Breast Cancer In Dogs

Breast cancer in dogs is the biggest cause of tumors in intact female dogs.  It is estimated that up to 60 percent of breast tumors in dogs are malignant.  There is also a great risk of the cancer spreading to other parts of the body.  Fortunately, mammary cancer is easy to prevent and easily treatable.


Breast cancer in dogs is believed to be caused by, or at least related to, estrogen and progesterone in the body of a female dog.  These are the female sexual hormones.  It has been shown that a dog’s risk of developing breast cancer is related to the number of estrus cycles the dog experiences.  Dogs that are spayed before they have their first estrus cycle have a 0.005 percent chance of developing breast cancer.  Dogs that are spayed after one estrus cycle have a 0.08 percent chance of developing breast cancer.  Dogs that are spayed after two or more estrus cycles have a 0.26 percent chance of developing breast cancer.

Mammary cancer is a common cancer among female dogs but spaying your dog before the dog is 30 months of age (2 1/2 years old), can do a great deal to prevent your dog from developing breast cancer for the rest of the dog’s life.

Some breeds are more prone to having mammary cancer than others.  Boxers, English Springer Spaniels, Cocker Spaniels, and Dachshunds are all breeds that seem to have a higher than average risk of developing breast cancer.


Approximately half of all mammary tumors are benign and half are malignant, so it is possible for your dog to have a mammary tumor that is not cancerous.  You may notice that your dog has a lump in a mammary gland and become concerned.  All lumps or changes to your dog’s mammary glands should be seen by a veterinarian.  If you feel lumps in your dog’s mammary glands they may feel like a solid mass.  If they are small they may feel like pea gravel.  They may feel very hard and don’t move when you feel them.  They may grow very fast and can double in size in just a few weeks.

Most dogs have five mammary glands on each side of their body.  Mammary cancer most often occurs in the fourth and fifth glands, at the lower end of the abdomen.  Many times a dog has more than one tumor.  A benign tumor will often grow slowly and be small and smooth.  A malignant tumor is more likely to grow fast, have an odd shape, and it may be open and bleeding.  Sometimes a growth that seems to be benign may change and begin to grow fast.


While the mammary tumor itself may be easy to palpate, it is usually necessary for a vet to perform a biopsy to be certain what kind of mammary tumor a dog has, or to remove the tumor.  If a tumor is more aggressive it may begin to spread to the lymph nodes or the lungs. Your vet may need to take chest x-rays and check your dog’s lymph nodes to see if the cancer has spread.


In most cases of breast cancer in dogs, surgical removal is used to treat tumors.  This treatment can completely remove the cancerous tumor more than 50 percent of the time.  Your vet may remove only the tumorous mass or he or she may remove some mammary tissue and lymph nodes as well.  This is a fairly simple surgery and recovery is usually quick.  It is not necessary to spay a dog when a mammary gland is removed and their is contradictory evidence about whether spaying an older dog will provide any protection from future recurrence of tumors.

Dogs that are older may not be good candidates for surgical removal of tumors or for spaying.

Chemotherapy and radiation have not been very successful or widely used to treat mammary cancer in dogs.  However, you may wish to consult a veterinary oncologist to see if these treatments would be advised in the case of your dog.  Surgical removal of mammary tumors is the most widely used and effective treatment at this time.

Long Term

Dogs that are spayed before they are 2 1/2 years old have a low incidence of malignant breast cancer.  However, mammary cancer is still a very common disease but one which is usually easily treatable.  If your dog has mammary cancer it is usually easily detected by feeling any lumps in your dog’s mammary glands.  A vet can usually easily remove lumps, whether they are benign or malignant.  Dogs that have breast cancer have an excellent prognosis for long term survival without any ill effects.  If you find that your dog does have a new lump in her mammary glands, see your vet right away and take action.

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Thursday, February 10, 2011 By: Sofia
Category: Cancer