Mast Cell Tumours in Cats

Mast cell tumours (mastocytoma) are skin tumours that have arisen from mast cells, which are a type of white blood cell formed in the bone marrow. These cells are found throughout the body but concentrate at points of contact between the cat and the outside world, especially the skin, respiratory tract, gastrointestinal tract, and conjunctiva.

Mast cell tumours can develop in different areas, including:

  • Skin (dermal or cutaneous)
  • Internal, such as the spleen, liver or gastrointestinal tract (visceral or disseminated)

Cutaneous tumours can in some cases, spread to regional lymph nodes and on to the internal organs (most commonly the spleen or liver), or tumours can develop in the organs with no skin involvement. Highly aggressive tumours can enter bone marrow (which makes all of the blood cells) and turn up in the bloodstream.

Mast cell tumours are the second most common skin tumour in cats, the most common splenic tumour and the third most common type of intestinal tumour. The majority of mast cell tumours are benign (non-spreading), but up to 10% are malignant (which can spread to other parts of the body).

Mast cells

Mast cells are a part of the immune system responsible for the allergic reactions many people (and animals) experience. They do this by releasing granules, known as degranulation. Granules contain several active substances which perform the following functions:

  • Histamine: Vasodilation (widening of the blood vessels), increase vascular permeability and stimulates the nerve ending.
  • Proteoglycans (Heparin): Prevents the formation of blood clots.
  • Interleukin 3: Promotes hematopoiesis, which is the production of blood cell formation.

If the cat (or person) has experienced an injury or a pathogen, degranulation is a good thing, but sometimes mast cells respond to ordinarily harmless substances, such as pollen, inducing allergy-related symptoms such as hives and itchy skin.


The cause is unknown, but it thought there is a genetic component because of a high incidence in Siamese cats.


Cutaneous (skin)

  • Single or multiple hairless nodules (lumps) on or beneath the skin. They can occur anywhere on the body, including the head, neck, torso, limbs and anogenital region. Tumours range in size from 1 to 4 cm and can rapidly change size (increasing or decreasing in size) due to inflammation around the tumour. They can be well or ill-defined. Ill-defined tumours are more aggressive.
  • From time to time nodules may become itchy and inflamed, due to the release of histamine from the tumour.
  • Up to 25% of tumours may have some degree of ulceration due to self-trauma.


Systemic symptoms may present with cats who have visceral mast cell tumours, which may include:


Your veterinarian will perform a physical examination and obtain a medical history from you.

Diagnostic workup:

  • Baseline tests: Biochemical profile, complete blood count, and urinalysis to evaluate the overall health of your cat.
  • Fine needle aspirate: Insertion of a needle into the tumour and drawing out cells) and cytology ( a study of the cells under a microscope), which applies to both cutaneous and visceral mast cell tumours, however, sedation and ultrasound guidance is necessary to obtain a fine needle aspirate from an internal organ. Your veterinarian may also take samples from local lymph nodes to determine if the tumour has spread.
  • Antihistamines: Administered before fine needle aspirate or biopsy as manipulation of masses can stimulate degranulation, which causes mast cells to release large amounts of histamines resulting in inflammation (Darier’s sign).
  • Imaging: X-rays and ultrasound of the chest and abdomen to look for visceral tumours and liver or spleen enlargement.
  • Buffy coat examination: A sample of blood is spun at high speed to separate it into its components. Red blood cells at the bottom, a small band of white blood cells and finally, the plasma. White blood cells are examined for the presence of abnormal mast cells.

Staging is necessary to determine the extent of the disease; there are three grades.

  1. Well-differentiated
  2. Moderately differentiated
  3. Poorly differentiated.


  • Surgical removal: Surgery is the treatment of choice. The surgery site must be large to get clean margins.
  • Lymphadenectomy: Surgical removal of local lymph nodes for grade 2 and 3 tumours.
  • Chemotherapy: Anti-cancer drugs which target rapidly dividing cells as a follow-up treatment for grade 3 tumours, non-operable tumours, or to target cancerous mast cells remaining in the blood.
  • Radiation therapy: This treatment uses high energy x-ray beams to treat tumours that can not be removed with a wide enough margin, such as the limbs.
  • Antihistamines: Famotidine (Pepcid) or diphenhydramine hydrochloride (Benadryl) can relieve symptoms associated with increased levels of histamine from non-treatable tumours.

Home care

It may be necessary to send the cat home in an Elizabethan collar to prevent self-trauma to the surgery site.

Antibiotics, painkillers, and antihistamines to relieve symptoms.

Keep a close eye on the surgery site and if you notice any swelling, redness or oozing, speak to your veterinarian.


  • 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