Mast cell tumours are the most common cutaneous tumour
in the dog, accounting for 16%-21% of all skin tumours. They are the second
most common cutaneous tumour in the cat. They tend to occur in older dogs
(mean age approximately 9 years). Boxers, Boston terriers, Labrador
retrievers, beagles and schnauzers appear to be at increased risk. Most
tumours occur on the trunk but up to 25% can occur on the limbs. Figures
A and B were taken from a firm, well circumscribed mass over the lateral
aspect of the left hind limb of a 6 year old Labrador.
Mast cell tumours involve the dermis and subcutaneous tissues.
They can be graded histologically as undifferentiated (high) grade,
intermediate and well differentiated (low) grade (grades 1, 2
and 3 respectively using the Bostock grading system). Histological grade
generally provides a good indicator of metastatic potential. High grade
tumours metastasise first to local lymph nodes and then to spleen and liver.
Bone marrow and peripheral blood involvement can occur with disseminated
anaplastic visceral mast cell tumours (disseminated mastocytosis)
which are almost always preceded by an undifferentiated primary cutaneous
lesion. Well differentiated mast cell tumours are usually solitary and slow
growing. Undifferentiated tumours tend to grow rapidly, are larger and very
often ulcerated. Surrounding tissues may become inflamed and oedematous.
The most common complication associated with mast cell
tumours relates to the release of histamine from the mast cell granules
which can result in gastrointestinal ulceration. Coagulation abnormalities
may also occur as a result of heparin release from degranulated mast cells.
Localised haemorrhage at the time of surgery can occur as a result of tumour
manipulation resulting in mast cell degranulation.
How useful is cytology?
The majority of mast cell tumours can be diagnosed on the basis of fine
needle aspiration cytology since the neoplastic cells exfoliate readily
(Figs A and B). The drainage lymph node can also be aspirated to
check for evidence of tumour metastasis (note however that a low number
of mast cells can be present in normal lymph nodes and in nodes responding
to antigenic stimulation).
Cytological grading of mast cell tumours has not been fully
assessed. The mast cells in poorly differentiated tumours tend to be larger,
are sparsely granulated, and contain nuclei with one or more nucleoli. In
contrast, mast cells from low grade tumours usually have densely packed
cytoplasmic granules which may obscure nuclear detail (note, however,
that some of these 'well differentiated' tumours have been shown to be malignant).
It is important to realise, therefore, that cytology at best provides only
a semi-objective assessment of malignancy and that histopathological examination
is always necessary to grade these tumours more precisely.
Prognosis and treatment
All mast cell tumours should be regarded as potentially malignant and wide
margin (at least 3 cm) surgical excision is indicated. Histological
grade is the most consistent prognostic indicator in canine mast cell tumours.
80%-90% of dogs with well differentiated mast cell tumours and approximately
60%-75% of those with intermediate grade tumours experience long term survival
following surgical excision. Recurrence or metastasis following surgical
excision of undifferentiated tumours is common. Dogs with regional lymph
node involvement and/or those which are showing systemic signs also have
a poorer prognosis.
Mast cell tumours in the cat
A relatively small percentage of cutaneous mast cell tumours in the cat
are malignant. Two types of cutaneoous mast cell tumour have been described;
mastocytic mast cell tumours and histiocytic mast cell tumours. The latter
typically occur in young cats. Siamese cats are predisposed to both histological
types. The histiocytic tumours are often multiple and may regress spontaneously.
Visceral mast cell tumours in the cat comprise approx-imately
50% of all mast cell tumours. These frequently involve the spleen, liver
and/or intestine and, in contrast to the cutaneous forms, widespread dissemination
and metastasis is common. Many cats are leukaemic (i.e. neoplastic mast
cells are in the circulation) at the time of presentation and the prognosis
is much more guarded. Peripheral eosinophilia and basophilia may be observed
and examination of a buffy coat preparation to check for the presence of
mast cells may also be potentially helpful in these animals, unless of course
they are overtly leukaemic at the time of presentation.
 
Figure A shows the typical
cytological appearance of a mast cell tumour. The mast cells in this instance
appear well differentiated and contain densely packed azurophilic cytoplasmic
granules which are obscuring nuclear detail. Free mast cell granules can
be seen in the background. Eosinophils are also present. In Figure B the
elongated spindle-shaped cell is a fibroblast. Fibroblasts are often seen
in fine needle aspirates from mast cell tumours.
References to the Mast Cell section
Small Animal Clinical Oncology. S.J. Withrow and E. Gregory MacEwen. 3rd
Edition. W.B. Saunders Co. 2001.
Diagnostic Cytology and Hematology of the Dog and Cat. R.L. Cowell, R.D.
Tyler and J.H. Meinkoth. 2nd Edition. Mosby, Inc. 1999.
Colour Atlas of Cytology of the Dog and Cat. R. Baker and J.H. Lumsden.
Mosby, Inc. 2000. |