An oncologist can give a percentage of dogs that survived for x months, years, and the average survival time, but they are nevertheless meaningless statistics.
When my dog was diagnosed with cancer, I was asked by my local vet at the time the news was given did I want a referral for a specialist opinion from an oncologist at the Animal Health Trust, which has the most up-to-date treatments, specialist staff and research which is focused on treatment and cures.
He was diagnosed with aggressive cutaneous melanoma, skin cancer, with an average survival post diagnosis of 6 – 12 months, with an excellent quality of life, with no dog having survived longer than two years. The options I was given was try one session of chemo to see how well he would tolerate it, go for surgery to excise the tumour with a wide margin followed by four sessions of chemo, or do nothing, he would have palliative care, and while he still had a good quality to his life, I could enjoy what I had while I had it.
He bounced back quickly from major surgery, had zero side effects from carboplatin (chemo) and jumped back happily into the car after each session and lived an active life for all but an addition 4 years. Diagnosed at four, he died at almost eight, it was a success.
Every case is individual and what if any treatment is in the best interest of a dog depends on the opinion of the oncologist and if the owner can afford or wants to go ahead with treatment.
Good luck with your dog. Keep the Animal Health Trust in mind if it is cancer, the worst case scenario, because the treatment is excellent.