I think it is great your vet referred you to UC Davis. I would definitely go to a specialist for something like this. I knew someone whose pup had a mast cell tumor on his leg. It was caught early and was surgically removed and he was fine.
Please keep us posted...we can all learn from each other's experiences. I personally have not dealt with a mast cell tumor in any of my pups.
Welcome to YT!
Prayers for Penelope...pretty name!