No, not necessarily.

CT scans are likely overused; my spouse has several chronic conditions and after receiving dozens a year for a while, we started asking if a CT was clinically indicated, or just precautionary. Mostly the response is "just in case". I wish EMRs did a better job of highlighting how many CTs someone's had recently for this sort of decision making.

There've also long been problems with kids getting unnecessarily high adult doses of radiation. My dad's a peds radiologist and was heavily involved in the founding of https://www.imagegently.org/.

My kid needed an abdominal ultrasound in the ER, but the ultrasonographer had gone home for the night. They did a CT instead, despite my concerns. CT missed the problem, which festered for several more years before being discovered during our next ER visit … on an ultrasound.

The folks round these parts that transfuse the blood of young healthy people to boost their lifespan are probably having a regular CT scan too.

I have a friend who went to an allergist last week for sinus issues. They did a CT scan within 5 minutes of coming into the office.

The "almost certainly" that you brush off is the entire point of the calculation. Is it 95% certain or 99.5% certain? The exact balance between benefit and risk in medical procedures is exactly how it is determined when to use them in the standard of care.

My first MRI about 30 years ago they did a precautionary CAT scan of my head because I had worked in machine shop (the place I worked had a machine shop and I used it for small personal projects), dunno if they do that anymore, but I was told ferrous bits in eyes had led to disastrous results for at least one patient.

In most cases instead of CT scan one could do the same diagnosis with MRI scan, which is only a bit more expensive.

Yeah, no. A doctor that I had a feeling wasn't paying too close attention to what I was saying ordered a pelvic/abdominal CT with and without contrast, ~30-40 mSv. Nothing turned up on the scans. When I went back he said "nothing's wrong" and, confused, I described my symptoms again. He just said "Oh! You need physical therapy."

Two weeks of physical therapy and I was fine.

At the time I was mad about the money, now I'm just thinking "what a dumb way to (maybe) get gut cancer."

I was young. I thought I was scheduled for an MRI like the one I had for my sinuses. I didn't even know what a was CT at the time.