Sorry to hear of this.
My dog also has cancer and is doing well for almost two years since his diagnosis/surgery.
Now he is too old to do much more than give him a new cancer inhibitor that has recently come on the market.
Sorry--but it is not for cancer's such as your dog has.
15 years old is nearly immortal for a dog.
I have seen people go hog-wild in treatment for dogs in advanced ages.
And honestly this is crazy, and a waste of money.
If your dog was 5 or 6 or even perhaps as young as ten, then I would say to have some advanced proceedures performed.
But in your case, I think your dog is way too old.
So old in fact that he/she will probably wake up one morning in dog-heaven from old age, rather than from the cancer... if you know what I mean.
If you really wanted to do something--then try chemo.
It does not affect dogs the way it ravages people.
Or try this stuff:
Essiac
http://essiacinfo.org/
Research this stuff.
I believe it has efficacy and this is the reason it was shut down decades ago.
But you'll have to brew it up yourself or order it from some outfit that produces it as a supplement.
I believe it changes the blood/body chemistry and works much in the way that these new inhibitor drugs work--but for much, much less.
I am going to give it a whirl with my pooch also--he has mast cell tumors.
There is also this stuff:
Neoplasene
The trick is to find a vet who is willing to use it, or knows how to use it.
This is tough.
It is a botanical agent derived from blood root.
It comes in a oral version that may prove useful to you.
http://holvet.net/neoplasene.html
http://www.buckmountainbotanicals.net/treatments/neoplasene.html
This stuff is not expensive to use.
You have to purchase 200 hondos worth at a time--and this last 4 months (depending upon the size of your hound).
So it works out to around 50 hondos per month to use (add the blood work costs).
Do your research on this stuff and let me know how it is going.