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Small Animal Section: No. 7
CANINE MAST CELL TUMOURS: HISTOLOGICAL
GRADING AND PROGNOSIS
Aim of communication
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To describe the histological grading of mast
cell tumours in the dog and its prognostic value |
General information
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Mast cell tumours in dogs are malignant and
account for 15-20% of all cutaneous tumours |
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Boxers, Boston Terriers, Bull Terriers, Fox
Terriers, Dachshunds, Labrador Retrievers and Weimeraners are predisposed.
The scrotal skin is commonly involved |
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Tumours are usually solitary, but may be multiple
at first presentation in 5-10% of cases; multicentric tumours in the skin
may occur sequentially |
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Average age is 8 years; occasionally seen
in dogs as young as 4 months |
Histological grading (classification)
Three histological grades are currently
recognised based on cellular and nuclear morphology, and mitotic activity.
Although a numerical grading system may lead to confusion, it has been
retained by WHO:
| Histological grade |
Prognosis after complete surgical excision
only |
| Grade 1 = well-differentiated |
83% survive at least 4 years |
| Grade 2 = intermediate differentiation |
44% survive at least 4 years |
| Grade 3 = poorly-differentiated |
< 10% survive more than 1 year |
The survival rate increases from 15
to 20% when radiation, chemotherapy and additional therapeutic measures are
taken.
Prognosis
The biological behaviour of mast
cell tumours in the dog is highly correlated to histological grade,
which is regarded as the most consistent prognostic indicator (several
surveys):
The
histological grading and clinical staging of the tumour [such as size (tumour
diameter of > 50mm is associated with less favourable medium and longterm
survival rate), growth habit and possible lymph node involvement] will decide
the therapeutic protocol. Treatment options, in addition to surgical excision
(stage 1 tumours), include: radiation therapy (stage 2 tumours), cryotherapy,
intralesional corticosteroid, deionized water therapy, chemotherapy (stage 3 and
4 tumours) and medical management of paraneoplastic disease
Diagnostic pitfalls
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These
tumours commonly have clinically indeterminate borders often frustrating
attempted surgical excision. Margins of 30mm are desirable |
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Cytology,
when positive, provides a rapid and inexpensive diagnosis but false negatives
may be problematic. Do not rely on negative fine needle aspirate cytology
results. Histopathology is essential to evaluate degree of infiltration,
tumour grading and evaluation of tumour margins |
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Severely
ulcerated/inflamed or poorly differentiated (anaplastic) mast cell tumours may
be difficult to diagnose histologically and special stains (Giemsa) may be
required |
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Grade 1
mastocytomas of the groin, prepuce, axilla and lips have a biological
behaviour similar to grade 3 tumours |
References
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JA Yager, BP Wilcock (1994) Surgical Pathology of the
Dog and Cat, 278-279 |
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MJ Hendrick et al (1998) Histological Classification
of Mesenchymal Tumours of Skin and Soft Tissues of Domestic Animals, WHO,
28-29 |
For further information contact:
VetPath Veterinary Pathologists
PO Box 8464, Pretoria 0001
Tel: (012) 529 8345/6
e-mail:
info@vetpath.co.za
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