One of our barn dogs has a mast cell tumor. It is located on one side of her lip right below her nose, which essentially makes it inoperable (have to remove at least 3 cm of tissue all around it, which means about 1/4 of her muzzle). Vet said we could try steroids and antihistamines to slow it down, antibiotics to fight the ulceration and infection, but basically that's all we can do. They don't metastasize but they tend to be aggressive (it has about tripled in size in a few weeks already, we originally thought it was just a hot spot or something but this week it looked so alarming I took her to the vet), so at best it will stay with what it is now (a big raw sore on her nose) and at worst it could continue to grow rapidly. She is not a young dog, either, and it seems to bother her when she is eating.
My smooth coated collie had a mast cell tumor on her sternum and a much smaller one on her hind leg. The chest one was surgically removed and then she had course of radiation on both areas. Six years later the tumor reappeared on her hind leg and she was euthanized due to complications from its removal. The one on her chest never did reappear. Is radiation a possibility?
Believe there is a new vaccine (or shot). Saw a bit on the Today Show a few weeks ago.
Having lost a dog last year to a large mast cell tumor on her rump, plus others internally, I took note when I heard this.
The story was done by the female reporter (can't recall her name) who has several adopted dogs and does the animal stories for Today.
If you can remove them, that's the best protocol. If you can't (as in your case) steroids can slow down the progression as your vet said. There's also chemo and radiation...but it doesn't sound like that's something you really want to get into.
There's got to be something in the water lately because two friends of mine have dealt with this in the last week. Poor pups!
I thought this was a pretty concise article...maybe it will answer some of your questions.