Breast Cancer in Dogs.
Did you know that dogs also develop Breast Cancer?
The risk of developing breast cancer is present in all dogs, but much higher in female dogs that are intact (unspayed) than those desexed. Furthermore, the statistics of female dogs developing breast cancer is even higher compared to human females!
But why dogs? Well, our canine companions are placental mammals, just like us humans. They develop mammary glands and their endocrine system functions just like ours. This is explained a bit more later in this blog.
Dr Scott Coonrod is Professor of Cancer Biology and Director at The Baker Institute for Animal Health in New York USA. Scott explains that the breast cancer found in dogs (and very occasionally in cats) is the same at a cellular level as human breast cancer, also research has shown that it occurs more often in dogs than in humans.
What causes breast cancer in dogs
Breast cancer occurs in the dog’s mammary glands (breasts). Intact (unspayed or undesexed) female dogs have fully developed mammary tissues.
- Estrogen is a hormone produced by the dog’s ovaries; it promotes the development of female characteristics in the body, such as mammary glands.
- Estrogen production in the female body has a stimulant effect on the growth of mammary glands as a female dog sexually matures. As the mammary glands grow, so too does the risk of developing breast cancer.
- Both Estrogen production and DNA/Genetics are linked to the development of breast cancer.
- Evidence-based, peer-reviewed scientific studies have found that if a female dog is desexed (spayed) before the onset of sexual maturity (around 6 months old), the spay surgery removes the ovaries’ ability to begin estrogen production. Estrogen is responsible for the full development of the mammary glands as well as being linked to the stimulation of cancer cells in the mammaries.
Whilst other contributing factors can include obesity from a young age, and breed of dog, the strongest predisposition and cause for canine mammary tumours (breast cancer) are a combination of genetic (DNA) and Estrogen production in intact female dogs (unpsayed). The longer the Estrogen ‘stimulant’ is ‘turned on’ during a female dog’s life, the higher her risk for developing breast cancer.
A 2015 study into mammary tumours in dogs published that;
“…It is widely known that females are predisposed to present this disease because of the tropism of natural estrogens in relation to the mammary gland that are capable of stimulating cell proliferation and generating carcinogenesis through potential cells.” (Salas Y, Márquez A, Diaz D, Romero L. Epidemiological Study of Mammary Tumors in Female Dogs Diagnosed during the Period 2002-2012: A Growing Animal Health Problem.)
Risks as dogs age
According to The American College of Veterinary Surgeons, the risk of a female dog developing breast cancer in her life is;
- 0.5% risk if spayed before her first heat cycle
- 8% risk if spayed after her first heat
- 26% risk if spayed after her second heat
- Furthermore, over 50% of breast tumours found in dogs are malignant.
Detecting and diagnosing breast cancer (canine mammary tumours)
Dog owners usually find a lump on their dog’s abdomen, where the mammary glands are. Other signs include
- Pain in touching the abdomen
- One or more swollen mammary/hard mass on the stomach/inflammation
- Discharge from one or more of the nipples.
- Ulcerations on the skin on the stomach
- Lethargy/malaise
- Significant weight loss at later stages of the cancer
- Mammary glands of undesexed females may present with lumps that dissipate after a heat cycle, but if these lumps do not disappear after a heat,
- or if any of these signs are found in an entire (undesexed) male dog.
- You should take your dog to the vet immediately if you find any of these symptoms on your dog, especially if she is undesexed. Your vet will examine the dog, determining whether the mass is in the mammary glands. Diagnostics will be required to determine if the mass is a tumour, and if so, what type of tumour (benign or malignant).
Veterinary Testing/Diagnostics
As well as a physical examination, your vet may perform any number of tests to diagnose your dog correctly;
- Pathology (histopathology, blood work and urinalysis)
- Radiology; Ultrasound of the abdomen, chest X-Rays to check for cancer spread.
- Any other diagnostic to determine if the cancer has spread elsewhere in the body, e.g., lymph nodes.
- Fine needle aspiration and Biopsy.
Treatment
If Canine Mammary Cancer is confirmed, surgery is the preferred method of treatment. Usually, the tumour is removed with the mammary (mastectomy) rather than just the tumour alone (lumpectomy). The mammary chain (glands either side) of the tumour may also be removed at times either as a precaution, or because the cancer has spread. However, if the cancer is metastatic and has already spread throughout the body, mastectomy surgery may not be an option.
