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Hello there ,
Thank you for the question ,
I am DrD , I will try to answer to your question ,
Mammary tumors are most commonly diagnosed in dogs ages 9 - 11 years. I do not want to scare you but If your dog is unspayed, was known to have had puppies, or was spayed in adulthood, she fits into the high-risk group for mammary cancer development . The good news, if there is some, is that approximately 50% of the tumors formed by female dogs are benign. Since you cannot tell by looking at a tumor which it is, the tumor must be removed or a part of the tumor must be sampled for biopsy. The laboratory can determine whether the tumor is benign or malignant based on the cells and their architecture within the tissue.
I recommend to take your dog for hands on exam ASAP.
I hope this helps. Good luck . DrD