Unfortunately I do know about Mast cell tumours as Duncan, my 11 year old black lab was diagnosed with one a couple of months ago.
The REALLY important thing is that the tumour is removed with wide margins (>2cm if possible). Duncan's was on his neck about midway down, 1" in size .... he came home with an incision running from above his ear right down to where his neck met his shoulder. The surgeon also ended up taking part of his cleidocervicalis muscle, leaving a sizeable hole in his neck.
As you may know, Mast cells are one of 3 grades (depending on how extensive they are and the characteristics of the cells within the mass) and 4 grades (depending on lymph node involvement, metastases etc) , which can only be determined after the mass is removed and other tests like ultrasound, buffy coat smear and bone marrow aspirates are performed. Grade I tumors, if removed with clean margins are typically curable. Grade II's even with clean margins have a 20% recurrance rate and are unpredictable in how they will act. Survival rates seem to vary between papers, but it seems about 75% of dogs with a Grade II live > 6 months unless you do radiation treatment when >80% live over 5 years. Grade III are aggressive tumours with a poor prognosis, only 15% living >6 months after diagnosis.
Now, after having said this, dogs don't read papers or textbooks. I have several colleagues who have dogs with Grade II tumours who are >1 year post mass removal and doing great.
Duncan also had a Grade II, stage I tumour. He was put on Benedryl and Famotidine prior to, and for a few weeks post surgery (Mast cells release histamine, that can cause GI upset). In fact many dogs with Mast Cell disease don't die from the tumours, but rather because they have GI side effects (e.g. ulcers) that affect their quality of life.
Unfortunately for Duncan he is going back for a recheck tomorrow as his incision has developed a new lump along it. I think it is a suture reaction, but Mast cells are known as great pretenders, so every new lump and bump needs to be checked out.
One other thing I did was to change Duncan to a low carb diet (I use Natures Variety Prairie) as cancer cells like carbs.
My dog had a grade one removed when he was 7. He died at 14.5 of something unrelated and the cancer had never returned.
Over the years he had many other lumps and bumps, all benign.
Prayers for some good news!
I'd have to look at our paperwork but I *think* that's the place (and lady) I saw with my Golden's osteosarcoma. If so, she was wonderful and I highly recommend her. Very smart and good at what she does as well as excellent dog side manner.