Mast Cell Tumors in Dogs

Mast cell tumors (MCT) are a common type of skin cancer in dogs, accounting for about 20% of all canine skin tumors.  They grow on or underneath the skin and can range from benign looking lumps to large, ulcerated, inflamed masses. 

Mast cell tumors are fairly easily diagnosed with a fine needle biopsy.  This involves taking an aspirate of the mass with a needle and syringe.  The cells that are drawn out are examined under a microscope- if lots of mast cells are seen, suspicion of an MCT is confirmed.

The bad news regarding MCT’s is that they can be quite aggressive.  They range from Grade 1 to Grade 3, with Grade 3 tumors being the most aggressive.  Grade 1 tumors have a very low chance of spreading and Grade 2 tumors have a low to moderate likelihood of spreading.  MCT’s will initially spread to the lymph nodes in the area, then to abdominal organs, such as the liver and spleen, then to other parts of the body.

Mast Cell Tumor treatment involves surgery in the first instance.  Because of their aggressive nature, MCT’s need to be removed with fairly large margins of seemingly normal tissue, in an attempt to ensure that all malignant cells are removed.  The piece of tissue that is removed should be sent to a lab for analysis.  The pathologist will determine the grade of the MCT and whether all the malignant tissue has been removed.  Grade 3 MCT’s, as well as the more malignant Grade 2 MCT’s, will likely require follow-up radiation therapy or chemotherapy to ensure the remaining MCT’s are destroyed.  Grade 1 and less aggressive Grade 2 MCT’s have a good prognosis following complete excision.

Like all cancers, MCT’s have a better prognosis the sooner they are treated, so it is important to have any skin masses investigated soon after they are discovered.

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