Chemotherapy and Medical Cancer Treatments in Pets: A Comprehensive Guide for Owners

Etiology and Pathophysiology
Tumor Types
The majority of tumors originating from the mammary glands in cats are epithelial neoplasms. Histopathological classification is based on the cellular characteristics of the tumor, growth pattern, degree of invasion, and tissue organization. Therefore, definitive determination of the histological subtype is often not possible based solely on gross appearance or cytological evaluation.
One of the most clinically important characteristics of feline mammary tumors is their high rate of malignancy. Approximately 80–96% of mammary neoplasms are reported to be malignant. Adenocarcinomas are the most frequently encountered malignant tumor group.
Adenocarcinomas may exhibit different histological growth patterns. Non-invasive (in situ) carcinomas and tubulopapillary, solid, and cribriform patterns are among the commonly described forms.
Less frequently encountered malignant histological types include squamous cell carcinoma, mucinous carcinoma, adenosquamous carcinoma, and carcinosarcoma.
Inflammatory mammary carcinoma (IMC) is rare in cats but is particularly important from a clinical perspective. Because it may cause diffuse swelling, redness, increased warmth, edema, and pain in the mammary tissue, its clinical appearance can resemble mastitis. Therefore, the possibility of neoplasia should be included in the differential diagnosis of rapidly progressing mammary lesions with an inflammatory appearance.
Sarcomas originating from the mammary glands are extremely rare in cats.
Not every mammary lesion detected in a cat represents a malignant neoplasm. Benign neoplastic lesions include ductal papillomas, simple or complex adenomas, fibroadenomas, and benign mixed tumors. In addition, non-neoplastic changes such as cysts, ductal ectasia, focal fibrosis, and various forms of hyperplasia may produce a mass-like appearance in the mammary tissue.
Lobular hyperplasia, in particular, may be evaluated under different histological patterns, including epithelial hyperplasia, adenosis, and fibroadenomatous hyperplasia. This demonstrates that classifying a mammary mass as benign or malignant based solely on palpation or imaging characteristics is not reliable.
Therefore, histopathological examination is the primary diagnostic method for definitive characterization of a mammary mass.

Reproductive Status and Hormonal Risk Factors
Hormonal exposure plays an important role in the development of feline mammary tumors. In particular, the cumulative duration of lifetime exposure to ovarian hormones is thought to influence the likelihood of developing mammary neoplasia.
Early ovariohysterectomy or ovariectomy significantly reduces the risk of mammary tumor development later in life. The greatest protective effect is achieved when sterilization is performed before the first estrus.
In cats spayed before the first estrus, the risk of developing mammary tumors has been reported to decrease by approximately 91%. When sterilization is performed before the second estrus, the reduction in risk is approximately 86%. When performed before the third estrus, the protective effect decreases substantially, with the reduction in risk reported to be approximately 11%.
Spaying after approximately two years of age has not demonstrated a significant protective effect against the development of mammary neoplasia.
These findings indicate that early-age sterilization in cats is important not only for reproductive control but also as a preventive approach against mammary neoplasia.

Other Risk Factors
In addition to endogenous hormones, exposure to exogenous hormones is also important in the development of mammary tumors.
Cats exposed to progestin-containing preparations, in particular, are known to have an increased risk of proliferative changes in the mammary tissue and mammary tumor development. The level of risk may be influenced by the specific compound used, dosage, frequency of administration, and total duration of exposure.
Therefore, the potential effects on mammary tissue should be considered in cats receiving long-term progestin therapy for estrus suppression or other indications.
The higher reported incidence of mammary tumors in certain cat breeds suggests that genetic predisposition may also play a role in disease development. However, the molecular and genetic risk factors associated with feline mammary cancer have not yet been characterized as extensively as those associated with human breast cancer or canine mammary tumors.
The role of obesity is another area of ongoing investigation. In dogs, there is evidence suggesting that obesity, particularly during early life, may be associated with an increased risk of mammary neoplasia later in life. A similar association may exist in cats, but this relationship has not yet been established with sufficient strength.

Metastatic Behavior
One of the most important considerations in the clinical management of feline mammary tumors is their high malignant and metastatic potential. Although malignant mammary tumors may differ in their biological behavior, the majority of mammary neoplasms in cats tend to follow an aggressive clinical course. Approximately 80–90% of mammary tumors are reported to be malignant, and the rate of metastatic disease is also considered high.
Tumor cells primarily spread through two pathways:

• Lymphatic spread

• Hematogenous spread

Regional lymph nodes are important sites of metastasis. In addition, the lungs and pleura are particularly important metastatic sites in feline mammary carcinomas. Liver metastases may also occur.
In advanced disease, metastatic lesions may be detected in various anatomical locations, including the adrenal glands, pancreas, diaphragm, peritoneum, kidneys, spleen, brain, skeletal muscles, and bones.
Because of this extensive metastatic potential, evaluation of a cat with a mammary tumor should not be limited to the primary mammary mass. Clinical staging is a fundamental component of determining both the treatment plan and prognosis.

Lymphatic Drainage of the Mammary Glands
The lymphatic drainage of the mammary glands in cats is clinically important; however, the drainage pattern is not always completely predictable. Particularly in neoplastic disease, normal lymphatic pathways may change and alternative drainage routes may develop.
According to the general anatomical distribution:

• The 1st mammary gland (cranial thoracic) primarily drains cranially toward the axillary lymph node.

• The 4th mammary gland (caudal abdominal/inguinal) primarily drains caudally toward the superficial inguinal lymph node.

• The lymphatic drainage of the 2nd and 3rd mammary glands is more variable and may occur in both cranial and caudal directions.

It should also be remembered that lymphatic drainage from different mammary glands to the sternal lymph nodes may occur.
Because lymphatic drainage can vary in neoplastic disease, it is not possible to definitively predict which lymph node may contain metastasis based solely on the anatomical location of the primary tumor.

Clinically Important Considerations
Mammary masses detected in cats should be taken seriously with regard to malignancy, even when they are small or appear clinically localized. Due to the high rates of malignancy and metastasis, early diagnosis, appropriate surgical planning, histopathological evaluation, and accurate clinical staging are critical steps that may directly influence prognosis.
In particular, tumor size, histological type and grade, surgical margins, lymphatic and vascular invasion, regional lymph node involvement, and the presence of distant metastases should be evaluated together to establish an individualized prognostic assessment for each patient.

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