So sorry to hear about your girls diagnosis.
I have been on the receiving end of that diagnosis over 16 times with my first girl Cami. She started at age 2 developing them and they continued to show up for the rest of her life. Sadly we lost her just shy of her 7th birthday to heart issues. Had she not been dealing with that she would have continued to fight (and win) her seemingly constant battle with MCT's.
There are so many options now. You need to get your hand on a copy of the pathology report. Don't just accept the vets phone call with "clean margins" (or not) and what the grade of the tumor is. The actual report includes far more info than what *most* vets will give the owners. They will gladly give you a copy! Just ask. Once you have it you can post the results here and we can help you decipher them. I have read so many I am pretty good at understanding what they say.
BoxerWorld is a great resource for info relating to MCT's. Try doing a search (if you haven't already) on:
MCT, MCT's, mast cell tumors, - my user name, Cami (I shared her journey for many years with various treatments). Also Vetalog and intralesional triamcinolone. You should have enough to read until you get the results!
Don't hesitate to ask questions if you have any.