Mammary gland tumors and mammary cancer can affect both dogs and cats. Learn how mammary tumors are treated and how spaying your pet can help prevent them.
Tumors of the mammary glands of female pets may be benign or malignant (cancerous). Benign tumor types include adenomas, fibroadenomas, mixed tumors, or duct papillomas. Malignant tumors, akin to “breast cancer” in humans, are carcinomas (including tubulopapillary, solid, or anaplastic carcinomas). Older animals are affected much more frequently than younger animals, especially unspayed females or those not spayed before 2 years of age. Male dogs may rarely be affected, and mammary tumors in males carry a worse prognosis than for females.
Mammary Tumor Statistics in Dogs & Cats
- Dogs
- Mammary tumors are common in dogs. Over 25% of unspayed female dogs will develop a mammary tumor at some point (that’s one in four).
- 50% are malignant, 50% are benign. Few of the malignant types are fatal.
- Any dog breed can be affected, especially poodles, dachshunds, and spaniels.
- Cats
- Mammary tumors are rare in cats.
- 85% are malignant. These tumors are very aggressive and tend to invade locally and spread by the time of diagnosis.
- Any cat can be affected, but Siamese and other Oriental breeds as well as domestic shorthairs are most commonly affected.
- Of those mammary tumors that are malignant, 50% will have metastasized (or spread to distant locations in the body) by the time of diagnosis, thus carrying a much worse prognosis.
Cause and Risk Factors of Mammary Tumors
The trigger of most mammary tumors is the influence of sex hormones. Therefore, spaying a dog or cat before her first heat cycle (which generally occurs by 6 months of age) greatly reduces the risk of developing mammary tumors later in life; this is especially true for dogs. The risk of mammary gland neoplasia increases with each subsequent heat cycle.
When a dog is spayed before her first heat (which can be a silent heat with no obvious signs), the risk of developing mammary neoplasia (or cancer) is only 0.5% likelihood; if she is spayed after her first heat, the risk rises to 8%; if she is spayed after her second heat cycle, the risk jumps to 26%.
For cats undergoing a spay before 6 months of age, her risk of mammary cancer is reduced by 91%; if she is spayed before 1 year of age, the risk is 86%; if she is spayed before 2 years old, the risk reduction is only 11%.
Once a cat has reached 2 years and a dog has reached 2.5 years of age, spaying has minimal effect to reduce the growth of mammary tumors; spaying is still recommended, however, to prevent other unrelated diseases, such as pyometra (uterine infection).
Regardless, spaying just prior to the time of mastectomy (mammary tumor removal surgery) is still recommended since removal of hormonal input may increase the survival of pets with malignant tumors. However, spaying at a young age can increase risk of degenerative joint disease (DJD) and osteoarthritis in large and giant breed dogs; to protect their joint health, spaying is not recommended until after 12-24 months of age. Therefore, speak with your veterinarian to carefully consider the advantages (reduction of mammary cancer) vs. disadvantages (higher risk of joint disease) of spaying your dog before her first heat cycle.
Obesity can also increase the risk of mammary neoplasia.
Learn more about the benefits of spaying or neutering your pet HERE.
Diagnosis of Mammary Tumors in Dogs & Cats
Diagnosis involves physical examination (a lump or ulcer is detected), fine needle aspirate or biopsy of the mass and nearby lymph nodes (for cellular/tissue analysis), and thoracic radiographs and abdominal ultrasound to evaluate the possibility of metastasis.
Treatment of Mammary Tumors in Dogs & Cats
Treatment of both benign and cancerous mammary tumors involves surgical removal of the mass. Following surgery, routine rechecks are recommended.
For dogs, surgery involves either a lumpectomy (removal of the mass), mastectomy (removal of the entire mammary gland from whence the mass is arising), regional mastectomy (several adjacent mammary glands are removed), or chain mastectomy (all mammary glands on one or both sides of the body are removed); no difference in recurrence rates or survival rates have been determined between the different surgical approaches. Nearby lymph nodes will also be removed.
Chemotherapy may also be considered along with surgery, but an oncologic role has not been fully determined for its effectiveness with mammary neoplasia.
If an inflammatory type carcinoma is involved, surgery is not helpful due to a poor prognostic outlook, and radiation with NSAIDs are recommended instead.
For cats, aggressive treatment is required; radical resection (unilateral or bilateral chain mastectomy) is recommended to decrease the risk of illness and increase the chance for survival (can decrease the risk of recurrence by up to 100% if metastasis has not yet occurred).
If metastasis has already occurred, surgery may not be an option, and palliative, hospice care may be recommended instead.
Prognosis of Mammary Tumors in Dogs & Cats
The prognosis for dogs following surgical resection is generally quite good depending on the tumor type. The prognosis is better for small, non-inflammatory tumors that have not yet metastasized. However, following mastectomy for a solo tumor, 58% of dogs will develop a new tumor in a mammary gland on the same side as the one that was removed. Therefore, pet owners need to feel the mammary glands monthly for new tumor development and alert their vet of results.
For cats, the prognosis is worse than for dogs since the majority of cats with mammary neoplasia have a malignant type.
Benign masses can be cured, and many pets can enter remission and live for several years following the early detection and surgical removal of a non-metastatic mammary tumor. Thus, make sure you’re taking your pet to the vet for routine check-ups and schedule a veterinary appointment at the first instance you notice a lump or ulcer on your pet’s belly.
– Maranda Elswick, DVM
