Feline Mammary Adenocarcinoma – NAVLE Study Guide

Overview and Clinical Importance

Feline mammary adenocarcinoma is a highly aggressive malignant neoplasm originating from the glandular epithelium of mammary tissue. It represents the third most common tumor type in cats (after lymphoma and skin tumors), accounting for approximately 17% of all feline tumors. Unlike dogs, where approximately 50% of mammary tumors are benign, 80-90% of feline mammary tumors are malignant, with adenocarcinoma being the most common histologic type. This disease predominantly affects older, intact female cats and carries a guarded to poor prognosis due to high metastatic potential.

Epidemiology and Risk Factors

Signalment

Feline mammary tumors typically affect older female cats between 10-12 years of age. While any breed can be affected, Siamese cats have twice the risk of developing mammary carcinoma compared to other breeds and tend to present at a younger age. Domestic shorthair cats are commonly affected due to their prevalence in the general population. Male cats can develop mammary tumors but this is rare and typically associated with exogenous progestin administration.

Hormonal Influence and Protective Effect of Spaying

Hormonal status is the most significant modifiable risk factor. Intact queens have a 7-fold higher risk of developing mammary tumors compared to spayed cats. The protective effect of ovariohysterectomy is age-dependent and diminishes rapidly:

Risk Reduction by Age at Spaying

Exogenous Hormones

Administration of exogenous progestins (such as medroxyprogesterone acetate for estrus suppression or behavioral modification) increases the risk of developing both benign and malignant mammary tumors by 3-fold. This risk applies to both males and females receiving progestin therapy.

Anatomy of the Feline Mammary Gland

Cats have four pairs of mammary glands (eight total) arranged in two parallel chains along the ventral thorax and abdomen:

• Thoracic glands (T1, T2): Located in the cranial thoracic region
• Abdominal glands (A1, A2): Located in the caudal abdominal/inguinal region

Lymphatic Drainage

Understanding lymphatic drainage is critical for surgical planning:

Pathophysiology and Tumor Biology

Histologic Classification

The majority of feline mammary tumors are malignant epithelial neoplasms (carcinomas). The most common histologic subtypes include:

• Simple tubular/tubulopapillary adenocarcinoma (most common)
• Solid carcinoma
• Cribriform carcinoma
• Mucinous carcinoma
• Inflammatory carcinoma (rare but carries worst prognosis)

Unlike in dogs, complex and mixed mammary tumors are rare in cats. Benign tumors such as fibroadenoma, simple adenoma, and duct papilloma comprise only 10-15% of feline mammary tumors.

Histologic Grading

Feline mammary carcinomas are graded using the Elston and Ellis (Nottingham) grading system, adapted from human breast cancer classification:

Grade Determination: Grade I (well differentiated) = 3-5 points; Grade II (moderately differentiated) = 6-7 points; Grade III (poorly differentiated) = 8-9 points

Metastatic Behavior

Feline mammary adenocarcinomas are highly metastatic. 50-90% of cases have metastasis at necropsy. Common metastatic sites include:

• Regional lymph nodes (axillary and inguinal) - most common initial site
• Lungs (often miliary pattern; may cause pleural effusion)
• Pleura (pleural carcinomatosis)
• Liver, kidney, adrenal glands
• Bone (distal extremities more common than axial skeleton)

Clinical Presentation

Physical Examination Findings

Mammary tumors typically present as:

• Single or multiple palpable masses (60% of cats have more than one tumor at diagnosis)
• Firm, nodular consistency
• Located within or adjacent to mammary glands along the mammary chain
• May be freely movable or fixed to underlying tissues
• Ulceration may be present in approximately 25% of cases
• Regional lymphadenomegaly (axillary or inguinal)

Important: Nearly 50% of cats with distant metastasis show no clinical signs of systemic illness. Many cats present in good body condition despite having advanced disease.

Inflammatory Mammary Carcinoma

This rare but highly aggressive variant presents with swollen, hot, painful mammary glands due to lymphatic obstruction. Clinical signs mimic mastitis but occur in non-lactating cats. Prognosis is extremely poor with median survival of only weeks.

Diagnostic Workup

Minimum Database

• Complete blood count (CBC): Usually within normal limits; may show anemia with advanced disease
• Serum biochemistry panel: Evaluate organ function; rule out concurrent disease
• Urinalysis: Baseline assessment before anesthesia and treatment
• FeLV/FIV testing: Assess viral status (not etiologically related but affects treatment decisions)

Tumor Assessment

• Fine-needle aspirate (FNA): Can confirm epithelial neoplasia; more reliable in cats than dogs due to high malignancy rate
• Histopathology: Gold standard for definitive diagnosis, grading, and assessment of lymphovascular invasion
• Tumor measurement: Critical prognostic factor - measure with calipers

Staging

Complete staging is recommended for ALL mammary masses in cats due to the high malignancy rate. Staging includes:

• Three-view thoracic radiographs: Evaluate for pulmonary metastasis (miliary pattern) and pleural effusion
• Abdominal ultrasonography: Assess medial iliac lymph nodes and abdominal organs
• Regional lymph node FNA: Sample axillary and inguinal lymph nodes even if normal size
• CT scan: More sensitive for detecting pulmonary metastasis than radiographs

TNM Staging System

The modified WHO TNM staging system is used to classify feline mammary tumors:

Clinical Staging and Prognosis

Treatment

Surgical Treatment

Surgery is the mainstay of treatment for feline mammary adenocarcinoma. Due to the high malignancy rate and bidirectional lymphatic drainage, radical (chain) mastectomy is strongly recommended over regional or local excision.

Lymph Node Excision: The inguinal lymph node is routinely removed with A2 gland as it is embedded within the fat pad. Axillary lymph node removal is recommended if enlarged or FNA-positive.

Concurrent Ovariohysterectomy: There is no evidence that OVH at time of mastectomy improves survival or prevents recurrence. However, it may be performed to prevent progestin therapy if needed for other conditions.

Chemotherapy

The role of adjuvant chemotherapy remains controversial with conflicting results in clinical studies. No prospective randomized controlled trials have demonstrated significant survival benefit. However, chemotherapy may be considered for:

• Cats with lymph node metastasis
• Grade III tumors
• Lymphovascular invasion on histopathology
• Incomplete surgical margins
• Non-resectable or metastatic disease

Common Chemotherapy Protocols

Prognostic Factors

Multiple factors influence prognosis in cats with mammary adenocarcinoma:

Differential Diagnoses

When evaluating a mammary mass in a cat, consider:

• Fibroadenomatous hyperplasia (mammary hypertrophy): Progesterone-dependent; typically affects young cats; may regress with OVH
• Mastitis: Typically in lactating queens; painful, warm, erythematous; systemic illness
• Benign mammary tumors: Fibroadenoma, adenoma (rare in cats - only 10-15% of tumors)
• Other malignancies: Mammary sarcoma, squamous cell carcinoma, carcinosarcoma
• Lipoma or other subcutaneous masses: Usually distinguishable by location and consistency