Mammary tumors are the third most common tumor type seen in the feline cat, and they account for approximately 20% of cancer in the female cat. These tumors arise from the mammary tissue and are typically very invasive with a high metastatic rate. Most mammary tumors in cats are classified as adenocarcinomas. Mammary cancer is often a disease of middle aged to older cats, with Siamese cats having a higher risk. Similar to breast cancer in women, hormonal influences are involved in the development of mammary cancer in cats. Several studies have found that cats spayed early in life had a 40-60% reduced risk of developing mammary cancer.
Surgery can be very effective at removing the masses, but the success of surgery may be hindered by the invasive nature of the disease. Therefore, an aggressive surgery is typically recommended, which involves removing all four mammary glands on the affected chain (or both chains if needed), called unilateral or bilateral mastectomy. Studies have shown that more aggressive (called radical chain resection) significantly reduces the chance for local tumor recurrence. Radical resection generally will entail the removal of the draining lymph node which is evaluated via histopathology and the presence/absence is prognostic. Due to the high rate of recurrence and metastasis, chemotherapy is recommended following surgery. Most effective protocols involve the use of the chemotherapeutic agent, doxorubicin, alone or in combination with other drugs. Doxorubicin is generally well tolerated with minimal side effects that are generally self-limiting in nature.
In human oncology, the use of targeted therapies like Herceptin (a treatment that targets certain molecular and genetic defects found in cancer cells) holds great promise for improving the outcome of therapy for the treatment of mammary cancer. The HER-2/neu protooncogene encodes a transmembrane tyrosine receptor kinase protein that has been associated with an increased metastatic propensity and a decreased prognosis. Approximately 25–30%of malignant cells in human breast cancer also overexpress this protein. This has been evaluated and shown to be over-expressed in feline mammary tumors. In veterinary medicine, two tyrosine kinase inhibitors exist Palladia (toceranib) and Masivet (masitinib) with anecdotal response noted in cats treated with Palladia.
Survival time for cats with mammary cancer is variable and depends on several factors including breed (Siamese = worse)size of the tumor, type of surgery (radical associated with better prognosis), tumor grade, proliferation indices (high indices – poor), Cox-2 expression (high-poor), presence/absence of metastasis, and is believed the addition of chemotherapy prolongs survival. The average survival times range from 4 months to over 3 years depending on these factors. Cats with tumors greater than 3 cm had a significantly worse overall prognosis, with survival times ranging from 5 to 10 months.
In canine dogs, mammary tumors are the most commonly diagnosed tumor in intact females older than 7 years of age. Male dogs can also develop mammary tumors, but rarely. Several breeds are prone to developing mammary cancer including Poodles, English Spaniels, English Setters, and Terriers. About 50% of mammary tumors are malignant and 50% are benign—this is very similar to the statistics for breast cancer in women. Hormonal influences have been found to be involved in the development of canine mammary cancer. It has been well documented that spaying (ovariohysterectomy) before the first heat cycle (typically before 6 months of age) dramatically decreases the risk of developing mammary tumors to a less than 1% risk. If spayed after the first heat cycle, the risk of getting mammary cancer increases to 8%. The risk increases to 26% if a dog is spayed after the third heat cycle.
As with felines, surgery is the treatment of choice for all dogs with mammary gland tumors. The type of surgery performed depends on the extent of disease. If only one or two tumors are present, removal of just those tumors is typically adequate. If there are multiple tumors present, a more aggressive surgery (removal of an entire chain, or both chains, of mammary glands) may be recommended. Chemotherapy may be recommended in some cases, such as with high grade, aggressive tumor type, very invasive and/or non-resectable tumors, or the presence of metastatic disease. Radiation therapy may be recommended for certain types of mammary cancers. This treatment is not as routinely used in veterinary oncology as it is for the treatment of breast cancer in people.
Survival time for dogs with mammary tumors is variable and depends on factors including the tumor size, grade of the tumor, tumor type, the presence of metastatic disease, lymphatic/vascular invasion as well as what treatment regimens are used. The average survival times range from less than a month (for aggressive inflammatory carcinomas) to several years.
- Borrego, J. F., J. C. Cartagena, and J. Engel. “Treatment of feline mammary tumours using chemotherapy, surgery and a COX‐2 inhibitor drug (meloxicam): a retrospective study of 23 cases (2002–2007)*.”Veterinary and comparative oncology 4 (2009): 213-221.
- Winston, J., et al. “Immunohistochemical detection of HER‐2/neu expression in spontaneous feline mammary tumours.”Veterinary and comparative oncology1 (2005): 8-15.
- Philibert, Jeffrey C., et al. “Influence of host factors on survival in dogs with malignant mammary gland tumors.”Journal of Veterinary Internal Medicine 1 (2003): 102-106.