[ RadSafe ] Allowable Dose Rates

[ROBBARISH at aol.com]

Mr. Ravello writes:
 
Dear Sirs,

I have to mention one thing about my consult in the  list: the dose rate that
we measured was at control booth. If we take the  NCRP recommendations, the
dose in a wk must be less than 0.1 mGy. We  calculate the dose in function of
the workload: the result was that the  shielding is good. But usually, the
dose rate we measured is less than 100  uSv/h [kerma]. So, when we measured
the 500 uSv/h, we asked: is there a dose  rate limit for practice purpose?
 
Here is my assessment: 
 
My daily work now consists almost exclusively in the design of  radiation 
shielding for medical facilities worldwide. In this regard,  I point out that in 
the United States the acceptable dose is only  determined by the workload and 
occupancy at the point of interest, not by dose  rate. In the United Kingdom, 
however, the regulatory authorities have  decided to consider the 
"instantaneous dose rate" (IDR) from medical x-ray  equipment in deciding on the 
acceptability of shielding.
 
The current standards that are expected to be met are elaborated in a  
document with the title: Medical and Dental Guidance Notes: A Good Practice  Guide 
on All Aspects of Ionising Radiation Protection In the Clinical  Environment. 
This document is published by the Institute of Physics and  Engineering in 
Medicine. My copy is dated 2002.
 
Briefly put, that document does not specify a maximum permissible dose rate  
in "supervised areas" (equivalent to what we in the USA call "restricted  
areas"). So the brief answer to Mr. Ravello is that there is no dose-rate-limit  
at the control booth. He needn't worry.
 
The situation is quite different for "unsupervised public areas" (what we  in 
the States call "unrestricted areas"). The guidance document calls for an  
IDR with a maximum of 7.5 uSv per hour in those public spaces. For  those of us 
who think in old units, that's an instantaneous dose rate of only  0.75 mrem 
per hour!  In those same areas the recommended annual dose is 300  uSv (30 
mrem) per year, about a third of the 1 mSv (100 mrem) annual dose limit  applied 
in US facilities. 
 
For a shielding design in the UK, the IDR plays a significant  role in the 
design criteria. Just as a brief example, if a 15 MV medical linear  accelerator 
is running at a dose rate of 6 Gy per minute at the  isocenter, and a 
horizontal beam impacts a person at a distance of 6.5 meters  away, the shielding 
required to meet the IDR standard would be just over  8.5 feet of concrete. Here 
in the USA, with identical geometry, for an annual  dose of 100 mrem and a 
typical workload of 500 Gy per week with a use  factor of 1/4 toward the wall, 
the required shielding would be only a bit more  than 7 feet of concrete. 
 
What's interesting about this, aside from the fact that the National Health  
Service apparently doesn't mind paying for this extra shielding, is that the  
document contains the following quote: "In estimating adequate protection at 
the  design stage, the following future developments should be considered:
(a) increases in dose rates."
 
In designing here in the USA, if a new breed of accelerator has double the  
dose rate of existing units, the patient treatment time would be halved, so the 
 shielding would still be adequate given the fact that actual beam-on time is 
a  small fraction of the total time devoted to each patient. In the UK, the 
IDR  from this hypothetical new machine would double, requiring an extra HVL of 
 shielding. The irony is in the request to estimate this technological leap 
in  the design stage. My psychic powers don't allow me to predict what the dose 
rate  of a future generation of linacs will be, so, I confess to this forum, 
I  really can't meet the requirements of the IPEM document when I design a  
British facility. I hope I don't lose any jobs there as a result of this  
confession.
 
Anyway Mr. Ravello, as long as your control booth is meeting the weekly  
integrated ALARA dose limit of 0.1 mGy (10 mrad), I think that it's more than  
adequate.
 
If there is a subscriber to the list from a country where there are IDR  
restrictions and the permitted IDR is less than 7.5 uSv/hr in unsupervised  public 
areas I would be interested in knowing where that is, and what those  limits 
are!
 
Robert Barish, Ph.D., CHP