Mammary tumors are the third most common feline cancer,1-3 accounting for 10.3% to 12% of all diagnosed tumors.1,2,4 Female cats are more frequently affected than males, with 25.4 out of 100,000 queens developing mammary tumors.5 Mammary neoplasia in male cats is rare; less than 1% of cats with mammary neoplasia are male.6 Regardless of a cat's sex, mammary tumors most commonly arise in older cats (average age 10 to 12 years). However, the diagnosis of feline mammary tumors is not restricted to geriatric patients, as mammary cancer in cats as young as 9 months old has been reported.1,6-11
Most cats have four sets of mammae, and malignant tumors most commonly arise from the thoracic and inguinal glands.7,12 Malignant mammary tumors readily spread to ipsilateral regional lymph nodes, and sites of local metastases are dictated principally by lymphatic drainage patterns. Thoracic mammary gland tumors drain to the axillary lymph nodes, and inguinal mammary gland tumors drain to the superficial inguinal lymph nodes. The cranial and caudal abdominal mammary glands can drain bidirectionally, so tumors arising from these sites may spread to both axillary and superficial inguinal lymph nodes. Additionally, mammary tumors developing within the thoracic glands or the cranial or caudal abdominal glands may also drain to the cranial sternal lymph nodes.
Recent evidence suggests that mammary tumors may not spread to adjacent mammary glands or contralateral lymph nodes through the lymphatics, as no interglandular lymphatic connections have been identified.13 However, because both mammary chains share a common venous system, the seeding of neoplastic cells between contralateral mammary chains is anatomically possible.7
Common metastatic sites include the lungs (diffuse or nodular metastasis) (Figure 1) and regional lymph nodes, but metastases may also occur in the liver, spleen, brain, and bone.4-8,14-17 Regional lymph node metastasis is reported to be present in more than 25% of cats at diagnosis.4,12 At necropsy, 76% of cats have pulmonary metastasis and 40% have pleural metastasis,12 with up to 93% of cats having one or more sites of metastasis (lymph nodes, lungs, pleura, liver).18
Figure 1. A lateral thoracic radiograph of a female cat presenting for evaluation of inappetence, behavioral changes, and rapid breathing. Radiographic findings are consistent with diffuse pulmonary metastasis (diffuse infiltrates) from a relatively large (> 3 cm) primary mammary tumor (white arrow).
About 90% of feline mammary tumors are malignant.6,7,15,19 Most are carcinomas or adenocarcinomas, with the most common histologic patterns being tubular, papillary, and solid carcinomas.6-8,20 Tumors are graded as well-differentiated (Grade I), moderately differentiated (Grade II), and poorly differentiated (Grade III) based on histologic features including tubule formation, nuclear and cellular atypia, and mitotic index.21 Histologically, invasion into the lymphatic or vascular system or both is common—noted in 27% to 57% of tumor samples.12,22,23 Additionally, infiltration of cancer cells into the surrounding stroma occurs at an even greater frequency, with a reported rate of 42% to 88%.12,23 Although most feline mammary tumors are of epithelial origin, malignant transformation may occur in mesenchymal tissues resulting in the development of mixed mammary tumors and sarcomas.8,21