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Re: QUESTION regarding radiation theraopy using linear accelerator that produces a 6 MEV photon
Well - actually not quite.
The accelerator accelerates electrons and does so all at the same energy
- it cannot work any other way. To get photons the electrons strike a
target and then produce a spectrum of photons.
The point was well made about 6 meV photons being a surprisingly high
for this therapy. I don't do this sort of work so I DON'T KNOW - BUT -
I would suspect that MAYBE they are using the electron beam at an energy
set so as to put the Bragg Peak at the depth of the cancer. I DO KNOW
that this is the method used when heavy ion accelerators are used for
breast therapies.
But then again - I don't know that this is not a generally distributed
cancer. If it were - then I would GUESS that the high energy would be
used in a lateral orientation in order to achieve a fairly flat dose
across the entire organ.
I was waiting to see if someone with more specific information would
weigh in on this subject before stepping in myself.
alstonchris@netscape.net wrote:
>
> 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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