Are they the only option? or is there a regular metal detector
beside it?

In the US, you can opt-out of the scan, at which point they do a
pat-down. Would be useful to know if the same option exists in
europe.

Exactly what are you scared of? If you are worried about radiation
then perhaps you shouldn't fly at all.

Are you saying unavoidable radition at altitude exceeds the exposure
of imposed and man-made radiation from the scanner?

BTW, radiation isn't the only concern. Some travelers are shy about
having their full nude body examined (as the scanner is good enough to
produce an image that closely resembles the nude body).

Travelers heading for a casino might not want their money-belt to
trigger cross-examination harassment.

It's not difficult to find issues with the scanner.