Re: QUESTION regarding radiation theraopy using linear accelerator that produces a 6 MEV photon

["Vincent A King/KINGVA/CC01/INEEL/US" <KINGVA@INEL.GOV>]

I guess I don't understand the clarification...



If I read it correctly, the initial posting by Alston said that a spectrum

of photons, rather than a beam of monenergetic photons, was being created -

essentially an x-ray spectrum with a very high peak energy.



Then the correction seems to be saying the same thing - that a spectrum of

photons is produced.



What is the distinction being made here?



I also don't understand the reference to the Bragg peak in discussing a

photon beam - I thought the Bragg peak was a feature of energy deposition

by a particle beam -?-  Do depth dose curves (at least that's what I

vaguely remember them being called a long time ago) for high energy photon

beams have a similar feature making them "tuneable" like particle beams?



V. King









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.



...





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

>













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