Canine Cancer - Mammary Tumors
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| Characteristics / Terms | Hemangiosarcoma | Lymphoma |
| Histiocytomas | Mammary Tumors | Testicular Tumors |
| Mast Cell Tumors | Bone Cancer |
Mammary Tumors in Dogs
Mammary tumors are the most common tumors in female dogs that have not been spayed. Mammary tumors can vary from small, simple nodules to large, aggressive, metastatic growths. With early detection and prompt treatment, even some of the more serious tumors can be successfully treated. Cats also suffer from mammary tumors, and they have their own unique set of problems that will be discussed in a separate article.
Dogs At Risk of Developing Mammary Tumors
Mammary tumors are more common in unspayed, middle-aged female dogs (those between 5 and 10 years of age), although they can, on rare occasions, be found in dogs as young as 2 years. (They are rare in dogs that were spayed at less than 2 years of age.) Occasionally, mammary tumors will develop in male dogs (in the same way that men can sometimes develop breast cancer), and these are usually very aggressive and have a poor prognosis.
Spaying greatly reduces the chances of a female dog developing this condition. In those females spayed prior to their first heat cycle, mammary tumors are extremely rare. The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dogs spayed after one heat, and 26% in dogs spayed after their second heat. The elimination or reduction of certain hormones causes the lowering of incidence of the disease in dogs that have been spayed—this is the scientific consensus. These hormones are probably estrogen, progesterone, a similar hormone, or possibly a combination of two or more of these.
Types of Canine Mammary Tumors
There are multiple types of mammary tumors in dogs. Approximately one-half of all mammary tumors in dogs are benign, and half are malignant. All mammary tumors should be identified through a biopsy and histopathology (microscopic examination of the tissue) to help in the treatment of that particular type of tumor. This is, of course, a role for the veterinarian.
The most common benign form of canine mammary tumors is actually a mixture of several different types of cells. Single tumors rarely possess more than one type of cancerous cells. 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.
The malignant mammary tumors include the following: tubular adenocarcinomas, papillary adenocarcinomas, papillary cystic adenocarcinomas, solid carcinomas, anaplastic carcinomas, osteosarcomas, fibrosarcomas, and malignant mixed tumors.
Symptoms
Mammary tumors present as a solid mass or as multiple swellings. When tumors do arise in the mammary tissue, they are usually easy to detect by gently touching and retouching 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.
The dog normally has five mammary glands, each with its own nipple, on both the right and left side of its lower abdomen. In half of the cases, more than one growth is observed. Benign growths are often smooth, small, and slow growing. Signs of malignant tumors include rapid growth, irregular shape, firm attachment to the skin or underlying tissue, bleeding, and ulceration. Occasionally tumors that have been small for a long period of time may suddenly grow quickly and aggressively, but this is the exception and not the rule.
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 whether 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 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. There are regional lymph nodes on both the right and left sides of the body under the front and rear legs. They are called the "axillary" and "inguinal" lymph nodes, respectively.
Prevention of Mammary Tumors in Dogs
Few cancers 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 risk of mammary cancer. Dogs spayed before 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 greater risk, but face less risk than dogs that were never spayed or spayed later in life. Early spaying is still one of the best things pet owners can do to improve the health of their dogs.
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