I'm sorry to hear this news. You may have already done a search, but there are several threads on here re: mast cell carcinoma. I had a scare with my galgo, Cruz, back in the winter. He had a lump come up on a foreleg that was suspicious, based on the needle aspiration at my vet's office. My vet removed it with wide margins. This was scary to me since there is very little extra skin on a foreleg. Anyway it was a long healing process to say the least. But when the full biopsy came back after the surgery it wasn't a mast cell tumor! It was some sort of bizarre fungal thing that was releasing histamines into his system similiar to a mast cell tumor. I was relieved, of course, that it wasn't cancer, but I felt horrible that I had put him through that surgery for no reason. It's so hard to know what to do sometimes.
Anyway if you haven't already, use the search function to find the previous threads on treatment and prognosis for mast cell cancer. Keep us posted.
Sending good thoughts to you and Valor.