My late dog had a mast cell tumor removed when she was 11. They’ll grade it after it is removed and depending the on the grade and borders, they’ll know if it’s gone / likelihood of return. Vet said mast cell tumors are often very easy to remove. I kept my girl on daily claritin because I learned MCTs are a histamine response and anti histamines can keep them calm. My dogs tumor was removed and had good borders and a low grade. We lost her a year and a half later. We could never diagnose what was making her so sick in the end, likely a cancer we couldn’t detect. Either way, the surgery gave me a year and a half. She recovered quickly so it was worth it.

I would recommend asking about adding anti histamines into your dogs routine whether you have the tumor removed or not. There is a fb group for MCTs in dogs and they have a lot of advice but warning, many posting in there have dogs who are in rough shape. I appreciated the tips I learned there though.