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Breast Cancer in Dogs (Mammary Gland Tumors)

Reviewed for accuracy on September 20, 2019, by Dr. Jennifer Coates, DVM

Benign and malignant tumors of the mammary glands occur fairly frequently in female dogs that aren’t spayed. In fact, mammary gland tumors are the most common type of tumor diagnosed in unaltered female dogs.

The mammary glands' function is to produce milk to feed newborn puppies. They are located in two rows that extend from the chest to the lower abdominal area; the nipples indicate their location.

While breast cancer in dogs primarily occurs in the female population, it does also affect male dogs, although rarely.

Spaying can largely reduce a dog’s risk of developing breast cancer, especially if the dog is spayed before she has an opportunity to go into heat.

Types of Mammary Tumors in Dogs

Mammary gland tumors are commonly categorized as being either benign or malignant.

About half of affected dogs will be diagnosed with the benign form of mammary tumors, which may be further classified as adenomas or benign mixed tumors. 

And the other half of dogs that are diagnosed will have a malignant form of tumor, which may be solid carcinomas, carcinoma in situ, or simple carcinomas like the cystic-papillary form, amongst others. 

Symptoms of Breast Cancer in Dogs

Here are the most common symptoms of mammary tumors in dogs: 

  • Single or multiple masses in the mammary glands—about half of patients have multiple tumors

  • Superficial loss of tissue on the surface of the skin over the mammary tissue, frequently with inflammation and/or drainage

  • Mass may be freely movable, which may imply benign behavior

  • May be fixed to the skin or body and hard to move, which may imply malignant behavior

  • Swelling of nearby lymph nodes

  • Breathing difficulties (if the cancer has already spread to the lungs)

Causes of Mammary Cancer in Dogs

The causes of mammary gland tumors are not fully understood, although hormonal and/or genetic influences likely play a role.

A genetic basis is possible in a number of breeds, and there are some genes that have been identified in dogs that are predisposed to breast cancer. 

For example, Toy and Miniature PoodlesEnglish Springer SpanielsBrittanysEnglish Cocker SpanielsEnglish SettersPointersGerman Shepherd DogsMaltese and Yorkshire Terriers have been reported to have an increased risk of developing mammary tumors compared to other breeds.

Exposure to female reproductive hormones are known to increase the risk of breast cancer in dogs. In fact, one study showed that when dogs were spayed prior to their first heat, they had 0.5% the risk of developing mammary cancer in comparison to unaltered females.

The median age at diagnosis is about 10.5 years (the range is 1 to 15 years of age); it is less common in dogs younger than 5 years of age.

Diagnosis

 

Several diseases cause symptoms that are similar to those seen with breast cancer in dogs. Your veterinarian will want to rule them out before arriving at a conclusion. You will need to give a thorough history of your dog's health, reproductive history, and onset of symptoms.

A blood chemistry profile, complete blood count, and a urinalysis will likely be run. Diagnostics generally also include X-rays of the chest and abdomen, which may detect metastasis.

It will be necessary to conduct a biopsy of the mass to identify it and determine whether it is benign or malignant.

In addition, the lymph nodes will be examined, and samples may be taken from them for analysis.

Treatment

Several treatment options are available for mammary tumors in dogs. Your veterinarian will decide the most effective treatment based on your dog’s circumstances. They may consult a veterinary oncologist (cancer specialist) for additional or updated information, or they may refer your dog for advanced care.

Surgery is the primary mode of treatment. Surgical removal of the tumor may be curative or it may be combined with other treatments to improve your dog’s prognosis. However, the extent of the surgery will depend on a few factors, like the type of tumor that’s present and whether it has obviously metastasized.

Some tumors are more invasive, moving deeper into nearby tissues, making them very difficult to remove. In these cases, partial removal of the cancerous mass may be performed. Chemotherapy and radiation treatments may also be recommended, usually in conjunction with surgery.

It is also common to spay female dogs at the time of surgical removal of the tumor as this may enhance survival times.

Your dog’s treatment plan will include managing your dog's health at home, with follow-up progress visits to the veterinarian or oncologist. Physical examinations and chest X-rays may be required on a regular basis following the initial treatment to check for recurrence.

Prevention

Spaying before the first heat cycle will greatly reduce your dog's risk for developing breast cancer.

Early spaying is the best method for prevention of this form of cancer. If spaying is delayed until after the first heat cycle, the risk of developing breast or mammary tumors increases from 0.5% to 8%.

If spaying is delayed until after the second heat cycle, the risk increases to 26%. If spaying takes place after the dog has reached approximately 2.5 years of age, there is no sparing effect on the risk of developing mammary tumors.

Life Expectancy for Dogs With Mammary Cancer

The prognosis and course the disease will take varies according to the size and type of mammary tumor (benign or malignant), as well as whether it has spread.

In general, dogs with smaller tumors survive longer than those with larger tumors, and dogs with disease that’s confined to the mammary gland do better than those who have metastasis to the lymph nodes. If the cancer has spread to distant sites like the lungs, the prognosis is worse.

The treatment method can also have a big effect on a dog’s prognosis. For example, one study showed that dogs treated with surgery and chemotherapy survived, on average, for 24 months, while those treated with surgery alone only lived for an additional 6 months. 

Why You Should Never Ignore a Lump on Your Dog’s Breast

You should never ignore a breast or mammary nodule, or adopt a wait-and-see attitude towards it. A breast or mammary lump will probably only get larger, metastasize and become more difficult to treat with time.

Early detection and finding an appropriate treatment is best.

Check for lumps in the mammary gland(s) if you have a female dog that is not spayed. If you’re not planning to breed your dog, early spaying will markedly decrease the likelihood of her developing breast or mammary gland tumors in the future.