Re: HPS objective

[David Scherer <scherer@uiuc.edu>]

Ron said:

>Unnecessary radiation exposures aren't all necessarily harmful.
>
>If I get a second chest xray because the first one got lost or is of 
>poor quality, the second isn'y harmful even though it should not have 
>been necessary.

This depends on what you mean by  "harmful" and "unnecessary."

On harm: It could be that low doses of radiation add an incremental risk of
cancer etc.  Is incremental risk "harmful" or do we need actual effects?
If someone drives drunk ten times and has only one accident, does that mean
that drunk driving was safe the first nine times?

It seems to me that "harm" has to do with the degree of risk with the
uncertainty factored in somehow, but it is a judgement call.  A chest x-ray
(a few mrad) is probably harmless, but an abdominal CT (a few rad) may not be.

On necessity:  At the time of the retake, the patient has not been
diagnosed.  You have to decide whether to take the picture again.  The
exposure is "necessary" to get the diagnosis.  If there was a diagnostic
quality film in the tech's hand while they take the second film, that would
be "unnecessary."

But I don't think the HPS Objective is addressing accidents and mistakes.
It appears to be based on the ICRP's principle of justification, perhaps
with some elements of optimization.  The intent is to address unnecessary
exposures in the sense that they provide no societal benefit.  Using Cs-137
in a self-fluorescent wall paint might be an example of something that is
unnecessary.  Or performing that chest x-ray at 70 kVp instead of 120.

Regards,
Dave Scherer
scherer@uiuc.edu