Mast Cell Tumours

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Skin tumours in dogs are very common. Most will get the odd benign wart or cyst appearing as they get older, which may or may not need treatment. Unfortunately, more malignant tumours can also occur in the skin, the commonest are mast cell tumours which account for 20% of all cutaneous neoplasms. Known as "the Great pretender", the difficulty with these tumours is that they can have almost any appearance from small, non-painful masses to a rapidly growing, inflamed ulcerated areas. They also can behave in different ways. Some are completely benign, some extremely malignant, while most are termed "intermediate"-a quarter to a third of this type will spread in a "cancerous" fashion: first to the local lymph nodes, and then to the liver, spleen and bone marrow.

Mast cell tumours generally appear in middle age dogs and a more common in particular breeds, such as Boxers, Staffies, labradors, golden retrievers, Weimeranas, and Schnauzers.

Mast cells contain histamine and heparin. In some tumours these inflammatory molecules can be released spontaneously or following trauma, causing the mass to increase and decrease in size. Histamine can also cause gut ulcers leading to vomiting or dark faeces.

The easiest way to diagnose a mast cell tumour is by the technique of a fine needle aspiration biopsy (FNAB for short). This pain-free procedure involves inserting a needle into the mass, sucking a few cells into the needle with a syringe, and then squirting the cells onto a microscope slide for a histopathologist to look at. This technique will give a result in 90 to 95% of cases. Unfortunately it is unable to tell us if the tumour is benign, intermediate or malignant. Other factors may give us a clue: aggressive tumours tended to be more likely to cause stomach ulceration, they are often found around the mouth or the anus and far more common in Shar peis and Labradors, whilst Boxers tended to develop benign tumours.

If we find a mast cell tumour on a dog, a process called "staging" (to ascertain if a tumour has spread) should be carried out before a treatment plan is decided. Initially, the local lymph nodes should be examined for any enlargement. If swollen, they should be biopsied for the presence of increased numbers of mast cells. If this test is positive the liver and spleen should be examined with ultrasound and also biopsied if necessary. The bone marrow may also need to be aspirated to check for abnormal cells.

The good news is that in most cases the local lymph nodes are normal, and if there is no evidence of a gut problem we can move on to trying to cure the problem.

For the vast majority of tumours surgery will be the first line of treatment and in all benign, 65 to 75% of intermediate and 10% of malignant cases will provide a cure. If surgery is going to be successful, a margin of apparently normal tissue 2- 3 cm around the tumour should also be removed. The mass and margin is then sent to the lab for histopathology to discover the grade of the tumour. If the local lymph nodes are enlarged these should be resected as well.

When the tumour is in an area like a limb or on the face where it may be very difficult to take a decent margin and still close the wound, radiation therapy can be useful: as much of the tumour as possible should be removed and then the wound left to heal. After 10 to 14 days, 3-4 doses of radiotherapy can be administered to the area to prevent the spread of any remaining cells. Radiation therapy can also be useful if the lymph nodes are affected.

If the tumour is too large to remove, or there is spread of the tumour to the liver, spleen or bone marrow, chemotherapy tends to be the only option although unfortunately no protocols are particularly effective. A combination of a steroid called prednisolone and an injection called Vinblastine is showing the most promise in recent trials.