Mammary gland tumors are located at the mammary glands. One or more mammary glands can be affected.

Behavior

Individual mammary gland tumours can start as benign but evolve to become malignant. The chances of malignancy increase with tumor size. The chances of metastases are smaller than 50% in malignant tumours. When metastases occur, they are usually found in the regional lymph nodes, liver, lungs and bones.

Prevalence

Mammary gland tumours are common in dogs (the most frequent tumour in non-spayed bitches). After all, in dogs the exposure to ovarian hormones is an important factor in the development of mammary gland tumours. Mostly middle-aged and older dogs are affected. It is very rare in dogs under 5 years of age.

Risk factors include:

1. hormones: hormonal exposure plays an important role in the development of mammary gland tumours. Spayed dogs have a lower risk of developing a mammary gland tumour than non-spayed dogs. If the spaying takes place before the first heat, the dog only has a risk of 0.5% of developing a mammary gland tumour, 8% before the second heat, 26% before the third heat vs a dog who hasn’t been spayed. Afterwards, when it comes to tumour development, it makes little difference whether the bitch was spayed or not. Dogs who are administered “the pill” (progestagens) are twice as likely to develop mammary gland tumours and often develop them at a younger age.
2. age: this tumour type mostly occurs in dogs at 7-8 years of age, after which the risk increases each year. The older the bitch gets, the higher the chances are the tumour is malignant.
3. diet and body weight also play a role to a lesser extent. Being overweight during puberty could increase the risk of developing a mammary gland tumour.

Breed predisposition

A higher risk to develop this tumour type has been reported for the following breeds: Cocker Spaniel, Chihuahua, Maltese, Poodle, Dachshund, Yorkshire Terrier (more in smaller breeds). Some larger breeds with a greater risk include: Boxer, German Shepherd, Dobermann, English Springer Spaniel, English Setter, Epagneul Breton, Pointer.

Symptoms

This tumour type manifests as a palpable lump in the mammary gland. Most lumps appear at the glands closest to the tail. More than 70% of non-spayed bitches have more than 1 mammary gland tumour at the time of diagnosis. The tumour size, disease stage and presence of general disease symptoms vary a lot.

Because of the risk of metastases, staging is always advised before initiating treatment.

Staging includes following tests:

1. Blood examination
2. Radiography of the thorax
3. Fine needle aspirate of the tissue mass and local lymph nodes

   A punction aspirate of the tissue mass can help to make a distinction between a mammary gland tumour and other tissue masses such as fat lumps or mast cell tumours. However, it can not distinguish benign and malignant mammary tumours!

   An aspirate of the local lymph nodes can indicate the presence of metastases. However, mammary gland tumours are capable of creating new lymph vessels. Therefore, metastases will not automatically occur in the nearest lymph node and it isn’t always possible to rule out metastases via this method. The veterinarian can find out which are the draining lymph nodes via scintigraphy of the lymph nodes, contrast ultrasound or applying dyes during surgery (coloring the draining lymph nodes).

4. A tissue sample will confirm the tumour type. It is important to examine ALL tissue masses, as some may be benign, while others are malignant.
5. Ultrasound of the abdomen can identify abnormal lymph nodes or clarify certain bloodwork anomalies.

If there are no metastases and only one malignant tumour present, a wide surgical resection (i.e. margin of 2 cm and a tissue layer beneath the tumour) is preferred. When multiple tumours are present, removal of the entire mammary chain should be considered.

If no visible metastases are present, spaying should also be recommended. The ovaries produce hormones that can help cancer cells survive. During the spaying procedure, the ovaries are removed which decreases the level of tumour feeding hormones. The efficiency of the spaying is associated with the presence of specific hormone receptors in the tumour. According to recent studies, the new formation of a malignant tumour cannot be prevented by spaying at older age. However, it lowers the risk for formation of new benign mammary tumours.

Hormone therapy such as Tamoxifen (an estrogen receptor antagonist) is not well tolerated by dogs (as opposed to humans).

Chemotherapy would be indicated when tumours are larger than 3 cm and/or metastases towards a lymph node are present, but to date there is no effective chemotherapy treatment. Tyrosine kinase inhibitors have shown some effect on carcinomas in general and could be considered.

For dogs with a tumour that expresses DER-2 receptors, research is ongoing to determine whether HER-2 vaccines are effective against the tumour. Via this vaccine, the dog will produce antibodies that will bind the receptors that control the tumour growth. Thusly, the tumour growth could be inhibited.

Very large variations of average survival times have been reported for malignant mammary tumours. This variation is in general dependent on the tumour size, involvement of the lymph nodes or other organs, and the histological grade.

The survival time is clearly longer for dogs with a tumour smaller than 3 cm vs those with a tumour larger than 3 cm. However, the size of the tumour is not important when there are metastases in the lymph nodes. The higher the staging score, the worse the prognosis.

Furthermore, ulceration of the skin, blood/lymph vessel invasion, high mitosis index on the tissue sample, lack of hormone receptor expression in malignant tumors and mammary gland sarcomas are associated with a worse prognosis.