Neutering and breast cancer in dogs, what’s the link?


One important reason that vets often give for neutering female dogs earlier is to reduce the risk of breast cancer. However, what’s the real risk of the disease? Our vet blogger Sian investigates…

How common is breast cancer in dogs?

Breast cancer is the most common cancer affecting female dogs. It is rarely seen in male dogs. The risk of a female dog developing breast cancer is three times greater than the risk to women. Risk increases with age, most tumours are seen between 7-13 years of age. All breeds are affected but predispositions are reported in Brittany, English Springer and Cocker Spaniels, Labradors, Boxers, English and Irish Setters, Pointers, Samoyed, Miniature and Toy Poodles, Dachsunds, Maltese and Yorkshire terriers.

Breast or mammary tissue is extensive in the dog. There are usually 10 mammary glands, each associated with a nipple. The glands are arranged in two lines underneath the abdomen, either side of the midline. The first pair are found on the chest wall inside the front legs and the last pair between the hindlegs, close to the vulva. 

How easy is it to detect breast cancer in dogs?

It can be difficult to spot early breast cancer in long coated breeds as the area is often hidden. The first sign is a lump or mass within the mammary tissue. There may be single or multiple lumps. Initially they are pea sized but can grow to 25 cm in diameter. The rate of growth is often important to the outcome in the dog. The lumps can be seen throughout the chain of glands but are more common in the back glands. 

How serious are they?

Cancer is always a frightening idea but cancerous lumps can be benign or malignant. Benign tumours can be cured by surgical removal, they are unlikely to recur and do not spread to cause disease in other areas of the body. Malignant tumours can invade and spread to normal tissue, form tumours in other parts of the body and recur after removal. Unfortunately, 50% of breast cancer in the dog is malignant. 

How can we minimise the risk?

Historically, vets have recommended neutering at a young age to reduce the incidence of breast cancer later in life. One study carried out forty years ago showed an impressive reduction in breast cancer if a dog was neutered before the first season. (1) The risk was found to be reduced to 0.5% of the risk of an entire female. The study also found that if she was neutered after the first season the risk was only 8% of that of an intact female. If she had two or more seasons before she was neutered but was under 2.5 years of age the risk was 26% of being intact and if over 2.5 it was found to be 40%. This and other studies were the basis for early neutering advice. 

That’s very old data – is it accurate?

As the relevant studies were carried out some time ago, researchers carried out a review of the data in 2010 to determine if the advice was valid. (2) They applied more modern research methods to the data. This review found evidence that neutering before 2.5 years was associated with a considerable reduction in breast cancer. Neutering before the first season was found to reduce the risk further. 

How easy is breast cancer to treat?

The diagnosis and treatment of breast cancer can be difficult. When a dog has a mass a vet can often make a diagnosis with an FNA, a fine needle aspirate. A needle is used to remove cells to send to the laboratory and obtain a diagnosis of what the mass is. Unfortunately, many breast tumours in the dog are mixed cell types so these results are often misleading. If malignant cells are found it is useful – but if they are not, it is still not a guarantee that the mass is benign.

Surgery is the only effective treatment for malignant breast cancer in the dog as chemotherapy has not been found to be effective. Surgery may not increase survival time as there can be secondary tumours or recurrence but it may make the dog more comfortable. There is evidence of longer survival times if a female dog is spayed two years prior to developing a breast mass. 

Why does it happen?

We do not know the exact cause of breast cancer in the dog. There are female hormone (oestrogen and progesterone) receptors on 70% of benign tumours and 50% of malignant tumours. However, the action of the hormones is not completely understood. It is believed that genes play a part as well as obesity. Obesity has been found to increase the risk of breast cancer in dogs and humans. 

Breast cancer represents 92% of the reproductive tumours in female dogs but ovarian and uterine cancer can also occur. Neutering prevents both if ovariohysterectomy is performed and ovarian cancer, if only the ovaries are removed. It is always important to consider pyometra. Pyometra is a severe condition that can be fatal if untreated. It is completely prevented by spaying. Diabetes also occurs more commonly in entire females.

Bottom line – does neutering prevent breast cancer?

In conclusion, there is evidence that neutering before 2.5 years of age significantly reduces the risk of breast cancer. The risk is further reduced by neutering before the first season. As more information comes to light about early neutering for large and giant breeds, the risks must be considered carefully. 

References and further reading: 

1 Schneider R, Dorn CR, Taylor DO. Factors influencing canine mammary cancer development and postsurgical survival. J Natl Cancer Inst. 1969;43(6):1249-1261.

2. Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs -a systematic review. JSAP. 2012

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