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Mammary Cancer in Dogs

Poppy's story and mammary cancer in dogs

“Poppy” was brought in to see me as her owner had found a small, mobile lump adjacent to one of her nipples.  

Poppy was four years old, a beautiful, but feisty, Black, and Tan Miniature Dachshund, with a lot of attitude!!  She was always tricky to examine, and she certainly would not allow a conscious needle biopsy of this lump. I could feel that the lump was a mammary tissue lesion adjacent to a nipple, and it was quite small and mobile. After discussions with her owner, I arranged for her to have this lesion surgically removed and for her to have a Chest Xray and be spayed under the same anaesthetic. I also advised that the lump was sent away for Histopathology to find out what it was.

Benign and malignant tumours of the mammary glands occur frequently in female dogs that are not spayed. In fact, mammary gland tumours are the most common type of tumour diagnosed in un-spayed, female dogs. 

The mammary glands' in dogs are located in two rows that extend from the chest to the lower abdominal area, their nipples reveal their location. A female dog normally has 8-10 mammary glands.  

Mammary tumours are most commonly found in un-spayed, middle-aged female dogs between the ages of 5 and 10 years. These tumours can present as a single soft, mobile lump, a large solid mass, or as multiple swellings in the mammary gland tissues. They are usually fairly easy to detect by gently palpating the mammary glands. In 50% of the cases, more than one growth is located. 

Benign growths are often smooth, mobile, small, and slow growing. Malignant growths may grow rapidly, have irregular shape, feel "fixed" to the skin or underlying tissue, and can become inflamed and ulcerated. This is not always the case though, so all mammary lumps are advised to be examined by your Vet. 

Prompt surgical removal of any mammary tumour is recommended, along with spay surgery if the female dog is not yet neutered. 

Spaying female dogs can reduce a dog’s risk of developing mammary cancer, especially if the dog is neutered before she has an opportunity to come into heat. Prevention of mammary tumours is maximised when the dog is neutered before this first heat. The benefit of the intervention then gradually decreases as the female dog ages. The risk of developing mammary tumours is very low: 0.05% in a female dog neutered before her first heat, compared with an unneutered dog. The risk is then 8% if neutering is done in between their first and second heat. This then increases to a 26% risk after their second heat. 

It has been shown then that early spaying may reduce the occurrence of mammary cancer in female dogs, because the procedure removes the source of the hormones that cause some mammary cells to change. Spaying a female dog prior to 2.5 years significantly decreases the risk for both benign and malignant mammary tumours. 

Mammary tumours are most commonly found in un-spayed, middle-aged female dogs between the ages of 5 and 10 years. 

Occasionally, mammary tumours will develop in male dogs and these are usually extremely aggressive and have a poor prognosis. 

It is not possible to determine the type of tumour just based on its appearance and feel alone. A fine-needle biopsy of the lesion and tumour removal and further Histopathology analysis are needed to determine if the tumour is benign or malignant. If able, a fine needle aspirate can be performed to help with the preliminary diagnosis. This procedure entails the withdrawal of some cells through a syringe and placing them on a slide. These slides are then sent to the Laboratory for Cytological examination. This can hopefully determine whether the tumour is benign or malignant with enough accuracy to determine how aggressive the surgical approach should be. Needle aspirates may be a helpful pre-operative procedure in many cases, however, it is the full tissue examination of the removed lump biopsy that is what is required for a Histopathology examination, to fully determine the diagnosis and the extent of disease. The type of tumour is important in determining the prognosis. After surgical removal, the biopsy sample will identify the tumour type and indicate whether the tumour was completely removed. A tumour removed with clean, clear margins carries a much better prognosis. Mammary gland tumours are commonly categorized as being either benign or malignant. Chest X-rays are done during the surgery procedure, and close monitoring of the mammary glands and adjacent Lymph nodes thereafter any mammary lump surgery is advised. 

Thankfully our wee, feisty Poppy had a benign type of mammary lump removed, and she recovered extremely well after her lump removal and Spay surgery. In fact, she became much more amenable and docile during her surgery check-up visits with me, and she even allowed a pat and a cuddle! 

Just as with us humans, never ignore a mammary lump. Early detection and appropriate treatment are always best, as they will grow. 

Check your pet’s mammary area for any lumps or bumps regularly, and if you have any concerns please contact your vet for an initial check-up appointment. 


Alison Laurie-Chalmers, 

Senior Consultant, 

Crown Vets

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