My crazy big ol' dog has had 2 cancerous tumors - both mast cell. One was 2 years ago and looked pretty bad - she was probably 5 at the time (she's a rescue lab, so I don't have her exact age). The second was about a year ago and looked really bad. We had surgeries to remove each. After the first, they went back to remove more to get a clean margin. After the second, the vet fully expected her not to live much longer - the second tumor grew extremely quickly and we thought for sure it had spread. With every month that went by without another tumor, the vet said our chances improved, but he didn't think it would be long. Well, she's proved him wrong again! No signs of a new one.
I'm going to guess yours is probably mast cell, too, if it is cancerous. It's more common than I realized - and it doesn't have to mean the worst news. You'd never know that Maisy had even had surgery - she's exactly the same - crazy.
Hang in there!