In the study, people were asked if they remembered having such an X-ray any time during their life. And people who had a brain tumor were more likely to answer yes.

I wouldn't be surprised if that just means that people are likely to forget about such exams, unless they get a brain tumor and start fretting about what might have caused it. Maybe they'll even start "remembering" radiation exposure in their youth that never actually happened. The study itself may produce such false memories by suggestive questioning.

I'll take note if the link can be shown with real data rather than questionnaires. Until then, I'm sceptical.