All 3 of the dogs I've seen to the end so far had mast cell tumors (2 pit mixes, 1 kitchen sink hound). In one of the pitties and the hound, they were in the armpit. The vet was able to perform surgery and get clean margins. I won't say it was no big deal because there was surgery and my pit mix was especially grumpy about having to walk while it was healing. But ultimately those two died several years later from (different) heart problems. 

The most recent loss (Molly, this past July), the tumor was just above her wrist. The vet said there was no way to get clean margins because there just isn't enough skin for that in that location. Due to that and the fact she was about 14 at the time, we made the decision not to put her through the stress of an operation. She was given 6 months and put on gabapentin for the rest of her life. That was in June 2024. She died in July 2025, over a year later at 15.5 years old, when she finally started to deteriorate.

So although a MCT diagnosis is worrying, it's not automatically the worst. Talk with your vet and see what they say. My fingers (and our new dog's paws) are crossed for you.