Cat Mammary Tumor Signs: Lumps, Staging, and Treatment
Feline mammary tumors are the third most common cancer in cats and carry a far worse prognosis than canine mammary tumors — approximately 85–90% are malignant. Intact female cats are at highest risk, but spayed females and even male cats can be affected. The key signs are firm nodules in the mammary chain, often detected first during grooming. Early surgical excision with wide margins is the most important factor in long-term survival.
Last reviewed: June 2026
Who Gets Mammary Tumors?
Intact (unspayed) female cats face dramatically higher risk than spayed females. As described in Withrow and MacEwen's Small Animal Clinical Oncology, cats spayed before their first heat cycle have approximately 9 times lower risk than intact females; spaying before age 6 months dramatically reduces lifetime risk, while spaying after 2 years confers minimal benefit. Siamese cats appear overrepresented in published series, and cats over 10 years of age account for the majority of diagnoses. Male cats occasionally develop mammary tumors, and these are almost uniformly malignant.
Signs of Mammary Tumors in Cats
The mammary chain in cats runs in two rows from the groin to the chest, with four mammary glands per side (eight total). Mammary tumors can occur in any gland and may be solitary or form a chain of nodules. They often feel firm or rubbery, range from a few millimeters to several centimeters, and may be attached to the overlying skin or underlying tissue in advanced cases.
What to look for:
- One or more firm, painless lumps in the mammary chain (under the belly, along each row)
- Redness, swelling, or ulceration of the overlying skin
- Nipple discharge (bloody or clear)
- Lymph node swelling in the armpit (axillary) or groin (inguinal) region
- Rapid growth of an existing nodule over 2–4 weeks
- In late-stage disease: weight loss, difficulty breathing (lung metastases), lethargy
Unlike benign adenomas (which are rare in cats), feline mammary carcinomas are typically not encapsulated and grow infiltratively. Any mammary mass in a cat — regardless of size — should be evaluated promptly.
Staging and Prognosis
The AAFP-AAHA Feline Life Stage Guidelines, 2021 emphasize thorough physical examination including mammary chain palpation at every wellness visit — because early-stage tumors (less than 2 cm) have a median survival time of more than 3 years with aggressive surgery, while tumors greater than 3 cm carry a median survival of less than 6 months. Staging includes chest X-rays (three views) to screen for pulmonary metastasis, abdominal ultrasound, and local lymph node sampling.
Treatment
Radical unilateral or bilateral mastectomy — removing the entire mammary chain on one or both sides — has lower local recurrence rates than lumpectomy or regional mastectomy. Cats tolerate chain mastectomies well and recover quickly. Adjuvant chemotherapy (cyclophosphamide-based protocols) is sometimes recommended for stage II–III disease, though evidence for benefit is less established than in canine mammary tumors. Spaying at the time of mastectomy is recommended for intact cats; it eliminates hormonal stimulation and may reduce contralateral tumor risk.
When to See a Vet
Call your vet today if:
- You have found any lump or nodule along your cat's mammary chain
- A known lump has grown noticeably over 2–4 weeks or become ulcerated
- Your cat has nipple discharge from any gland
Go to the ER immediately if:
- Your cat has difficulty breathing (possible lung metastasis)
- A mammary mass is bleeding continuously and you cannot control it
- Your cat collapses or is extremely lethargic
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Frequently Asked Questions
Can spaying prevent mammary cancer in cats? Early spaying (before first heat, ideally before 6 months) dramatically reduces risk. As described in Withrow and MacEwen's Small Animal Clinical Oncology, the relative risk of mammary tumor development is approximately 0.11 compared with intact cats when spayed before the first estrus. Spaying after 2 years of age provides minimal protective effect on mammary tumor risk.
How are feline mammary tumors treated? Radical chain mastectomy (removing the entire mammary chain) is the treatment of choice. Adjuvant chemotherapy is considered for large, high-grade, or lymph-node-positive tumors. Radiation is used for incompletely excised tumors. Prognosis correlates directly with tumor size at surgery — tumors under 2 cm have the best outcomes.
How much does mammary tumor treatment cost in cats? Diagnostic workup (X-rays, ultrasound, blood work) typically runs $400–$900. Unilateral chain mastectomy costs $1,200–$2,500; bilateral mastectomy ranges from $2,000–$4,500+. Adjuvant chemotherapy adds $1,500–$3,500 for a full course. Histopathology (biopsy) to confirm the diagnosis adds $150–$300.
What is the survival rate for cats with mammary cancer? Cats with tumors under 2 cm that undergo complete excision have median survival times exceeding 3 years in multiple published series. Those with tumors 2–3 cm have median survival of approximately 18–24 months. Tumors over 3 cm or with lymph node involvement carry median survivals under 6 months even with treatment.
Should I worry about mammary lumps in male cats? Yes — mammary tumors in male cats are rare but nearly always malignant. Any firm lump detected in a male cat's mammary tissue warrants immediate biopsy. Do not apply a "wait and see" approach to lumps in male cats.
Still Not Sure if Your Cat Needs a Vet?
When you're not sure if this is wait-and-see or call-tonight, Voyage AI Vet triages in under 2 minutes. Describe what you're seeing in chat, share photos of any lumps or skin changes along the mammary chain, or hop on a live video call if you want a second pair of eyes. Every answer comes with citations to the actual veterinary literature it's pulling from — so you see exactly where the guidance comes from, not just a chatbot's word.