As chi-grandma notes above, there are numerous considerations to think of when deciding to treat cancer in our dogs. For me, the most important thing is the dog's quality of life vs. quantity of life. The last thing I'd want to do is make my dog's last days a horror of sickness from treatment. Your dog is young, so that's something in her favor. How has she handled the chemo as far as nausea, vomiting, diarrhea or weight loss is concerned? How much does the mass impact her life?
I don't envy you being in this position, but I think there is no right or wrong road to take at this point. Treatment may buy some time or even a remission. Not treating may result in a shorter life, but better quality of time. There's just no way of knowing.
You might consider asking Dr. Cheng on the Ask a Vet forum about various treatments available to keep your dog in top condition during treatment. Dr. Cheng practices holistic medicine as well as western veterinary medicine, and I'm sure she'll have some good ideas for you.
I have a neighbor with a Chihuahua and did chemo and the dog a year later in remission...I beleive like humans its an individual recovery process...and the radiation to boot, this owner was a single older woman who just could not let go of her pet w/out a fight and sure enough she won the battle, but, watching the situation from afar, it was a hard thing for the dog and her....
I am not sure how I would handle that situation for myself...I am very attached to my dog, there would considerations to place, age, money, longivity, recovery, odds of recovery against odds of putting a pet thru more distress...
So you have a huge difficult challenge ahead of you and I am very sorry for your most unfortunate situation and I know how difficult this must way on your heart...
But, welcome to site, there are wonderful people here that will hold your hand via internet.