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  • Dr. Frans Pretorius BVSc

Mammary Gland Tumors in Dogs

Did you know that your dog can get mammary gland tumors (same as breast cancer)?

And that 50% of them are malignant, meaning that they are aggressive tumors that can spread easily to other organs such as the lungs, nervous system (brain) and other systems or organs. And 50% of the dogs that have these malignant mammary tumors also have more than one of these tumors.

It’s mostly female dogs that get these tumors and extremely rare in male dogs.

These tumors are influenced by hormones and the risk of your dog getting these might be vastly reduced if she’s spayed before her first heat (or estrus cycle). *see prevention

All breeds are affected evenly and we mostly see these tumors in dogs about 10 years of age.


Usually slow-growing single or multiple masses or lumps in the mammary gland. You should feel for any lumps when you give your dog belly rubs. Consult your vet if you are not sure and definitely don’t wait too long to have any lumps looked at. The longer you wait the bigger the chance these tumors have to spread to other organs and the bigger the surgical procedure becomes, longer surgery time, more tissue that needs to be removed.

It’s never okay to just observe a mammary lump or leave in place


Your vet might take needle aspirates of lumps but these results can sometimes be misleading and look like infection or inflammation.

The best way to get a definitive diagnosis is to do an excision biopsy - This is where the whole mass is removed with wide margins and sent to a lab for histopathology. The lab will then confirm diagnosis.

Your vet might also want to take survey x-rays of the chest and abdomen to check if the cancer has spread to these areas.


Surgery is the primary mode of treatment and delivers the best results. The sooner the better. Sometimes follow-up chest x-rays are done every three months with very aggressive tumors.


  • If you have your small or medium size bitch sterilized before the first heat the risk is 0.5% when compared to intact females.

  • If you have her sterilized before the second heat the risk is 8% when compared to intact females.

  • If you spay her after her second heat the risk is 26% when compared to intact females.

  • If she’s spayed after 2.5 years of age there is no sparing effect on the risk.

  • Large and Giant breed dogs are sterilized after their first heat to prevent urinary incontinence later on in life.

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