A mammary tumour is one that originates in the mammary gland. Mammary glands are modified sweat glands. Larger breeds typically have 4 - 6 pairs of glands (one of each pair on either side of the midline), whereas smaller breeds typically have 4 pairs. When 5 pairs are present, the glands are identified according to their location (see diagram): cranial thoracic, caudial thoracic, cranial abdominal, caudial abdominal and inguinal.

Each mammary gland has its own nipple. Although breast cancer can and does occur in all of the glands, it usually occurs most frequently in the 4th and 5th. In half of the cases, more than one growth is observed. Benign growths are often smooth, small and slow growing. Signs of malignant tumours include rapid growth, irregular shape, and firm attachment to the skin or underlying tissue, bleeding, and ulceration. Occasionally tumours that have been small for a long period of time may suddenly grow quickly and aggressively, but this is the exception not the rule.


No. There are multiple types of mammary tumours in (all) dogs. Approximately 50% of all mammary tumours in dogs are benign (not life threatening), and the other 50% are malignant (likely to spread and/or cause death).

Mammary tumours are more likely to occur in unspayed, middle aged female dogs (those between 5 and 10 years of age). Spaying a female prior to 2½ years of age significantly decreases risk for both benign and malignant mammary tumours. Spaying after this time reduces risk for benign tumours, but appears to have no advantage for prevention of malignant tumours. Interestingly, pregnancy and lactation appear to have no influence on mammary cancer risk. However, evidence suggests that females bred extensively, beginning at an early age, have a slightly lower risk for mammary cancer. The condition is extremely rare in male dogs.


Mammary tumours present as a solid mass or as multiple swellings. When they do arise in the mammary tissue, they are usually easy to detect by gently palpating (examine by feeling with the fingers) the mammary glands. When tumours first appear they will feel like small pieces of pea gravel just under the skin. They are very hard and are difficult to move around under the skin. They can grow rapidly in a short period of time, doubling their size every month or so.

It is very difficult to determine the type of tumour based just on physical inspection. A consultation with your Vet may lead to him/her deciding to perform a biopsy (removal of living tissue) or tumour removal, and analysis is almost always needed to determine if the tumour is benign or malignant, and to identify what type it is. Tumours which are more aggressive may metastasize (spread elsewhere in the body) and invade the surrounding lymph nodes or the lungs. A chest x-ray and physical inspection of the lymph nodes will often help in confirming this.


Mammary cancer spreads to the rest of the body through the release of individual cancer cells from the various tumours into the lymphatic system, which includes special vessels and lymph nodes.
Upon finding any mass within the breast of a dog, surgical removal is often your Vet’s first choice of treatment, unless the dog is very old, and he considers invasive treatment to be contradictory to the dog’s wellbeing. However, some Vets elect to first perform a fine needle aspiration to ascertain breast tumour classification. This gives them the opportunity to carry out cytology (examination of cells) on mammary tumours before surgery, in order to first rule out mast cell cancer and cystic disease.

Should surgery be performed, the area excised (cut away) depends on the judgement and preference of the Vet performing surgery. Some choose to remove only the mass itself; others, taking into consideration how the cancer can spread, will remove the mass and the rest of the mammary tissue and lymph nodes that drain with the gland.

It is important for you to discuss with your Vet all the options open to you, before deciding on what is best for your dog’s future health and welfare.

Surgical removal of a mammary gland in the dog should not be confused with a radical mastectomy in humans. In humans, this type of surgery would affect the underlying muscle tissue, which complicates recovery. In the dog, however, all of the breast tissue and the related lymphatics are outside of the muscle layer, so an incision through the skin and the mammary tissue is all that is required. This makes surgery much easier and recovery much faster for the dog. A radical mastectomy in a dog means all the breasts, the skin covering them, and the four lymph nodes are all removed at the same time. Although this is truly major surgery, removal of the sutures (stitches) usually takes place within 10 – 14 days, with normal activity resuming at that point.

