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
Histopathology
Regional lymph node assessment
Chest radiographs
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
"All mammary lumps are cancerous"
Approximately half of canine mammary tumours are benign; determination requires tissue examination.
"Spayed dogs cannot develop mammary tumours"
While risk is reduced, spayed dogs can still develop mammary tumours, particularly if spayed after multiple heat cycles.
"Small tumours are always benign"
Size alone does not predict behaviour; even small tumours may be malignant.
Related conditions
Mammary Adenocarcinoma in Cats
Mammary adenocarcinoma in cats shares the same tissue of origin and many of the same investigative approaches as mammary tumours in dogs, though the biological behaviour tends to differ between species. Owners with both dogs and cats may find it useful to understand how these growths are assessed and managed in each.
Lipomas
Lipomas are benign lumps that can appear in similar locations to mammary tumours, and both may be discovered during routine handling of an older dog. Distinguishing between the two typically requires examination and sometimes sampling, as appearance alone does not reliably separate one from the other.
Mastitis
Mastitis involves inflammation of the mammary glands and can present as firm, warm swellings along the mammary chain, sometimes prompting concern about the possibility of a tumour. The two conditions arise from different processes—infection or immune response versus abnormal cell growth—but both warrant veterinary assessment when a lump appears.
Cryptorchidism
Cryptorchidism, in which one or both testicles remain undescended, is associated with a higher risk of testicular tumours in male dogs, and some hormonal influences may also affect mammary tissue. Female dogs are far more commonly affected by mammary tumours, but the relationship between reproductive hormones and tumour development is an area of overlap in both sexes.
Mast Cell Tumours in Dogs
Mast cell tumours in dogs are another common skin growth that can appear as lumps anywhere on the body, including near the mammary chain. Both conditions require similar initial steps—examination, imaging, and often biopsy—to determine the nature of the lump and guide decisions about treatment.
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