ulcerated.
  • Diagnosis: Evaluation of cells or a tissue sample under a microscope.
  • Treatment: Treatment depends on the stage of cancer and may consist of surgery and/or chemotherapy.


Mammary gland (breast) tumours are similar to breast tumours in people, and it is the third most common tumour in the cat. 85-95% of mammary gland tumours are malignant, and adenocarcinomas are the most common type of malignant neoplasm of mammary gland cancer.

The average age of cats with mammary gland tumours is 10-12 years old. Intact females are at the highest risk. Spaying, especially before their first heat dramatically reduces their risk of mammary cancer, before the mammary glands are exposed to the sex hormones estrogen and progesterone.

Mammary cancer affects mostly female cats, but can also occur in males, affecting less than 1% of cases. The underlying cause is not known, hormone birth control in unspayed females has been found to triple the risk of mammary cancer. There is a seven-fold increased risk in unspayed cats. Genetics can also be a factor, domestic shorthairs and Siamese cats have an increased risk of developing mammary gland tumours, and at a younger age.

Fifty per cent of cats have tumours in multiple glands. Metastasis (spread of the tumour) is common and will often involve regional lymph nodes, lungs, and the liver.


Cats have four pairs of mammary glands, the four on the left-hand side are a chain and the four on the right-hand side are a chain. The glands are numbered one to four, with one being closest to the head, and gland four closest to the tail.

Typical symptoms of mammary gland tumour include:

  • Firm, palpable mass underneath the skin of the mammary gland(s), one study found 60% of cats had more than one tumour at the time of diagnosis
  • Ulceration of the skin which does not heal
  • Unexplained weight loss
  • Milky or bloody discharge from the nipple
  • Infection, swelling, and pain


Your veterinarian will perform a complete physical examination, including carefully examining the mass and palpitating the regional lymph nodes to determine if the tumour has spread. A thorough history is important including:

  • Onset of symptoms
  • Has the cat been spayed
  • How old was the cat when she was spayed?
  • Is the cat on any medications, including contraceptives?

Diagnostic workup:

  • Baseline tests: Biochemical profile, complete blood count, and urinalysis to evaluate the overall health of your cat and determine its pre-surgery status.
  • Fine needle aspiration (FNA) and cytology: A fine-gauge needle is inserted into the mass to obtain a sample of cells that are stained and examined under a microscope. Cytology is helpful to differentiate mammary cells from other tissues.
  • Histopathologic examination: Sample biopsy or the entire mass to determine if it is benign or malignant.
  • Radiographs, ultrasound and sometimes a CT scan: Radiographs or CT scan of the chest to look for signs of metastasis to the lungs as well as an abdominal ultrasound.


A modified WHO staging system is used to stage feline mammary tumours, which is based on the T (tumour size), N (lymph node status), and M (metastasis) system. See Table 1.

Staging is the evaluation of the extent of cancer, to determine how large it is, and if it has spread to the lymph nodes and beyond.

Size Lymph node status Metastasis
Stage I T1: < 2 cm max diameter N0 M0
Stage II T2: 2 – 3 cm max diameter N0 M0
Stage III T1, T2: < 3 cm N1 M0
Stage IV T3: > 3 cm N0 or N1 M0
Stage V Any Any M1


Lymph nodes

  • N0: No regional lymph node involvement
  • N1: – 1-3 lymph nodes involved


  • M0: Cancer has not spread to other parts of the body.
  • M1: Cancer has spread to other parts of the body.


The veterinarian may refer your cat to a board-certified veterinary surgeon or veterinary oncologist for specialist treatment. As 80-90% of mammary gland tumours are malignant, surgical excision and histopathology (examination of the removed tissue) are the procedure of choice.

  • Radical mastectomy (removal of the entire mammary chain), at least on the affected side, but your surgeon may also recommend removing both chains as well as associated lymph nodes.

Chemotherapy with doxorubicin and cyclophosphamide or carboplatin will be necessary if the cancer has metastasised or is inoperable.


Prognosis depends on the size of the tumour, early diagnosis and aggressive surgery has a better prognosis.

  • Smaller than 2 cm: Median survival rate of 3 years.
  • Between 2-3 cm: Median survival rate of 2 years.
  • Greater than 3 cm: Median survival rate of 6 months.


The best way to reduce a cat’s chances of developing mammary glands is to spay or neuter it. One study found that spaying cats before six months reduce the incidence of mammary cancer by 91 percent, and 85 percent for cats spayed before one year of age.

Perform monthly checks on your cat (male and female); this should include evaluation of the mammary glands to feel and feel for changes which include lumps and/or areas of ulceration. See your veterinarian immediately if you notice any changes.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

    View all posts