After doing some research on mast cell tumors a couple of years ago for someone, I noticed that quite a few people were having great success with something referred to as TCVM, or Traditional Chinese Veterinary Medicine with regard to these tumors, both the cancerous and non-cancerous versions. Here is a link that may be useful to you. There is a locator for a vet near you on the left side of the page. http://www.tcvm.com/
Ghilly
mass cells are bad news my golden died from grade 3 despite having surgery ask your vet about a drug pailadia.....prednisolone will stop working.
Hope you get some inputs from experience.
My little knowledge is with what I have called "fat pocket" which a large Golden Retriever had. I understand they can return, and in the case of the fat pocket, which I believe is a tumor, they can be cancerous. In this case, removal before it becomes cancerous is needed. Our Golden did develop cancer and that led to his death (put down).
Again, I don't know my experience directly relates to your question, but do discuss with the vet, I would hope the vet would not recommend surgery just to keep busy, make money... but who knows, some might.