[ RadSafe ] Dose rate limitations

[ROBBARISH at aol.com]

Fellow RADSAFERS:
 
The current (2002 edition) "Medical and Dental Guidance Notes: A good  
practice guide on all aspects of ionising radiation protection in the clinical  
environment" used in Great Britain as the basis for regulatory compliance does  
indeed contain instantaneous dose rate limits (IDR) for medical linear  
accelerators of the type mentioned here. IDR is defined as being averaged over 1  
minute. 
 
According to the Guidance Notes, the IDR for a public area adjacent to the  
primary barrier of a medical linear accelerator must be less than 7.5  
microsievert per hour. (0.75 mrem/hr) This makes the typical primary barriers in  
Great Britain a TVL thicker [about 18" (45 cm ) of concrete] than here in  the 
USA, where instantaneous dose limits are not used. (Don't get confused by  the 2 
mrem in-any-hour rule, which is not the same as 2 mrem per hour.)  Other 
values are established for controlled areas, but there are IDR limits  everywhere.
 
As an example, a linear accelerator here with a total workload of 500 Gy  per 
week and a use factor of 1/4 toward a primary wall operating at 6  Gy per 
minute might have a dose rate during a protection survey of 60  microsievert per 
hour (6 mR/hr). The linac will be "on" pointing in that  direction for 
approximately 20 minutes per week (500 x 1/4)/600 = 21 minutes. So  the total dose 
will be 20 microsievert for the week; an acceptable 1 mSv for the  year. In the 
UK that dose rate would be 8 times too  high, requiring the additional TVL of 
shielding.
 
In Britain, the National Health Service foots the bill for the additional  
protection, so it's always at taxpayers expense.
 
What's really interesting about this is the fact that new technologies  might 
come along that provide for higher instantaneous dose rates from the  therapy 
unit. Here, that would be irrelevant since, for example, doubling the  dose 
rate would, of course, half the treatment time so the integrated  exposure 
would remain constant.
 
There, the higher dose rate would require additional shielding. The  document 
states (on page 65) that treatment rooms should be designed considering  
possible future developments including "increases in dose rates." So a shielding  
designer for a British radiotherapy facility possibly has to  have psychic 
powers in order to predict, years in advance, the  dose rates that might come 
from future generations of  medical accelerators.
 
Robert Barish, Ph.D. CHP
_robbarish at aol.com_ (mailto:robbarish at aol.com) 
 
<Dear colleagues

<As we are installing now a new Clinac  2100C/D accelerator with 6 and 18
<MV X-rays , I would like to know if in  your countries are there
<limitations on the max dose rate in the walls of  the installation ,
<specifically for the direct beam walls. I am perfectly  aware of the
<calculations for the max annual integral dose with the use  factor,
<occupancy factor and so on. My question is related to the  radiation
<protection survey of the competent authorities, and if there  are
<guidelines concerning also the dose –rates measured for the max FS ,  for
<direct beam radiaton.

<Thank you in  advance

<Sergio Faermann,Ph.D.