Hi Jessica,
I am Spirit Maggie's mom that Shari mentioned. Sorry I am so late to post but I am working two jobs right now and can rarely get to the computer. Shari was going to send you my email address, you can PM me as well if you want to talk through email.
Maggie lost her leg to a mast cell tumor and was given 6 to 9 months because of lymph node involvement. She lived almost 4 years and did not pass from mast cell (we did do chemo). She had several more tumors pop up, I think she topped out at 7 (one before her amp, HUGE incision!). Maggie developed kidney failure then oral melanoma which is why I lost her. I chose not to aggressively treat the melanoma, she passed 3 months after diagnosis.
Mag's little sis Tani also has mast cell cancer, first diagnosed in 2007 (the week before Mag finished her chemo). She has had 5 tumors removed over the years. Against all odds Tani will be 13 on Sunday. In addition to cancer she has hydrocephlus, a collapsing trachea, IBD, and horrible arthritis. Last year I had two tumors removed and two new ones popped up before the path report came back! Considering everything I decided to not remove any more tumors unless they became a ulceration problem.
All cancers suck, but mast cell is particularly unpredictable. We don't know why Mag seemed to beat mast cell, lymph node involvement usually is really bad. Some of Tani's tumors come and go, I can't always find them, sometimes they almost stick through her fur. Reactive tumors can indicate an aggressive form of cancer, yet at this point I'm much more worried about the neurological episodes and arthritis.
All of that as background so I think I can safely say that I understand where you are. Some people thought it odd that after lopping off a leg to treat cancer I wasn't willing to put Mag through another surgery. It is a little dicey to not remove Tani's tumors or treat with chemo. But people making judgements don't have all the facts. For me it is about quality of life for the pugs. Tani still plays with her toys every day, bosses her bro around, and overall enjoys her life. That is as good as it is going to get for her. One of the things my vet said when I was deciding not to treat Maggie was " at some point the treatment becomes worse than the disease".
On the mast cell- I don't think anyone can tell you what will happen. Lots of stats out there, in my experience none of them hold up. Who knows how much time you have. My journey with Maggie taught me to just appreciate each day that we has and not look too far down the road.

Karen