Dog - Breast Cancer

Mammary neoplasia (breast cancer) is very common in the bitch. Its frequency is only second to skin cancer. Both benign and malignant forms occur but sixty percent of mammary lumps in the bitch are benign.

Are any particular breeds more prone?

Incidence in mongrels appears to be less than in purebred dogs. High risk breeds include Spaniels, Poodles and some terrier breeds. Beagles, Greyhounds and Boxers are also fairly prone but in these breeds the chances of the tumour being malignant is lower.

Does it occur at any particular age?

The majority of mammary tumours occur in bitches over the age of eight years. It has only rarely been reported in bitches of less than two years of age.

How is it diagnosed?

Initially on clinical examination. Often owners will notice a breast swelling during routine grooming. Very occasionally there may be an abnormal secretion from the teat of the affected breast. In cases involving malignant tumours there may be signs of spread to other organs, e.g. shortness of breath due to secondary tumours in the chest.

Are any particular breasts more affected than others?

Seventy percent of mammary tumours develop in the posterior (caudal or inguinal) glands- the ones at the tail end. The remainder are equally distributed through the anterior glands.

How are they detected?

The masses can vary from firm mobile nodules, either isolated or in clusters, to diffuse soft lumps that appear to be stuck to underlying tissues. Sometimes multiple small nodules only a few millimetres in diameter are noticed shortly after a season. If at all in doubt, consult us without delay.

What is the cause?

Like many forms of cancer at present the cause is speculative but we do know that most mammary tumours are hormone dependent.

What is the treatment?

Just as with human breast cancer, treatment can involve mastectomy, which is the removal of the affected breast (and in the case of the bitch those either side) or 'lumpectomy', which is removing only the lump. The treatment will depend very much on the stage the tumour has reached when diagnosed.

Will my bitch have to have radiotherapy and chemotherapy afterwards?

Approximately 60% of canine mammary tumours are benign. Therefore following adequate excision (surgical removal), which is not always easy, recurrence is less of a problem than with malignant tumours. In a bitch with malignant breast cancer chemotherapy is more effective than radiotherapy.

What is the prognosis?

There is great variation between benign and malignant tumours. It is always wise to have adequate laboratory tests (histology) after removal. These will give an indication of prognosis.

Can breast cancer be prevented in the bitch?

The best prevention is neutering before the first season. The protective effect of spaying diminishes with each cycle. After the second or third season it has little effect.

Why, then, has it been advised that my elderly bitch is spayed when she is operated on for her breast cancer?

As discussed, breast cancer is more common in older bitches. Probably more common than breast cancer in older bitches is pyometra (inflammation of the womb). Therefore if there is a risk of womb problems developing it is obviously prudent to complete both surgeries at the same time, thus reducing anaesthetic risk. However sterilisation at this age will not have any effect on subsequent possible development of breast cancer. We will be more than happy to discuss the pros and cons with you.

What if my dog is too old to undergo major surgery?

This is a situation with which we are very familiar. We will discuss the situation carefully after examination of the patient. If she is ill with another condition or, after assessment, it is considered that the risk of major surgery is too great, careful monitoring of the lump can be carried out. If the tumour is very slow growing no action may be taken. Otherwise we may suggest biopsy under a local anaesthetic in order to arrive at an informed decision.

Why is the wound so large following surgery?

In order to try to ensure that all the abnormal cells are removed it is good surgical practice to remove as large an area as practicable. In this way the chances of regrowth and further surgery is reduced.