Mammary Tumors

Most owners discover mammary tumors while petting their dog or cat. The signs range from barely noticeable nodules to large, painful masses:

• A firm or soft lump along the mammary chain — anywhere from the chest to the groin
• Multiple lumps in the same chain or across different glands (both are common)
• A mass that has recently grown faster than before
• Skin over the lump that appears reddened, ulcerated, or has discharge
• Swelling or thickening of an entire gland rather than a discrete lump
• In advanced cases: swelling of the hind limbs from lymph-node involvement, difficulty breathing if the tumor has spread to the lungs, or weight loss

Small, early-stage tumors are easy to miss if pets are not regularly checked. Running your hands along the underside of the body during grooming or nail trims is a practical way to detect lumps early.

Mammary tumors are driven primarily by reproductive hormones — estrogen and progesterone stimulate mammary gland cells, and repeated hormonal exposure over multiple heat cycles increases the cumulative risk of malignant transformation. Key risk factors:

• Intact (unspayed) status — by far the strongest risk factor; the relationship between spay timing and risk is steep (near-zero risk if spayed before first heat, rising with each subsequent heat cycle)
• Progestin-containing medications — use of hormonal contraceptives or hormone therapy in intact dogs markedly increases mammary tumor risk
• Age — most dogs are eight to ten years old at diagnosis; cats six to twelve
• Obesity at a young age — excess body fat elevates circulating estrogens
• Breed — Toy and Miniature Poodles, English Spaniels, Maltese, and Yorkshire Terriers are at higher risk in dogs; Siamese cats appear predisposed in that species

Male dogs can rarely develop mammary tumors; when they do, the prognosis is generally poor and the workup is similar.

The definitive answer about any mammary mass requires tissue, not just a needle aspirate — cytology of mammary tumors is often non-diagnostic, and histopathology (formal biopsy or excisional pathology) is required to grade the tumor and assess margins. Our standard work-up includes:

• Physical examination — size, texture, location, number of masses, and regional lymph node assessment (inguinal and axillary nodes are palpated for enlargement)
• Fine-needle aspirate — while less diagnostic for mammary tumors than for some other cancers, it can identify inflammatory carcinoma and occasionally other cell types; performed in-house
• Chest radiographs (3 views) — screens for pulmonary metastasis before any surgical decision; done in-house with digital radiography
• Abdominal ultrasound — evaluates regional lymph nodes and abdominal organs for spread
• CT scan — for cats and dogs with suspected nodal or pulmonary involvement where precise staging guides surgical planning
• Histopathology — tumor grade and surgical margin status from the pathology lab are the most important prognostic indicators after surgery

Surgery is the primary treatment for mammary tumors in dogs and cats. The extent of surgery depends on tumor size, location, and the number of affected glands:

• Lumpectomy (nodulectomy) — removal of the mass alone; appropriate only for very small (<0.5 cm), well-defined, superficial tumors with low suspicion for malignancy
• Simple mastectomy — removal of the affected gland(s); standard for discrete tumors in one or two glands
• Chain mastectomy (radical mastectomy) — removal of an entire mammary chain; performed when multiple glands are affected or when aggressive biology warrants wider resection; regional lymph nodes may be removed at the same time
• Concurrent spaying — recommended if the pet is intact; whether spaying at the time of mastectomy improves survival is debated for established tumors, but it prevents future hormonal stimulation and removes the risk of pyometra
• Chemotherapy — considered for high-grade malignant tumors or those with positive lymph nodes; coordination with a veterinary oncology specialist guides protocol selection

Inflammatory carcinoma — a particularly aggressive subtype where the entire gland is red, swollen, and painful — has a poor prognosis and surgery is typically not recommended; palliative care and systemic therapy are discussed in these cases.

Any new lump along your dog's or cat's belly — even one that seems small and feels soft — should be evaluated within a few weeks of discovery. Mammary tumors in cats especially should never be watched without biopsy; their aggressive behavior means delay reduces options. If a mass has grown quickly, the skin over it looks abnormal, or your pet seems unwell, seek evaluation sooner. Our Calabasas surgical team can stage, remove, and submit tissue for pathology in one procedure, and we coordinate adjuvant therapy planning based on the pathology results. Book an appointment today.