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Radiology/Occupational Radiation exposure


Hello, apologies for the long, multi-part question.    

I understand the annual maximum dose for a worker in medical imaging (x-rays) is 50 mSv a year, or 100 mSv averaged over 3 years. I understand this is monitored by dosimeter badges worn by workers, and that these badges record "absorbed dose" in mGy. This energy is then converted into equivalent dose by a radiation weighting factor of 1 (for x rays) then converted into effective dose in mSv. My question is: "what tissue weighting factor is used to convert from absorbed dose in mGy and equivalent dose in mSv to "effective dose" in mSv?" in other words, how are these maximum allowable dosages determined?

Is it the same as saying an employee working with medical imaging radiation could theoretically annually receive; 25 head CT scans (at 2 mSv each) or 5 abdominal CT scans (at 10mSv each) or 30 Lumbar spine x rays (at 1.5 mSv each) ect. or a combination of these, and still be within the allowable perimeters? Or is occupational exposure measured differently than this?

For example: Lets say a CT scanner was set up to administer a specific amount of radiation for, say, a Head CT (for example purposes, lets say, has an absorbed dose of about 50 mGy) lets just say that there isn't a patient lying on the table, the worker stands near the scanner and this specific amount of radiation is released uniformly into the air instead of specifically into a patients head. Does this mean the worker received a "whole body effective dose" of 50 mSv (50 mGy x 1 (tissue weighting factor for x rays)= 50 mSv equivalent dose, whole body exposure so: equivalent dose=effective dose? Now we have a whole body exposure instead of a specific organ, does this now mean an effective dose of 50 mSv, and the worker has now met their yearly exposure limits? or, would that 50 mSv be spread out over the body and each organ received much less than 50 mSv each? Resulting in a much smaller effective dose.

When determining effective dose in medical imaging a tissue weighting factor is multiplied by equivalent dose. Which tissue weighting factor would be used in this case, since multiple organs are exposed? I understand that when the whole body is exposed then equivalent dose=effective dose if all organs received uniform radiation. Does this mean that the above example of 50 mGy spread out evenly would result in only a fraction of the original dose reaching all organs (ie: each organ receives a tiny amount).

Along the same lines, how is "background radiation" determined? ie: we are exposed to approximately 3 mSv each year from natural sources. What tissue weighting factor is used? Does this mean we all are exposed to, say, a Head CT and a few x rays, for example, every year?

Thank you for clarifying. Markus, BC, Canada

Dear Marcus,
You bring up very interesting topics.  They require a lot of time that I don't have right now to address them appropriately.  When I find the time to address them I'll reply to this message again and you will receive another email. Sorry but between my daily commitments,  I am unable to devote that time right now.


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Michael K.


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