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Small Animal Section: No. 4
MALIGNANT MAMMARY TUMOURS (MMT) IN
THE DOG: CLASSIFICATION & PROGNOSIS
Introduction
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Mammary tumours rank 2nd
to skin neoplasms in prevalence in the dog and are the most frequent neoplasms
in the bitch. The risk of developing mammary tumours is reported to be 0,05%
in dogs spayed before first oestrus while 26% of dogs spayed after the second
oestrus will develop neoplasms. |
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These tumours are notorious for
their variation in structure and biological behaviour |
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Histopathology of incisional or
excisional biopsies is the most reliable diagnostic method allowing assessment
of (1) classification of the lesion; (2) details of histomorphology such as
degree of differentiation, pleomorphism and mitotic rate; (3) infiltration
into surrounding skin; and (4) completeness of excision |
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Between 41-53% of mammary
tumours in bitches are reported to be MMT |
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42-59% of dogs with mammary
tumours may have multiple lesions |
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Age distribution rises sharply
by 6-7 years of age, with a peak by 10-11 years |
Aim of communication
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To describe the
classification system we use for malignant mammary tumours in the dog
(following the most recent WHO classification, 1999) and provide details on
the prognosis of these neoplasms. |
Classification of MALIGNANT
mammary tumours (MMT) in the dog
The first three types of carcinoma are listed in an
INCREASING ORDER OF MALIGNANCY
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Non-infiltrating (in situ)
carcinoma |
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Complex carcinoma : lymphatic
invasion in about 10% of cases |
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Simple
carcinoma : 3 types are described (poorly differentiated 90% show recurrence
rate while well-differentiated MMT show 24% recurrence rate)
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Tubulopapillary carcinoma:
metastasis between 32-46% |
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Solid carcinoma |
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Anaplastic carcinoma:
extremely high tendency to recur and metastasize |
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Special types of carcinoma
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Spindle cell carcinoma,
squamous cell carcinoma, mucinous carcinoma and |
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lipid-rich carcinoma |
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Sarcoma (compose only 3-4% of
MMT) |
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Fibrosarcoma, osteosarcoma –
poor prognosis with these MMT |
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Carcinosarcoma : tumour with two
malignant components: epithelial/myoepithelial (carcinoma) and
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mesenchymal (sarcoma) |
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Carcinoma or sarcoma in benign
tumour : the term ‘malignant mixed tumour’ is discarded : insufficient
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data available on prognosis
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M.M.T. information:
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Malignant mammary tumours (MMT)
should be “staged” to determine metastatic spreading which include radiographs
of the thorax, clinical evaluation, cytology and ultrasonography if indicated.
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Surgery is indicated for all
dogs with MMT except those with distant metastasis and inflammatory
carcinomas. |
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Because of the slow spreading of
MMT in bitches approximately 50% are cured by surgical excision. Tumours
larger than 3cm and of higher grade malignancy show a recurrence rate of 70%
in one year compared to smaller tumours with low histopathological grading
where the recurrence rate is 30%. |
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After surgical excision of a MMT
clinical evaluation is essential at 1,3,6,9,12,18 and 24 months post surgery
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Reference
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W Misdorp et al (1999)
Histological Classification of Mammary Tumors of the Dog and Cat, WHO |
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Benjamin SA et al
Classification and Behaviour of Canine Mammary Epithelial Neoplasms Based on
Life-span Observations in Beagles. Veterinary Pathology 36:423-436 (1999) |
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Kritzinger et al (1986)
Prognostic Factors in Canine Mammary Tumours : Seminars in Veterinary Medicine
and Surgery : Vol 1 |
For further information contact:
VetPath Veterinary Pathologists
P.O. Box 8464, Pretoria 0001
Tel: (012)
529 8345/6
e-mail :
info@vetpath.co.za
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