I wonder how many years had passed from the time of PR's last ovarian cancer treatment and JonBenet's death? After having my husband go through chemo for colon cancer last year, I could see a shift in his personality. And although he hasn't had chemo in about 8 months, he still has "chemo brain", which is a real thing.
I don't know the answer, but she was diagnosed when JB was 3, and she probably was receiving medical care including surgery and chemo for around a year is my thought.