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RE: Re: QUESTION regarding radiation theraopy using linear accelerator that produces a 6 MEV photon
Ted
I'm not sure that I understand your response. For one thing, I thought that it was clear that we were talking about the photon beam energy. Also, FYI, 6 MV *photon* beams are used routinely to treat breast cancer. There are no lower energy machines available, unless someone has an older 4 MV linac. To the best of my limited knowledge, the latter are no longer being built. Maybe it would help to remember that kerma increases with depth, while absorbed dose decreases. Also, in order to minimize morbidity, i.e., spare healthy tissue, the beam is highly collimated (especially in IMRT) and enters the patient through a multiplicity of portals, or even in continuous rotation.
I happily await the slings and arrows
cja
Ted de Castro <tdc@XRAYTED.COM> wrote:
>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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