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RE: QUESTION regarding radiation theraopy using linear accelerator that produces a 6 MEV photon
A linac is basically a super-powered xray machine. It's not producing a "6
MEV photon". It's producing a beam of a continuous range of energies, up to a max E of 6 MEV. So, it's a spectrum like, in diagnostic xray language, a 6 MVp beam, but it's accelerated by RF, not a voltage potential. The effective E might be, nominally, 2 MEV. Hint: remember that what delivers the *dose* is electrons.
The RHH is not a great deal of help, in matters of radiological physics, that's not what it's designed for. I highly recommend: Khan, Faiz M; The Physics of Radiation Therapy; Williams & Wilkins. For a question like this, the edition number is of no account, I should think.
Cheers (and best wishes for your friend)
cja
"Sandy Perle" <sandyfl@EARTHLINK.NET> wrote:
>I am forwarding this question for Michael Coogen. Please respond to
>Michael directly at: michael_coogen@hq.dla.mil
>
>I have a very dear friend whose wife has been diagnosed with
>adenocarcinoma of the breast. The radiation therapist is using a
>linear accelerator that produces a 6 MEV photon. Looking at Figure
>5.3, page 129, in the RHH, June 84 edition, which is for water
>absorption (it's the closest graph that is comparative for tissue),
>and using the I=Ioe-(u/p)(p)(t) equation, I calculate that 0.8 of the
>photons pass right on through the breast. So I ask why use such high
>energy photons when most of them are not absorbed. Then looking at
>the same graph for Compton scattering, at 6 MEV, the scattering
>calculates to be only 7%. At 180 centigrays (rads/day), I conclude
>that they need the very high energy photon to minimize compton
>scattering to the rest of her body. I also suspect that the cone
>created by the scattered photons is less as the energy goes up. Am I
>right with these assumptions? Can anyone shed some light on the type
>of therapy?
>
>Michael Coogen
>michael_coogen@hq.dla.mil
>
>-------------------------------------------------
>Sandy Perle
>Director, Technical
>ICN Worldwide Dosimetry Service
>ICN Plaza, 3300 Hyland Avenue
>Costa Mesa, CA 92626
>
>Tel:(714) 545-0100 / (800) 548-5100 Extension 2306
>Fax:(714) 668-3149
>
>E-Mail: sandyfl@earthlink.net
>E-Mail: sperle@icnpharm.com
>
>Personal Website: http://sandy-travels.com
>ICN Worldwide Dosimetry Website: http://www.dosimetry.com
>
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