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Mast Cell Tumors

by Karyn Oz (Karyn is finishing her PhD in vet pathology in Australia.)

What do I know of Mast Cell Tumours (MCTs)? ... I can recognise them in sections down the microscope..... MCTs are a common tumour of dogs & the most common skin malignancy, usually occurring on or just under the skin. Any breed or mix can develop a MCT, but certain breeds are predisposed - bulldogs and related breeds (Boxers, Bostons), Labradors, Shar Peis, Weimaraners. Given there is a breed incidence, an underlying genetic basis has been theorised, but the cause is unknown. Because something is common does not mean it is necessarily genetic - for example, bowel cancer in people. Although genetic alterations have been found to be present in certain bowel cancers, most bowel tumours are not (yet) attributable to the inherited changes.

Average age of dogs diagnosed with MCTs is 8-10 years, but dogs as young as 3 months & as old as 16 have been recorded. The normal role of mast cells in the body is to respond to allergic reactions where they release histamines, heparin and other products. For instance, when a bee stings you the area becomes red, swollen and itchy because of products released by mast cells. MCTs often have fingers of malignant cells spreading into surrounding tissue. This is the reason a margin of at least 3-5cms of normal tissue surrounding any visible tumour is removed - so the microscopic fingers are removed. Sometimes this is enough, and a wider excision is required before the histopathology report says the edge of tissue resected does not contain any tumour cells. Many owners think this 2-inch margin is over the top, yet if it were a malignant melanoma on a person, they would not flinch as they expect heaps of tissue to be removed for these. MCTs often spread to the lymph nodes, spleen, and the bone marrow. Recurrence or metastasis is difficult to predict. There are a couple of grading schemes used & I think what is grade I in one is reversed in the other. Bottom line is we really can't predict biologic behaviour of these tumours very well at all - a fair % of dogs with really nasty looking tumours survive for many years, & other tumours look fairly bland, yet metastasise. Here in Aus, very few places offer any "treatment" other than excision.

Two years ago Delphi had a MCT present as a lump on the skirt, or fold of flank. It was smaller than thumbnail size, and every now and then, she would lick at it. The Vet expected it would subside by itself, but advised if it should grow or alter to come back and see him. She kept licking it and I decided as it was annoying her, so "one day soon" I would organise to have it removed. A little while later "one day soon" was not soon enough, the lump blew up to about 8-10cms across, stayed that size for two days, and went down again. I was in the country nursing my mother at the time and decided the minute I was back in Melbourne, it was coming off! An aspirate of the lump was performed prior to her surgery and it was pronounced that Delphi had a MCT. Even though we knew what it was prior to surgery & they took enormous margins, the Pathology report found she had tumour cells to the edge of this excision, so about a month later further surgery was performed. This time they went down to the fascia over the muscles of the stomach wall. Luckily the new margins were clear of tumour. Two years on & no signs. Keep fingers crossed.

Histiocytomas are completely different - they are derived from epidermal Langerhans (antigen presenting) cells. They usually spontaneously regress if left on an animal long enough, otherwise, excision is curative. Very common in young dogs.

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