Download Information Sheet – Mast cell tumours What are mast cell tumours

Information Sheet – Mast cell tumours
What are mast cell tumours?
Mast cells are found in all body tissues, and are involved in normal inflammatory responses in the
body. They are particularly active in situations such as allergic responses or response to injury such
as a bee sting – and the release of histamine from the mast cells in the skin causes the swelling,
itchiness and redness that is seen. Mast cell tumours (MCT) are cancerous proliferations of these
cells in the skin. Occasionally we see MCT arising in internal organs, and this is much more serious
than when they are in the skin.
What do they look like?
MCT can look like anything! They may be a soft mobile swelling in the skin, or a little hard nodule, or
even a hairless nodule. There is no way of knowing on examination whether the lump is a mast cell
tumour or not. Sometimes they release histamine after a bump or handling and this causes them to
swell up suddenly, so owners may think they are a ‘bee sting’. They can appear anywhere on the
body and be any size.
About 1/3 of tumours in dogs are skin tumours, and up to 20% of these will be MCT. About half of the
MCT are found on the main body of the dog, and about 40% on the limbs. The remainder are found
elsewhere such as on the head/ under the tail or in internal organs.
What causes MCT?
We do not know what causes MCT in dogs, they are rare in cats, and very rare in humans. Some
breeds are predisposed to MCT (eg Boxers, Golden Retrievers, Bulldogs, Weimeraners – although
this is not a complete list). Dogs tend to be slightly older, between 7.5-9 years when they get MCT
and we do not know of any way to prevent them. Some dogs, especially those breeds that are
predisposed, may have several MCT in their lifetime. However it is important to be aware that MCT
may be found in any age of dog or cat.
Are mast cell tumours malignant?
MCT are very common in dogs, and they can be relatively innocent or highly malignant. In cats, MCT
in the skin are usually benign, but in other tissues may be malignant. Up to 25% of dogs may present
with more than one MCT, although this does not mean that they are malignant.
How do you diagnose them?
You can only diagnose a MCT by taking a sample to be looked at in the laboratory. This can be a very
tiny sample taken with a small hypodermic needle (a fine needle aspirate) and this is the most
common way to diagnose a MCT. Sometimes a small slice of tissue (biopsy) is taken or the MCT is
removed and all of the tissue is submitted to the laboratory. If a biopsy is taken, we get more
information about the type of MCT and how it is likely to behave before we start treatment.
How do you know how aggressive the MCT is?
We cannot tell how aggressive the MCT tumour will be from its appearance or whether the patient has
more than one MCT. The only way to determine its behaviour is to have a histopathology report from
a tissue sample. Mast cell tumours are usually graded and the most commonly used systems are the
Patnaik and Kiupel grading system.
What is the prognosis associated with a MCT?
The prognosis depends on the grade of the MCT. It is also determined by whether or not there is
secondary spread of the tumour, and the location of the tumour which will affect the likelihood of
successful surgery. The reported median survival time for Patnaik Grade I tumours after surgery is
>40 months; Grade II tumours 25-40 months and Grade III tumours 9-12 months. The reported
median survival time for Kiupel low grade tumours is >24 months and the Kiupel high grade is < 4
months.
There are other tests that may be done to look at how the MCT is likely to behave such as Ki67,
AgNOR stains and these are done on tissue samples taken at the time of surgery. Information about
MCT is changing all the time and you should talk to the clinician who is attending your pet about the
most up to date information.
What other tests are necessary?
We like to ensure that we know all about the patient before we make any treatment decisions. So we
will take a blood sample to check that the red and white cells (haematology) are normal and there a
no other concurrent problems (biochemistry). We also do tests to check that the MCT has not spread
to other parts of the body. This involves ultrasound examination of the abdomen and needle aspirates
of the liver and spleen. We also take needle aspirates of the lymph nodes nearest to the MCT. This is
usually all done under a light sedation.
What are the treatment options?
There are three treatment options; Surgery, Chemotherapy and Radiotherapy. The treatment of
choice is surgery, as this aims to be curative, by removing the tumour with a wide margin of healthy
tissue (at least 2-3 cm). However this is not always as easy as it sounds. MCT on the body wall may
be easy to completely remove with ‘clean’ tissue all around the tumour, as there is lots of loose skin
available to close the surgery site. MCT on the head, limbs or feet, or around body orifices may be
very difficult to remove completely without specialist techniques. Sometimes, we recommend
chemotherapy for a short time before surgery to make the tumour shrink. If it has not been possible to
completely remove the MCT, then we recommend another treatment to reduce the likelihood of the
MCT growing back again. In some circumstances, radiotherapy may be used after the surgery to kill
any remaining cancer cells. This treatment is not appropriate for all sites on the body however, and in
some cases, chemotherapy is recommended.
Chemotherapy may be also used to treat MCT that cannot be removed surgically, or when we find
that the cancerous cells have spread to other parts of the body. In this instance, we aim for quality of
life rather than longevity, but remission times can be long. There are different protocols and you will
need to discuss these with a medicine specialist who will help you decide on the most appropriate
protocol, depending on the temperament of your pet and the costs involved.
Will my pet experience side effects with chemotherapy?
Some animals experience side effects with chemotherapy but these are generally mild and either
resolve spontaneously or with minimal treatment. It depends on the medications that are used and
how your pet responds to them. The clinician looking after your pet will always try to choose the drug
or combination of drugs that will cause the fewest side effects, whilst being the most effective. The
aim is that the pet should feel as well as possible during treatment, and most patients do not seem to
feel unwell whilst receiving chemotherapy. The potential for drug related side effects must always be
balanced against the benefit of the chemotherapy and the consequences of the cancer itself.
What else do I need to know?
Every dog has the potential to develop another MCT or recurrence at the site of surgery at a later
date. Even if the surgery has been successful, you should have checks done on a regular basis to
ensure that you do not miss any new mast cell tumours or a recurrence.