When we think of breast cancer, we usually think of people. But dogs (and cats) get breast cancer too. According to the ACVS (American College of Veterinary Surgeons), “more than a quarter of unspayed female dogs will develop a mammary tumor during their lifetime. The risk is much lower for spayed female dogs, male dogs, and cats of either gender. In female dogs, 50% of mammary tumors are benign and 50% are malignant. However, few of the malignant mammary tumors are fatal. In contrast, over 85% of mammary tumors in cats are malignant and most of these have an aggressive biologic behavior (i.e., mammary tumors in cats tend to be locally invasive and spread elsewhere in the body).”
It’s comforting to know that not all mammary tumors are cancerous. The most common benign form of canine mammary tumors is actually a mixture of several different types of cells. For a single tumor to possess more than one kind of cancerous cell is actually rare in many species. This combination cancer in the dog is called a ‘benign mixed mammary tumor’ and contains glandular and connective tissue. Other benign tumors include complex adenomas, fibroadenomas, duct papillomas, and simple adenomas.
Nevertheless, there are multiple types of mammary cancers in dogs. All mammary tumors should be identified through a biopsy and histopathology (microscopic examination of the tissue) to help in developing the treatment plan for that particular type of cancer. The malignant mammary tumors include: tubular adenocarcinomas, papillary adenocarcinomas, papillary cystic adenocarcinomas, solid carcinomas, anaplastic carcinomas, osteosarcomas, fibrosarcomas, and malignant mixed tumors.
It is important to know that mammary cancer does not refer to a single disease but there are many different subtypes, each responding differently to treatment and each with a different prognosis. Pet Education identifies these sub-classifications of mammary tumors is as follows:
Benign tumors
Adenoma
Simple adenoma
Complex adenoma
Basaloid adenoma
Fibroadenoma
Low-cellularity fibroadenoma
High-cellularity fibroadenoma
Benign mixed tumor
Duct papilloma
Malignant tumors
Non-infiltrating carcinoma
Complex carcinoma
Simple carcinoma
Tubulopapillary carcinoma
Solid carcinoma
Anaplastic carcinoma
Special types of carcinomas
Spindle cell carcinoma
Squamous cell carcinoma
Mucinous carcinoma
Lipid-rich carcinoma
What are the symptoms of mammary cancer in dogs?
Mammary tumors are observed as a solid mass or as multiple swellings. When tumors do arise in the mammary tissue, they are usually easy to detect by gently palpating the mammary glands. When tumors first appear they will feel like small pieces of pea gravel just under the skin. They are very hard and are difficult to move around under the skin. They can grow rapidly in a short period of time, doubling their size every month or so.
It is very difficult to determine the type of tumor based on physical inspection. A biopsy or tumor removal and analysis are almost always needed to determine if the tumor is benign or malignant, and to identify what type it is. Tumors, which are more aggressive may metastasize and spread to the surrounding lymph nodes or to the lungs. A chest x-ray and physical inspection of the lymph nodes will often help in confirming this.
Mammary cancer spreads to the rest of the body through the release of individual cancer cells from the various tumors into the lymphatics. The lymphatic system includes special vessels and lymph nodes.
What is the treatment of mammary cancer in dogs?
Surgical Removal: Upon finding any mass within the breast of a dog, surgical removal is recommended unless the patient is very old. If a surgery is done early in the course of this disease, the cancer can be totally eliminated in over 50% of the cases having a malignant form of cancer. The area removed depends on the judgment of the veterinarian. Sometimes only the mass itself will be removed. Other times, taking into consideration how the cancer spreads, the mass and the rest of the mammary tissue and lymph nodes that drain the gland will be removed.
Owners may confuse a surgical removal of a mammary gland in the dog with a radical mastectomy in humans, with all of the associated problems. In humans, this type of surgery would affect the underlying muscle tissue which complicates the recovery. In the dog, however, all of the breast tissue and the related lymphatics are outside of the muscle layer, so we only need to cut through the skin and the mammary tissue. This makes the surgery much easier and recovery much faster. A radical mastectomy in a dog means all the breasts, the skin covering them, and the four lymph nodes are all removed at the same time. Although this is truly major surgery, suture removal usually occurs in 10 to 14 days with normal activity resuming at that point.
How can I prevent mammary cancer in my dog?
There are few cancers that are as easily prevented as mammary cancer in dogs. There is a direct and well-documented link between the early spaying of female dogs and the reduction in the incidence in mammary cancer. Dogs spayed before coming into their first heat have an extremely small chance of ever developing mammary cancer. Dogs spayed after their first heat but before 2.5 years are at more risk, but less risk than that of dogs who were never spayed, or spayed later in life. We all know the huge benefits of spaying females at an early age, but every day, veterinarians still deal with this easily preventable disease. Early spaying is still one of the best things pet owners can do to improve the health and ensure a long life for their dogs.
Conclusion
Mammary cancer is a very common cancer and can often be successfully treated, if caught early. If all non-breeding dogs and cats were spayed before their first heat this disease could be almost completely eliminated. If you find a growth or lump in the mammary tissue of your dog, you should inform your veterinarian immediately and not take a “wait and see” attitude.
To learn more about breast cancer in dogs, and treatment and surgical options, there are several helpful and detailed sites on the Internet you may consult:
The Bark http://thebark.com/content/breast-cancer-dogs
Mammary cancer in dogs: http://www.peteducation.com/article.cfm?c=2+2087&aid=460
Petwave: Symptoms of breast cancer in dogs: http://www.petwave.com/Dogs/Health/Breast-Cancer/Symptoms.aspx
Another article on symptoms can be found at VetInfo: https://www.vetinfo.com/dog-breast-cancer-symptoms.html
AVCS has helpful information, including photos, on mammary tumors here. Mammary tumors: https://www.acvs.org/small-animal/mammary-tumors
Pet Cancer Center is another good source, with detailed diagrams, prognosis, and outcomes. http://www.petcancercenter.org/Cancer_Types_Breast_Dogs.html
