CONDITION

Mammary Tumours in Dogs

Mammary tumours are growths that develop in the mammary glands — the tissue that produces milk. They can appear as single lumps or multiple masses anywhere along the mammary chain, which runs from the chest down to the groin. About half of mammary tumours in dogs are benign, meaning they grow locally but do not spread, whilst the other half are malignant and can spread to other parts of the body. Most owners first notice a firm lump under the skin, often during grooming or stroking. These lumps can vary widely in size, texture and growth rate. Some remain small and stable for months or years, whilst others grow quickly or become ulcerated. The significance of any individual lump cannot be determined by appearance alone. This page explores what mammary lumps can look and feel like, what determines whether a tumour is benign or malignant, how these growths are investigated and typed, and the range of approaches used to manage them. It also looks at factors that influence prognosis and the patterns seen in different types of tumour.

Why this matters now

Mammary tumours most commonly develop in middle-aged to older female dogs. The risk is significantly influenced by reproductive history, with intact females and those spayed later in life being at higher risk. Early detection through regular home examination may allow for prompt evaluation.

Benign tumours typically grow slowly and remain localised. Malignant tumours can vary widely in behaviour, with some growing rapidly and others progressing more gradually. Metastasis, when it occurs, most commonly affects the lungs and regional lymph nodes.

Signals & patterns

Early signals

Small firm lump in mammary area

A single nodule may be noticed when petting or grooming, often along the mammary chain running from chest to groin.

Multiple small nodules

Several small lumps may be present, sometimes affecting more than one mammary gland.

Changes detected during routine examination

Regular palpation of the mammary area may reveal changes in texture or small masses.

Later signals

Rapid increase in lump size

A previously stable mass may begin growing more quickly.

Skin changes over the mass

Redness, ulceration, or discharge from overlying skin may develop.

Multiple masses developing

New lumps may appear in other mammary glands over time.

Click to read about the biological mechanisms

How this is usually investigated

Evaluation typically involves characterising the tumour through sampling while also assessing for potential spread to other sites.

Fine needle aspirate

Purpose: Obtaining cells from the mass for initial characterisation.
Considerations: Can provide preliminary information but may not definitively distinguish benign from malignant.

Histopathology

Purpose: Microscopic examination of tissue samples to determine tumour type and grade.
Considerations: Typically performed on surgically removed tissue; provides the most definitive diagnosis.

Regional lymph node assessment

Purpose: Evaluating nearby lymph nodes for evidence of spread.
Considerations: Important for staging malignant tumours.

Chest radiographs

Purpose: Looking for evidence of lung metastasis.
Considerations: Commonly performed when malignancy is suspected or confirmed.

Options & trade-offs

Management approach depends largely on tumour characteristics, extent of disease, and individual patient factors.

Surgical removal

Excision of the affected mammary gland or chain is the primary treatment approach.

Trade-offs: Extent of surgery varies; wider margins may reduce recurrence risk but involve more extensive recovery.

Ovariohysterectomy

Spaying at the time of tumour removal may be considered if not previously performed.

Trade-offs: May influence prognosis in some tumour types; adds to surgical extent.

Monitoring small benign tumours

Some confirmed benign masses may be monitored rather than removed.

Trade-offs: Avoids surgery but requires regular assessment for changes.

Adjunctive therapies

Chemotherapy or other treatments may be considered for certain malignant tumours.

Trade-offs: Depends on tumour type and stage; benefits vary.

Common misconceptions

Misconception:

"All mammary lumps are cancerous"

Reality:

Approximately half of canine mammary tumours are benign; determination requires tissue examination.

Misconception:

"Spayed dogs cannot develop mammary tumours"

Reality:

While risk is reduced, spayed dogs can still develop mammary tumours, particularly if spayed after multiple heat cycles.

Misconception:

"Small tumours are always benign"

Reality:

Size alone does not predict behaviour; even small tumours may be malignant.

Regular home examination of the mammary chain can help detect changes early. Noting the size, number, and characteristics of any lumps provides useful baseline information for tracking any changes over time.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS