So, I've also been through cancer (ditto of 'cancer is the worse thing ever' feeling; I hate that), and my mother and father had somewhat similar beliefs. My dad I think had it more so. We don't have the best relationship, to say the least, so I looked at his worry with a very biased opinion.
It could just be a parent thing, because my mom brought this up to me once or twice before. Both of my parents are mentally ill and I was ready to blame their beliefs on that. I think, for some reason, a parent will always find a way to blame themselves when their child gets sick. If their minds are a little off kilter, then it is likely to turn into irrational thought. My dad, I don't know about my dad.... My mom, she still feels it was her fault.

I don't know if I answered your question or fed your confusion. Just know that I can kind of relate.