Small Animal Section: No. 3

BENIGN MAMMARY TUMOURS IN DOGS

Mammary tumours are the most common tumours in bitches, and, after skin tumours, the most common tumour of dogs in general.  The risk of developing mammary tumours is highly correlated with the age of spaying – 0.5% bitches spayed before their first heat develop tumours, while 8% of those spayed after their first heat develope mammary tumours.   The proportion of canine mammary tumours that are malignant varies widely in the literature but in most studies is approximately 50%. Malignant tumours are covered under our separate information sheet (Malignant mammary tumours of the dog). Histological classification of these tumours is a valuable tool, since histological type and grade are of prognostic value.  Benign mammary tumours are classified as follows:

1. Adenoma:  an adenoma is a benign neoplasm of well-differentiated epithelial or myoepithelial cells.

2. Simple adenoma:  a benign adenoma of simple tubular type, which may contain secretions; or solid masses of myoepithelial cells.  These tumours are rare in cats and dogs.  Some tumours are cystic.

3. Complex adenoma:  benign adenoma with both epithelial and myoepithelial components.  This tumour forms a continuum with fibroadenoma, ductal papilloma, benign mixed tumours, and lobular hyperplasia, all of which are common in the bitch.

4. Basaloid adenoma:  benign adenoma with monomorphic basal cells which palisade along a basal lamina.  So far, these are only described in Beagles treated with progestins.

1. Fibroadenoma:  a benign tumour containing a mixture of epithelial and stromal cells, sometimes with a myoepithelial component.  This tumour may be similar to a complex adenoma, and is fairly common in the dog and cat.

2. Benign mixed tumour:  a tumour containing a mixture of epithelial (adenoma) myoepithelial, and mesenchymal cells.  The latter produce a combination of cartilage, bone, fat and fibrous tissue.  This is the most common benign tumour of canine mammary glands.  Mixed mammary tumours are usually lobulated, and may be hard (bony) or contain glassy, blue foci (cartilage) on section.  They may contain red, spongy (haemorrhagic) areas, or cysts filled with straw-coloured fluid (dilated ducts).  Papillary growths may protrude into these cysts. 

In addition, some forms of mammary hyperplasia, dysplasia and adenosis may be mistaken for neoplasia, and more than one type of tumour can co-exist in the same mammary gland.  Ductal ectasia, multiple mammary cysts and focal fibrosis (fibrosclerosis) have also been recorded in bitches.  Well-demarcated, encapsulated lesions, that are slow growing and are not attached to the overlying skin or deep tissues are more likely to be benign, than fast growing, ulcerated tumours that are attached to the surrounding tissue (invasive).  Firm cords that radiate from the tumour (invaded lymphatics), enlarged lymph nodes and emaciation are usually signs of malignancy.  Benign lesions usually shell out easily during surgery.  While the macroscopic and clinical appearance of mixed mammary tumours in dogs is often characteristic, it is always worth submitting the entire tumour for histopathology since occasionally, malignant mixed mammary tumours occur, and these behave as carcinomas.  In addition, the grade of the tumour, invasion of lymphatics or blood vessels and completeness of excision can be assessed so that the prognosis for the dog can be calculated.   Cytology is not a reliable method for differentiating between benign and malignant lesions.

REFERENCES:

W. Misdorp, R.W.Else, E. Hellmen and T.P. Lipscomb (1999)  Histological Classification of Mammary  Tumours of the Dog and Cat.  Second Series (Vol VII), Armed Forces Institute of Pathology,   Washington DC.

J.E. Moulton (1978) Tumours of the Mammary Gland.  Chapter 11, in Tumours in Domestic Animals edited by J.E. Moulton, University of California Press, Berkeley

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Last modified: Friday June 25, 2004