MAST CELL TUMOURS

DESCRIPTION
Mast Cell Tumours (Mastocytomas) are one of the true chameleons of the cancer world. Their appearance can vary tremendously and a tumour can change in size and appearance tremendously over a very short period. It is common for a mast cell tumour to triple in size over night if it had been somehow traumatized that day. This occurs because a mast cell tumour is made up of a large numbers of common white blood cells called mast cells. These cells exist throughout the body and they respond to foreign invaders such as parasites. The mast cell possesses granules of especially inflammatory biochemicals meant for use against invading parasites. Unfortunately mast cells will sometimes overreact to other foreign invaders that enter the body, such as pollens or bee stings. The mast cells when stimulated by these foreign substances, will release histamine and create a “histamine” or allergic reaction, often causing intense swelling and itchiness and sometimes even an anaphylactic reaction with severe consequences. Tumours made of mast cells will also release histamine and if a small skin mast cell tumour is “bumped” accidentally, the released histamine can make it swell to many times its original size in a matter of a few hours. Mast cell tumours are especially common in dogs accounting for approximately one skin tumour in every five. The Boxer is at an especially high risk, as are related breeds: English bulldog, Boston terrier. Also at higher than average risk is the Shar pei, labrador retriever, golden retriever, schnauzer, and cocker spaniel. Most mast cell tumours arise in the skin but technically they can arise anywhere that mast cells are found. The mast cell tumour does not have a characteristic appearance though because of the tumour’s ability to cause swelling through the release of granules, it is not unusual for the owner to notice a sudden change in the size of the growth or, for that matter, that the growth is itchy or bothersome to the patient. Mast cell tumours are notoriously invasive and difficult to treat.

DIAGNOSIS
Diagnosis can often be made with a needle aspirate, which collects some cells of the tumour with a needle, and the cells are examined under the microscope. The granules have distinct staining characteristics and can be recognized easily. An actual tissue biopsy, however, is needed to grade the tumour; grading is crucial to determining prognosis. The veterinarian will also want to perform blood work, chest xrays and a needle aspiration of the local lymph nodes and of the spleen to determine if the tumour has traveled to other organs/tissues. GRADE I TUMORS: This is the best type of mast cell tumour to have as it tends not to spread beyond its place in the skin. Surgery should be curative. About half of all mast cell tumours are grade 1 tumours and can be cured with surgery alone. GRADE II TUMORS: The behaviour of this type of tumour is somewhat unpredictable. Surgical excision alone may be enough to cure the patient but recent studies have shown that radiation therapy administered to the site of the tumour can cure greater than 80% of patients as long as the tumour has not already shown distant spread. GRADE III TUMORS: This is the worst type of mast cell tumour to have; they behave very invasively and aggressively. If only surgical excision is attempted without supplementary chemotherapy, a mean survival time of 18 weeks (4-5 months) can be expected.

TREATMENT
Mast cell tumours arising from the nail bed, genital areas, muzzle, and oral cavity carry a poorer prognosis. Mast cell tumours that originate in deeper tissues such as the liver or spleen carry a particularly grave prognosis. Tumours that have been present for months or years tend to be more benign. Therapy for mast cell tumours consists of surgery, radiation therapy, and chemotherapy. What combination of the above is chosen depends on the extent of spread and malignant characteristics of the tumour. If the tumour can be cured with one or even two surgeries, this is ideal. Mast cell tumours are highly invasive and very deep and extensive margins (at least 3 cm in all directions) are needed. This can be a problem for tumours located on the neck or in the mouth. Currently three anti-cancer drugs have been particularly helpful in combating mast cell disease: prednisone (steroid), lomustine, and vinblastine. To decrease the histamine effects of mast cell tumours that frequently cause inflammation and increased stomach acid secretion, antacids and antihistamines can be used.

Other cancer types.