I am sorry about your dog's disease.
Biopsy is the only definitive way to diagnose this cancer. MRI, Ultrasound and other diagnostic procedures would still have to be confirmed with biopsy.
Some cancers go into remission for no known reason. These are labeled as "miracles" and maybe you and your dog are lucky. I sincerely, hope so!
There are many non-chemotherapy holistic treatments for cancer that are effective in some cases patients are worth a try. If you are interested please google: PolyMVA, Essiac tea, Neoplasene or blood root extract, Hoxsey Formula, Medicinal Mushroom Extracts, Maqui berry and more. The Chinese herbal formulas, which are available by prescription from your vet from Dr. Xie's Jing Tang Herbal are also helpful. They are: Stasis Breaker, Max's Formula, and Wei Qi Booster. A homemade diet consisting of high protein, and no starch or grains is also helpful since cancer lives best on carbohydrates. If you are interested in the anticancer diet please post again and I will post a copy for you.
Thank you for your response and recommendations. Perhaps our dog, Fin, and we have simply been lucky. I hope so. Fin has been on a high protein, meat based, diet for a long time. He gets almost no starch. Though we put him on a meat based diet for different reasons, it is good to know that it is better for fighting cancer. My wife and I are receptive to holistic and herbal remedies so we are interested in the rest of the details for an anticancer diet. If you will please post a copy to me, I'd greatly appreciate it. Thank you again for your help.