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Re: shielding requirement for existing vault
Firstly, even if the "dose limiting values" were the same, if you were going
to use the higher-E beam, there would be photoneutrons flying around in
significant numbers. Unless someone was very farsighted, when doing the
original shielding design, it's unlikely that anyone allowed for the
potential for dose equivalents originating with neutrons. Even so, remember
the changes in the radiation weighting factors (we don't know how old the
facility is). Neutrons aside, I think that most xray control regulators
would regard the installation of a 20MV machine in a room designed
(presumably) for a 10MV one as a significant change in usage, and, thus,
require a new plan review. I suspect that most of them would require that
you meet the 100 mrem/y (sometimes interpreted as 2 mrem/wk) limit, unless
you could make an excellent case against it, on ALARA engineering grounds.
The only way to know for sure is to ask them.
>
>
>At 10:05 11/20/98 -0500, you wrote:
>>It's been a while since I've done shielding calculations for a therapy
>>unit. If we want to replace a unit with 10MV x rays for one with 20MV x
>>rays, can I use the same dose limiting values (10mR/week, uncontrolled and
>>100mR/week, controlled) used for the original shielding calcs? Or are we
>>required to add shielding to meet more recent and more stringent dose
>>limits? Such as 2mR/hour in any one hour? etc.
>>
>>Kent Krugh, M.S.
>>Intercommunity Cancer Center
>>Cincinnati
>>
>> *********** ************
>>Frequently Asked Questions (FAQ) for Medphys at:
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>>
>
Chris Alston
alstonc@odrge.odr.georgetown.edu
I am not here a representative of my employer.
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