The best course of treatment depends upon a variety of factors for each individual case; the diagnosis by the veterinarian, the cancer stage, size and type of the tumour, the best prognosis for the dog’s recovery, the age of the dog, balanced with the realistic and viable options and wishes for the owners of the dog (e.g., financial, age of the dog, level of cancer, chances of survival, palliative options). Ultimately, the most important factor is that the dog does not needlessly suffer.
Where surgery is considered as the best course of treatment, veterinary surgeons may likely recommend also spaying the dog at the same time as tumour removal. This is because spaying (ovariohysterectomy) will reduce the chance of a recurring mammary tumour.
Other cancer treatment options:
VSSO does not recommend hormonal therapy due to it lacking effectiveness, pointing to ‘a lack of steroid receptors in malignant mammary tumours’. Likewise, if the diagnosis of mammary cancer for your dog is Inflammatory Mammary Carcinoma, surgery is not recommended, but rather Radiation therapy coupled with nonsteroidal inflammatory medication (NSAIDS). Chemotherapy is usually reserved for very advanced cases of the disease, but the cost is exorbitant. Furthermore, some veterinarians may dispute the ethics of allowing pets to undergo the ongoing side-effects of chemotherapy treatment.
Post-surgery and recovery
The best chances of complete remission for a dog recovering from a mammary tumour is if it is
Detected early, and
Removed completely.
The size and spread of a tumour will often determine your dog’s recovery from cancer. There is a poorer prognosis for the dog if the tumour is larger and has already spread (metastasised) throughout the body.
After removal by a veterinarian, tumours are sent for testing (histopathology) to test that the surgery had ‘good margins’, that is, all the tumour mass was removed from the body successfully. Some tumours are worse than others, if the dog’s tumour is a sarcoma or a carcinoma, the prognosis is much poorer for the dog. As recurrence of breast cancer after surgery is possible in other mammary or spreads to other parts of the body, regular testing is recommended by way of chest X-Rays and abdominal Ultrasounds, every 3 to 6 months.
It’s important and sobering to note that half of all mammary tumours in dogs are malignant and sadly 50-75% will likely kill the dog through recurrence or spread to the lungs within 2 years. (Source, Canine Cancer Awareness)
What about breeding dogs?
If all female dogs on the planet were spayed by 6 months, it wouldn’t take long before there were no dogs at all. Internationally, reputable and registered dog breeders are generally aware of these risks and take precautions to breed responsibly; they monitor their dogs for symptoms of breast cancer (canine mammary tumours), they breed with reputable lines, and the accepted practice is for female breeding dogs to retire from breeding (being mated) by approximately 4 to 5yrs of age, at which time they are also spayed.
Conclusion
The best prognosis for dogs with canine mammary cancer is early detection, but science shows that the best preventative measures owners can take is having their female dog desexed before their first heat, about 6 months of age.
Facts at a glance:
- The usual age for a female dog to develop a mammary tumour is between ages 6 to 10 years. Rarely occurs in dogs younger than 4 years.
- Lifetime risk for an undesexed female dog in developing mammary cancer (breast cancer) is 23 to 34%.
- Multiple mammary tumours can occur at once and are common (more than one mammary gland).
- Mammary Tumours account for 82% of reproductive tumours in female dogs, and 42% of all tumours in female dogs. 41-53% of these being malignant.
- Common areas of breast cancer spread (metastasize) are lymph nodes, lungs, adrenal glands, kidney, heart, liver, bone, brain, skin.
- Evidence points to dogs being fed a high fat diet/obesity at a younger age are at an increased risk of developing mammary cancer.
- Male dogs can develop mammary cancer, it is usually malignant.
- Breast cancer accounts for about 42% of all diagnosed cancer tumours found in female dogs and has a lifetime risk of 23-34%. Compare this to female humans, whose lifetime risk is 12.4%
- © Mittagong Vet Hospital 2023. All rights reserved.
Sources:
Cornell University College of Veterinary Medicine, Riney Canine Health Center.
Ohio State University Veterinary Medical Center, Hummel Trueman Hospital for Companion Animals
Epidemiological Study of Mammary Tumors in Female Dogs Diagnosed during the Period 2002-2012: A Growing Animal Health Problem. PLoS One. 2015. Salas Y, Márquez A, Diaz D, Romero L.
Veterinary Society of Surgical Oncology
American Breast Cancer Organisation.