Re: To BSF or not to BSF.

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

Doesn't BS from deeper tissues contribute significantly to the dose to the
skin? If you have concerns about deterministic effects, you'd need to know
what that increment is (assuming that it exists); even if you're not really
worried about skin doses, I'd think that you would want to know the
contribution, just for the sake of accuracy. On the other hand, if there
were no significant BS dose to the skin, then all you'd need would be the
free-air entrance exposure (or air kerma). See "Evaluation of an On-Line
Patient Exposure Meter in Neuroradiology", Gkanatsios N and Huda W, et alia,
presented at the 81st RSNA Meeting in November 1995; I think that it was
also published in Radiology, last year, but I don't have the reference, sorry.

chris alston


At 08:47  09/08/98 -0500, you wrote:
>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.
>
>If you had to provide an absorbed dose number to a person, why should the
number be artificially inflated by the BSF (to as much as 50%) when no
patient-equivalent phantom is used in the measurements (i.e., measurements
made free-in-air).  There exists enough uncertainty in diagnostic dosimetric
calcs.)  I understand the reasons for the f-factor, %DD, Cpt but not the BSF
in the above eqn.
>
>If I understand correctly, there are tables out there and one table =
another table x BSF.  But I still don't understand why the BSF should be
incorporated in the calcs.
>
>Anyone have references?  (I assume the Rosenstein's tables are in reference
to _Handbook of Selected Tissue Doses for Projections Common in Diagnostic
Radiology_ published by the DRH of the FDA).  Regarding the mammo. ref.
provided by Joel, do the Wu conversion tables printed in the ACR MQSA manual
incorporate BSF?
>
>Yep, I guess I'm wondering what the physics involved in this is.
>
>Thanks again,
>
>Bill
>
>William R. Geisler, M.S.
>Medical Physicist
>Marshfield Clinic
>1000 N. Oak Ave
>Marshfield, WI 54449
>(715) 389-3826
>FAX (715) 387-5240
>email: geislerw@mfldclin.edu
>
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Chris Alston
alstonc@odrge.odr.georgetown.edu
I am not here a representative of my employer.

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