If it helps, one of mine got those tumors, as well. She had been spayed at about 5-6 months.I adopted this cat when she was about a year old, she wasn't spayed, when she went into heat later but I never got her fixed because she was an indoor cat & I thought it would be hard on her, I had no idea about breast cancer in cats, now I hate myself because I feel responsible for her suffering.
That vet said it didn't have anything to do with it. Of course, this was about 25-30 years ago...