What a beautiful picture of Jack--the strong silent boy.
What a terrible disease.
My old dog had a nerve sheath tumor--it was one of several types. He had been covered with fatty lumps--all of which were benign, but we knew we hadn't tested all of them. One day I was poking at the big one, and realized it seemed like a lump on a lump, and as I did so, he tried to bite me. Not a warning nip--and I KNEW. I knew he had cancer the second that happened.
The options all sucked, so because of his age, I elected just to enjoy what time he had left. When it became clear to me he had enough, I let him go. It was hard, but it was right.
Something my vet told me (as we sat on the treatment room floor with him, both crying) was that she's never had a patient regret putting a dog out of pain sooner, rather than later. You will know because Jack will tell you.
I hope you have a good friend who will be there for you--actually be there--your "innerweb" friends will certainly be in spirit.