Canine Mammary Gland Tumors – NAVLE Study Guide

Overview and Clinical Importance

Mammary gland tumors (MGTs) are the most common neoplasm in intact female dogs, representing 50-70% of all tumors diagnosed in unspayed bitches. They are the second most common tumor in dogs overall (after skin tumors). Understanding the epidemiology, classification, staging, treatment options, and prognosis is essential for NAVLE success and clinical practice.

Approximately 50% of mammary tumors are malignant, and of those malignant tumors, about 50% will metastasize and cause patient death within 1-2 years if not treated. The most common sites of metastasis include regional lymph nodes and lungs.

Epidemiology and Risk Factors

Patient Demographics

Age: Mean age of presentation is 10-11 years (range 5-15 years). Tumors are rare in dogs younger than 5 years.

Sex: Female dogs account for greater than 99% of cases. Male dogs represent less than 1% and typically have worse prognosis.

Breed Predisposition: Poodles, Dachshunds, Spaniels (especially Cocker and English Springer), German Shepherds, Pointers, Yorkshire Terriers, Chihuahuas, and Boxers show increased incidence. Mixed breeds may have lower incidence in some studies.

Risk Factors for Mammary Tumor Development

Anatomy and Lymphatic Drainage

Dogs have 4-5 pairs of mammary glands extending from thorax to inguinal region. The glands are numbered cranial to caudal: T1 (cranial thoracic), T2 (caudal thoracic), A1 (cranial abdominal), A2 (caudal abdominal), and I (inguinal).

Key Anatomical Points

• Caudal glands (A2 and I) are most commonly affected (60% of cases)
• 50-70% of dogs have multiple tumors involving more than one gland
• Both mammary chains are affected with equal frequency
• Multiple tumors may have different histologic types within the same patient

Lymphatic Drainage Pattern

Histological Classification

The WHO/Goldschmidt classification system (2011) is the current standard for classifying canine mammary tumors. Tumors are classified based on cell type of origin (epithelial, myoepithelial, or mesenchymal) and behavior (benign vs. malignant).

Tumor Classification Summary

Histologic Grading (Elston-Ellis Method)

Histologic grade is determined by three criteria, each scored 1-3 points:

Grade Interpretation: Grade I (3-5 points) = Well-differentiated; Grade II (6-7 points) = Moderately differentiated; Grade III (8-9 points) = Poorly differentiated

Clinical Presentation and Diagnosis

Clinical Signs

• Palpable mass(es) in mammary chain (most common presentation)
• Single or multiple nodules, ranging from mm to greater than 10 cm
• May be well-circumscribed (benign) or poorly defined/fixed to underlying tissue (malignant)
• Ulceration, discharge, or skin involvement suggests malignancy
• Regional lymphadenopathy (axillary or inguinal)
• Advanced disease: respiratory signs (cough, dyspnea), weight loss, lethargy

Inflammatory Mammary Carcinoma (Special Subtype)

Inflammatory mammary carcinoma (IMC) is a highly aggressive subtype accounting for 4-18% of malignant mammary tumors. Key features include:

• Rapid onset of painful, firm swelling with erythema and edema
• May mimic mastitis clinically (key differential)
• Invasion of dermal lymphatics by tumor emboli causes skin changes
• 96% have lymph node metastasis at diagnosis
• Surgery is NOT recommended (does not improve survival)
• Median survival: less than 1 month (25 days with palliative care)

Diagnostic Workup

TNM Staging System

The modified WHO TNM staging system is used to classify mammary tumors based on Tumor size (T), regional lymph Node involvement (N), and distant Metastasis (M).

Treatment Options

Surgical Treatment (Mainstay of Therapy)

Surgery is the treatment of choice for most mammary tumors (except inflammatory carcinoma and metastatic disease). The goal is complete tumor removal with adequate margins.

Role of Ovariohysterectomy (OVH)

The role of spaying at the time of tumor removal is controversial. Key points:

• OVH at time of mammary surgery may reduce development of NEW tumors
• Most studies show NO survival benefit for dogs with existing malignant tumors
• Exception: Secretory carcinoma (hormone-dependent) - spaying IS recommended
• OVH eliminates pyometra and ovarian disease risk; simplifies future mammary monitoring

Adjuvant Chemotherapy

The role of chemotherapy is NOT well-established in canine mammary tumors. Limited prospective studies show variable efficacy. Consider chemotherapy for:

• Stage III-IV disease (large tumors, lymph node metastasis)
• High-grade (Grade III) tumors
• Incomplete surgical excision
• Lymphovascular invasion on histopathology
• Mammary sarcomas (treat like appendicular osteosarcoma)

Common Chemotherapy Protocols

Prognostic Factors

Multiple factors influence survival in dogs with mammary tumors:

Post-Operative Monitoring

For dogs with malignant mammary tumors, recommended follow-up includes:

• Physical examination every 3 months for the first year, then every 6 months
• Palpate surgical site and regional lymph nodes for recurrence
• Thoracic radiographs every 3-6 months to monitor for pulmonary metastasis
• Abdominal ultrasound periodically if concern for abdominal metastasis
• Monitor for development of new mammary tumors (50-60% of dogs with malignant tumors develop new tumors)