So, not cancer, but I can speak from recent experience.
A couple of weeks ago I woke up on Monday with major abdomen cramps that stayed with me all day. Tylenol gave minimal relief. I barely ate, for fear I was constipated and already feeling gassy... but nothing happened, just pain. I barely slept, and Tuesday the pain was worse. I lay in bed Tuesday morning trying to decide what to do. I want to die. But I do not want death to be painful, and whatever was wrong with me might not have been terminal, but it definitely was painful.
I knew it was crazy to seek treatment for something when I was hoping to die in a couple of weeks... but I thought, "what if" this pain just continues and I can't sleep or eat and then what if my method fails?
So, I called for an ambulance, and went to the hospital. I was in the hospital from Tuesday morning through Saturday afternoon of last weekend. They found I had a kidney stone, at least partially blocking my left kidney. They had to give me pain killers while I was in there, until they inserted a temporary stent in my left kidney to open it up for it to drain more normally and hopefully help the stone to pass. I also had been running a low fever and had evidence of a decent infection brewing inside of me.
In the week since coming home... no pain, but not completely normal. I also have a temporary stent that the doctor said needs to come out within a month of putting it in there... and if the stone hasn't passed by then he would try and break it up while he removed the stent. They even called on Thursday asking about scheduling the procedure, but I didn't answer and haven't called them back.
When I was in pain, I wanted relief... but if I can die before I need to get this stent out of me, and not have pain in the meantime... then I'm thinking why bother with that needless procedure just to die.
I don't know if that exactly answers your original question, but that's sort of my very recent experience with making a similar decision.