Dear Angry,
I am sorry to hear of the loss of your Mom. Mom's are special and we dearly miss them. My Mother died many years ago.
I can only suggest that you stop beating yourself up as to whether it was a good thing or bad to have your Mom go for the Radiation. If she were still here and healthy, you would say that was the best thing, if you didn't take her in, she could have died just as fast. A distant family member decided not to have Radiation and they seemed fine for awhile and then, in 2 weeks, they were dead.
Each person is different, I have known people that had one dose of Chemo and Died that night. I know people that went thru the full treatments for cancer and they are 11 years out. One lady I know was told to get all her things in order because she wouldn't be here in 2 years. That was 6 years ago. The Doctors do the best they can, but they really don't know. They can guess from the averages of things that happened before.
Your Mother is in your heart, my parents aren't here anymore, but I still talk to them, I'm sure you do too.
My Best to Everyone here