Breast cancer has distinct cellular, genetic, and molecular factors involved in its progression and growth. Mammary tumors are seen in the canine population with a low frequency, but similarities of the canine and human patient regarding disease biology and treatment options are evident.
Canine Breast Cancer
Mammary tumors are among the most common tumors in female canines. The incidence of these tumors is correlated with overall life expectancy and hormonal influence. Mammary cancer is usually seen in canines between the ages of 10 and 11 years. Between 41 and -53 percent of tumors are considered malignant. The most common histological types include carcinomas, solid carcinomas, tubular carcinomas, papillary carcinomas, and anaplastic carcinomas. Carcinoma in situ is not predominantly recognized in the canine population.
Hormonal influence plays a large role in canine mammary malignancy. It is known that dogs undergoing ovariohysterectomy prior to the first estrus have a 0.05 percent risk for developing malignant tumors. This risk increases to 26 percent if an ovariohysterectomy is performed after the second estrus. Normal canine mammary tissue contains progesterone and estrogen receptors, as do many benign tumors. Approximately half of malignant tumors have these receptors expressed. This finding suggests that a loss of steroid dependency may occur during the cancer progression. Increases in growth hormone (GH), acting through insulin-like growth factor 1 (IGF-1) that increases, are induced by progestins. These increases may stimulate mammary cell proliferation as well.
Many genes are known to be involved in breast cancer formation and progression. Oncogenes expressing for growth factor receptor c-erb-2 (c-neu) have overexpression of mRNA in the majority of canine malignancies, but not benign growth. BRCA1 occurrence, a genetic alteration, is seen in some canine mammary malignancies. It is thought that obesity and diet may also play a role in the development of canine mammary cancer.
Tumor size (greater or less than 3 cm.)