What are dog mammary gland tumours?
Mammary (breast) tumours are common in female dogs, especially those that are unspayed or spayed after two years of age. They are rare in male dogs and cats. While the exact cause is unknown, hormonal imbalances are thought to play a role.
Written by Dr. Mariella Roberts, Veterinary Surgeon, Animal Trust Vets CIC | Published May 2025 | Review date May 2027 | This advice is for UK pets only and is not a replacement for seeing a vet
Symptoms of mammary tumours in dogs
Tumours usually are found by owners who feel a lump. Other symptoms include:
swollen glands
painful abdomen
discharge from one or more glands
ulceration of the skin on the abdomen
lethargy
weight loss (at later stages)
A benign tumour is usually small and firm, with a well-defined border. Malignant tumours are often fast-growing, with ragged edges, fixated to the skin or tissue. They may cause ulcers or inflammation.
When to contact your vet
If you find a lump on the underside of your pet, anywhere between the front legs and your pet’s book a physical consultation with your vet.
Diagnosis
Physical exam and history - these tumours are typically found as either singular or multiple nodules along your dog’s abdomen during a physical exam. Also, your dog’s history if entire or late spayed (which might present lumps within tissue) would be quite significant in the diagnosis of mammary cancer.
A biopsy, either a fine needle aspiration (FNA) or surgical removal of the tumor, is usually necessary to determine if the mass is benign or malignant.
Basic blood work, including a complete blood count and chemistry panel, is often done to assess the dog's overall health and to rule out other conditions.
X-rays and ultrasound may be used to rule out other causes and assess the size and whether the tumour has spread.
If the tumour is found to be malignant, staging may be done to determine the extent of spread to other parts of the body.
Treatment for mammary tumours in dogs
Surgery is an essential treatment for mammary tumours. Generally, one or more mammary glands are removed. If a tumour is found in multiple mammary glands, removal of the entire mammary chain may be recommended.
For dogs with larger tumours or evidence of spreading to other areas of the body, chemotherapy is typically recommended.
Prevention
The most effective way to prevent mammary cancer in dogs is through early spaying (ovariohysterectomy) before the first heat cycle, ideally before sexual maturity. This is because spaying removes the ovaries, which produce the hormone estrogen, crucial for the development of mammary glands. Other preventative measures include maintaining a healthy weight and regular home exams for lumps.
Outlook
The prognosis for most dogs with mammary tumours is generally good. Early treatment is important, as tumour size is directly related to prognosis; dogs with tumours over 3cm in size are much more likely to have tumour spread elsewhere, and a potentially shortened lifespan as a result. Most tumour surgery is straightforward and, if the tumours can be completely removed and there is no evidence of metastasis (spread), surgery is often curative.