Mammary (breast) Tumours in Cats & Dogs

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Diagnosis
- Treatment
- Outlook

What are Mammary Tumours?

Mammary (breast) tumours are the most common type of cancer in female dogs. Tumour development is thought to be linked to hormones released by the ovaries. Mammary tumours are often seen less commonly in dogs who were spayed (neutered) before their second season, however there is currently no definite evidence linking neutering with a reduced incidence of mammary tumours. The average age for development of mammary tumours is eight years old and any breed can be affected. 

Approximately 50% of mammary tumours are benign (not aggressive). However, there is evidence that tumours can develop from pre-cancerous types to more aggressive tumours, so it is recommended to treat all mammary masses. Of the 50% of tumours that are malignant (aggressive), roughly half of these will spread around the body (metastasise), most often to the lungs. 

Mammary tumours are much less common in cats, but the majority are malignant, so any masses found should be treated as soon as possible. 

What are the symptoms of Mammary Tumours? 

Mammary tumours often start as small “pea-sized” lumps under the skin near to the nipple. These may not be easy to see but can be easily felt. Some tumours remain small while others grow quickly and can become very large. Mammary tissue develops in a chain running along the line of nipples meaning multiple tumours may be down along one side of the body. Some pets may have tumours on both the left and the right side of the body; these may be the same or different types of tumour. 

Generally mammary tumours do not cause any symptoms. Most often they are found when the pet is having a tummy rub, by the groomer or during routine examination. Uncommonly, very large mammary tumours can become inflamed and painful. 

Which tests are used to diagnose Mammary Tumours?

Mammary tumours can contain both benign and malignant cells (known as mixed type), so the only reliable way to diagnose them is with analysis of a solid tissue biopsy (known as histopathology). The lab will examine the individual cells and how they are structured in relation to each other. Since mammary tumours only affect mammary tissue, which has very well-defined limits, we generally do not need biopsy results to perform surgical planning as we know which tissue will need removing. For this reason, mammary tumours are one of the few types of tumour where a biopsy is usually carried out as part of treatment, rather than as a separate procedure. 

Since a considerable number of mammary tumours will spread elsewhere in the body, it is often recommended to examine the lungs prior to treatment. Chest x-rays are quick and easy to perform and will show tumours that are more than 6mm in size. Unfortunately, x-rays will not detect micro-tumours in the lungs, but are an excellent screening tool. The most sensitive method of examining the lungs is using a CT scan as this can detect much smaller masses. However, CT scans are more expensive and are not available at every practice. Your vet will advise you on the most appropriate option. 

If evidence of tumour spread to the lungs is found, it is generally not recommended to proceed with surgery. This is to prevent the pet going through a surgery which will not be curative. Occasionally, if the primary mammary tumour is affecting quality of life (e.g. ulcerated or infected), then palliative surgery may be considered as an option. 

How are Mammary Tumours treated? 

Surgery is the treatment of choice for mammary tumours, whether they are benign or malignant (often not known before surgery). 

A single mammary mass is usually treated by removing the affected mammary gland, including the nipple, known as a local mastectomy. This is a straightforward surgery, and pets usually recovery quickly. 

If more than one mammary mass is present, it is recommended to remove them all. If they are in glands next to each other, it is likely that multiple mammary glands will be removed in one block. This is known as a regional mastectomy and is most commonly performed on the two or three glands nearest to the pelvis. Removing the glands together means any tumour cells spreading along the mammary chain will hopefully be removed as well. A regional mastectomy is a little more invasive than a local mastectomy, so recovery may be a little longer. 

If there are masses along the whole mammary chain, or a pet has more tumours develop after having previous mammary tumours removed, it may be recommended to remove the entire mammary chain. This is known as a mammary strip and is quite a big surgery. In cats, this is often the first choice of treatment as their tumours are much more likely to behave aggressively. Both dogs and cats will usually benefit from staying in hospital for one or two nights for pain relief, and a drain may be fitted for a few days to help with healing. Occasionally, a mammary strip is performed on both the left and right side at the same time, but more commonly if this is needed, it is staged into two separate surgeries. 

If your pet hasn’t been spayed, your vet may recommend neutering at the time of mammary tumour removal, or as a separate procedure afterwards. This does not reduce the likelihood of further mammary tumour development; however, older female animals are at significantly increased risk of pyometra (infection in the uterus), uterine cancer and ovarian cancer, all of which are prevented by spaying. Spaying a healthy pet is always safer than needing to spay due to ill health. Your vet will advise if this is appropriate for your individual pet. 

Unlike in humans, chemotherapy has not been shown to increase survival times in dogs with mammary cancer and is no more effective than surgery alone. In cats, where mammary tumours are typically more aggressive, there may be some benefit, however this is generally assessed on a case-by-case basis. 

What is the outlook for pets with Mammary Tumours?

The prognosis for cats with mammary tumours is unfortunately very poor, as they are often malignant and diagnosed later (as most cats don’t like their abdomens being touched). Cats with tumours less than 2cm in size treated surgically can do well for many years. However, cats with tumours more than 3cm in size at diagnosis often survive for less than a year, even with surgical management. 

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. 

Members of the The Healthy Pet Club get 20% discount off the cost of neutering. 


Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.

Page last reviewed: 7th August 2023
Next review due: 7th August 2025

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