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Vizsladogs, Ltd.
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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