[ RadSafe ] Prostate Radiation Treatment

[Robert Barish]

It is very likely that the person who heard a dose of  58 Gy over eight 
weeks as the radiation therapy prescription for his prostate cancer simply 
reversed the numbers: 85 Gy  is a reasonable value for a curative 
external-beam approach using IMRT (Intensity Modulated Radiation Therapy).

Also, for a number of reasons, many sites do indeed use 6 MV rather than the 
10 MV, 15 MV or 18 MV that their linear accelerators can also produce.  The 
most important of these is that the dose distributions obtained with 
multiple 6 MV fields are very conformal to the required treatment volume 
while sparing the surrounding sensitive normal structures. So the 
"horsepower" needed years ago when four-field "box" treatment were the norm 
no longer is necessary. Another reason is the secondary neutron production 
in the accelerator that occurs with those higher-energy beams. Since IMRT 
treatments typically involve 3-5 times more "beam on" time (expressed as 
increased monitor units) for an identical treatment dose when compared with 
conventional (3-D) radiotherapy, the leakage radiation from the accelerator 
contributes that much more whole-body dose to the patient. The photon 
component is limited to a value of about 0.1% of the treatment dose in the 
form of direct leakage. Added to this is the room and patient scatter dose, 
the value of which is a more complex function of the room size and geometry. 
At 6 MV there is no neutron production since that photon energy is below the 
binding energy of the materials in the accelerator structure. At the higher 
energies available with these linacs, the photon dose is supplemented by the 
neutron component. At 18 MV, that value is about 0.15 percent of the monitor 
units, so with IMRT, for the 85 Gy treatment described here, a patient's 
whole-body dose can be as much as 450 mGy from photons with a 6 MV treatment 
where the "IMRT factor" is about 5, and more than 1 Sv if a 15 MV beam is 
used. That high whole-body dose troubles those concerned about induced 
malignancies in patients who are cured of their cancer and might survive 
long enough to reach a point where secondary tumors caused by the radiation 
treatment might occur.

Robert Barish, PhD, DABR, CHP
Radiation Shielding Consultant


> ----------------------------------------------------------------------
> Date: Tue, 28 Jul 2009 15:11:37 -0600
> From: Brian Rees <brees at lanl.gov>
> Subject: [ RadSafe ] External radiation treatment for Prostate Cancer
> To: radsafe at radlab.nl
> Message-ID: <7.0.1.0.2.20090728150330.02a2e8e0 at lanl.gov>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
>
> A colleague and friend of mine is receiving external radiation
> treatment for prostate cancer.  He had some questions regarding his
> dose that I was unable to answer, so I figured I'd turn to Radsafe:
>
> He has been told that the total dose over 8 weeks of treatment is 58
> Gy, is that reasonable? (I KNOW that the Doctor prescribes treatment,
> and does what they think is best, but this seems a bit low to me)
>
> What is the probable energy of the radiation?  He was told 6 MeV,
> that seems kinda high to me.  Or is this just Emax, with E ave ~1/3 Emax?
>
> Is there an approximate whole body dose for each treatment or an
> integrated WB dose equivalent?  Even approximately?  Since blood is
> irradiated, and there's some scatter, I figure there's some sort of
> pseudo-WB dose....
>
> Thanks,
> Brian Rees
>