[ RadSafe ] Allowable Dose Rates
ROBBARISH at aol.com
ROBBARISH at aol.com
Tue Mar 28 04:41:25 CST 2006
Mr. Ravello writes:
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
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
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
Robert Barish, Ph.D., CHP
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