Re: To BSF or not to BSF.

[Chris Alston <alstonc@odrge.odr.georgetown.edu>]

>> Behalf Of William Geisler
>> Walter and Joel,
>>
>> I said "what's what with using or not using back scatter
>> factor in determining absorbed dose of diagnostic examinations"
>>
>> You said:
>> >Entrance exposure = (exposure free in air) X BSF
>> ...
>> >dose at depth = (air exp)x(BSF)x(f-factor)x(% depth dose)x(temp &
>> >pressure & chamber energy correction factors)
>> ...
>>
>> Thanks much for the inputs, but WHY would you want to inflate
>> the entrance exposure value?  The radiation scattered off the
>> surface of the skin ain't gonna rescatter back to the patient
>> (at least not much--low Z of air); although perhaps some from
>> the table.
>Bill,
>
>You ask a good question. The issue of scatter is difficult to pinpoint
>because it is a coplex process that has many factors (Just a remainder, I
>know you know that). Since we possibly cannot hope for doing Monte Carlo
>simulations for all possible combinations of SID, kVp, HVL, patient
>thickness, etc, we settle for simple, ball-park estimates. Who cares if we
>are off by 20%?
>
>Now, the way I look at it, the BSF is a simpleton's answer to both scatter
>and backscatter issues. If you look at tissue close to the surface,
>backscatter will dominate (forget scatter in air, although if you want to be
>precise, you must look at iut, too). If you look at tissue close to the beam
>exit area, the backscatter will be negligible, but scatter from surrounding
>tissue will be significant. So, averaging two effects - one diminishing, the
>other one growing - we
>get an average, ballpark factor that someone eons ago decided to call BSF.
>
>Now, don't me get started on tissue-to-air ratios...
>
>The Wu tables convert entrance exposure in air to mean glandular doses. So
>all the above factors are included. You need to remember that Wu data are
>based on Monte Carlo calculations, so he counted everything, without calling
>it any particular name...
>
>Hope this helps.
>
>Wlad Sobol
>
>                    *****
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Chris Alston
alstonc@odrge.odr.georgetown.edu
I am not here a representative of my employer.

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