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
BIOLOGIC BEHAVIOR
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
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).
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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
PATHOLOGY
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
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