Sometimes your Veterinary Oncologist might elect for your dog to have chemotherapy. Chemotherapy is likely to be recommended for cancer that has already spread to other areas of the body (metastatic disease), for tumours that occur at more than one site (multicentric disease), or for tumours that cannot be removed surgically (nonresectable disease). It may be used as the sole treatment for certain cancers, or may be used in combination with other treatments, such as surgery and radiation therapy.
Compared to people who receive chemotherapy, dogs experience fewer and less severe side effects because of the use of lower doses of drugs, and a combination of drugs is not used as in humans. Chemotherapy drugs attack cells in the process of growth and division, while individual drugs may work through many different mechanisms, such as damaging a cell's genetic material (DNA) or preventing the cell from dividing.

Chemotherapeutic drugs cannot distinguish between malignant cancer cells and normal cells. All rapidly dividing cells are potentially sensitive to chemotherapy. Toxicity to normal, rapidly growing or self-renewing tissues in the body is the reason for most of the side effects seen with chemotherapy. Fortunately, these normal tissues continue to grow and repair themselves, so the injury caused by chemotherapy is rarely permanent.

The normal tissues that typically are most sensitive to chemotherapy are the intestinal lining, the bone marrow (which makes red and white blood cells), and hair follicles.

Toxic effects to the gastrointestinal tract are responsible for decreased appetite, vomiting, and diarrhoea. These effects may be mild, moderate, or severe. In most cases, these signs are mild and usually resolve on their own, or with oral medication given at home. Although infrequent, some dogs may develop severe diarrhoea requiring hospitalisation and fluid therapy. In many cases, the gastrointestinal side effects from chemotherapy are not seen on the day of treatment. They often occur 3 to 5 days later.

Suppression of the bone marrow by chemotherapeutic drugs may cause a drop in the white blood cell count, leading to increased susceptibility to infection. Severe infections may require hospitalisation for intensive supportive care, including intravenous fluid and antibiotics.

Hair follicle cells in dogs which are wire-haired or non-shedding may be particularly susceptible to chemotherapy. The hair will re-grow once chemotherapy is stopped, but may initially result in a modest change in colour or texture.

How a chemotherapeutic drug is administered, how often it is given and how many treatments are given varies from case to case. The type of cancer, the extent of disease, and general health of the animal help the oncologists to put together a treatment protocol (type of drugs, dose, and schedule used) appropriate for your dog. Some drugs are given orally (pills) at home. Others are brief injections that your dog gets from your Vet in hospital. In some instances, slow infusions or repeated treatments throughout the day may require your dog to spend the day in the hospital. The treatments are typically repeated from weekly to every third week. Blood tests may be needed to monitor the effects of chemotherapy during the weeks between drug treatments. The duration of chemotherapy depends on the type of cancer and the extent of disease. Some animals need to receive chemotherapy for the rest of their lives. In others, treatments may be spread out or discontinued after a period of weeks to months provided that the cancer is in remission, i.e., there is no detectable evidence of cancer in the body.

Chemotherapy treatment has not always been successful or widely used in the treatment of mammary tumours in dogs. However, with the constantly changing and improving drugs available, your Veterinary Oncologist might consult to find out if there is an effective drug available for your dog’s particular type of mammary cancer. The decision of whether to pursue chemotherapy treatments can be complex. Medical information, practical concerns (such as need for repeated visits, your dog's temperament, etc.), and financial responsibility all play a part in this decision. In such circumstances, you should never be concerned about discussing your fears with your Vet and/or Oncologist when making any decisions.
The effectiveness of other treatments, such as radiation therapy, has not, as far as we are presently aware, been thoroughly researched. Some anti-hormonal drugs are being tested for their effectiveness in treating mammary cancer in dogs.

With early detection and prompt treatment, even some of the more serious tumours can be successfully treated.


  individually or in collaboration with other researchers across Europe, is looking to identify genes that, when mutated (changed/altered), are associated with an increased risk of developing various common cancers. Within this project, the AHT is specifically planning to investigate the genes associated with Mammary Tumours in English Springer Spaniels.

ESS Breed Clubs Health Co-ordinators:

Lesley Bloomfield (Tel: 01923 823579 Email:
Louise Scott (Tel: 020 8427 3396 Email:

For details of Mammary Tumour Research Project click HERE

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