NAVLE Reproductive

Canine Mammary Gland Tumors – NAVLE Study Guide

March 28, 2026 25 min read 17 views
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).

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.

High-YieldRemember the "Rule of 50s" for canine mammary tumors: 50% of intact female dogs will develop mammary tumors, 50% of those tumors are malignant, and 50% of malignant tumors will metastasize. This makes early detection and spaying critical.
Risk Factor Clinical Significance
Spay Status (Most Important) Spayed before 1st heat: 0.5% risk Spayed after 1st heat: 8% risk Spayed after 2nd heat: 26% risk Intact: 7x greater risk than spayed
Hormonal Exposure Estrogen and progesterone stimulate mammary epithelial proliferation Synthetic progestins (e.g., medroxyprogesterone) increase risk of benign tumors Growth hormone and IGF-1 contribute to tumor development
Obesity Obesity at 1 year of age increases risk Adipose tissue produces estrogens via aromatase Associated with higher-grade tumors and shorter survival
Diet High-fat diet and red meat consumption associated with increased risk

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.

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