I always ask the vet in such situations what he or she would do if it was their pet. I have asked that when putting the pet down was already stated as an option and when the vet had not yet mentioned it. Every time the vet has said they would put the pet down.
Our vet told us not to even bother getting the tumor biopsied; because of its location and the fact it was wrapped around various structures in her neck, active treatment was not going to do much. We did do antibiotics (she had infected teeth; a cleaning and extraction were how we discovered the tumor), which helped reduce some of the swelling for a time, and some liquid non-steroidal medication which had shown to be somewhat helpful in bladder tumors.
We got three months of her being happy, eating chicken cutlets daily and having good energy til about the last ten days. We probably could have held on a little longer, but she was struggling to breathe at night when the tumor pressed on her windpipe. (When she was on her feet, she was fine, until the last 2-3 days when she quit doing stairs. She was eating well some days, and not on others.) The vet told us we did the right thing at the right time. Her feeling is that many people wait too long and the animal starts suffering.
It’s such a personal decision. I couldn’t put her through chemo because her QOL would have been lousy and she’d have no idea why. It’s a different decision when I make that choice for myself or talk to a loved one about courses to pursue.
The only reason we had the tumor biopsied was because the vet thought it was an abscess. He went in to have it drained and that’s when she found the tumor. 
Previous previous dog: biopsy was done when mass on toe was amputated. Cancer turned out to be an unusual type that was not fatal. Dog died four years later from another illness. Previous dog: biopsy was done because the mass was small but suspicious. It wasn’t obstructing anything at that point; it could have been something else. Current dog: biopsy wasn’t done until after the amputation. The veterinarians were that sure that taking the leg off was the best way to stop the pain (which it did) and that the diseased leg bone (visible on the x-rays) was from cancer.
I think your choice is fine, OP. Your cat is old, the prognosis is grim, and the treatment would be painful and difficult.
Two cats, two different choices in my recent experience. An older, eight years old at the time, cat who seemed to have suffered a stroke. Half his face and body were paralyzed. My vet said more testing would be required to understand what had happened and to take him to The Veterinary Industrial Complex vet, but if I didn’t like the answer/choices, to come back. The specialist said he could give the cat a CAT scan (lol), but the anesthesia might kill him and then we might know. Might. And it would cost $1000. So I said thank you and went back to the local vet who did some simple, cheap tests (BP) and gave the cat some steroids. The cat slowly got better and is still alive ten years later.
I found a kitten who had been hit by a car. His femur was broken. I went first to my local vet, who ascertained that the broken leg was the only problem. He said he could amputate the leg for about $100, or I could go to the orthopedic vet and see if it could be pinned. Because this was a kitten, maybe eight weeks old, I paid the $2000(!) to have the leg repaired. That cat is now eight yo and completely normal.
^^^True – Our sweet girl was hit by a car at age 6 and had numerous injuries (broken bones, temporary blindness, collapsed lung, etc.). We paid for four days in doggie ICU and I nursed her at home (and work) for a month afterwards as she healed. No question about what we would do, though we did tell the specialty vet we had a budget (and amazing how the bill came out to exactly that amount…).
We did draw the line at a doggie ventilator – $1500/day and not a good survival rate when a dog came off of one. If she needed a ventilator, she was suffering, so that was our line in the sand. But she was a very young six yo, in excellent health – and she recovered beautifully and lived another eight years. You’d never know how injured she was.
We did have a running joke about how she used up her Doggie Training 529 account after the accident.
We recently had a cat with a sarcoma. Prognosis is very poor. Eventually, we knew when it was time to let him go. I miss him so much and wish with all my heart that had not happened to him.
@Massmomm, I’m sorry you didn’t get better news. It’s so hard.
I do have a positive cat jaw tumor experience, though. My cat had a tumor on her jaw when she was just 5. The vet was pretty pessimistic and everything I read online sounded dire. For some reason (can’t remember what), the vet didn’t do a biopsy, but went straight to surgery. Amazingly, the tumor was benign. The surgery was still $1,500+ and with the pet insurance I had then, it would have been covered if she was diagnosed with cancer. But since she was OK, I had to pay.
My kitty turns 16 next month. 
@Massmomm So sorry. Like many others, I have been there several times. We made different decisions depending on the situation.With a 15 year old and a likely poor outcome, I think I would opt for palliative care. Hugs to you.
One other thing: keep your kids informed. My parents had two of my dogs put down without my involvement or even knowing they were sick, and it does not make for trust.
@Consolation , I appreciate your wisdom regarding the children. We’ve told our son, who is too far away and probably won’t be home to say goodbye. He asked us to call him when the vet comes to put our sweet kitty down so that he can talk to him one last time.
We will tell our daughter when she visits this weekend.
My dog story. Shih tzu developed problems the summer kid was far away. Hurting and sweet dog became untouchable for awhile. Turned out to be something cerebellar (not initial lumbar disc as time and steroids did not change and symptoms evolved). Kept him alive until son returned to say goodbye. He died the following Monday before euthanasia was to be done. I’ll never forget his last day- let him lie outside in the grass and on my lap (my comfort, I finally decided I had to let go sometime, not after hours). I think he understood and I thank him for letting me keep him going. My elderly father then recalled how he still remembered being away at college when his childhood dog died. That reinforced my feelings to make our dog wait.
We went into pet ownership having made decisions about the extent of treatment we were willing to undergo. Considered what would happen if a pet were in the wild and favored letting nature take its course more than with humans. I also remember elderly relatives who gradually lost interest in the world and stopped seeing doctors, getting more done. Both died at home and were discovered. Peaceful.
Our cat lasted a bit beyond the prognosis of 1-2 months. We had him euthanized at home yesterday evening. Our son got to say goodbye before heading back to college after his Thanksgiving break, and our daughter and son-in-law were still visiting. I know we did the right thing, but it still feels terrible. It is so hard to part with a little creature whose life has been part of yours for 15 years, and who never brought you anything but joy and the occasional flayed mouse carcass.
I’m very sorry, @Massmomm. I agree with you: you did the right thing and it feels terrible. I felt the same about the deaths of our dogs by euthanasia. Please be kind to yourself. You’ve had a stressful, sad few months, and grief hurts.
I’m so sorry. Have been down this road more than once before and am a cat person. FWIW, I agree you did the right thing. I know the sadness though. (((Hugs)))
I’m so sorry @Massmomm. You did the right thing but I know it feels horrible. It is like losing part of the family. Big hugs and lots of love.
Thanks for the update. You did the right thing, it hurts to lose a family member even if a pet. You will always have the memories.
Big hugs. You did the right thing for him.
I’m sorry @Massmomm . Losing a pet is so hard, especially when it’s tied to a holiday.
It sounds like it was a blessing that your cat survived longer than the initial prognosis, long enough for your son to see him one more time over Thanksgiving, to say goodbye in person. Having to euthanize a pet when a child is away at school is not easy, either, but sometimes is necessary.