Re[4]: Lead Aprons in Dentistry

[tdc@ehssun.lbl.gov]

>This is per a posting from a member on the MedPhysics LiustServer, a 
>state inspector.. He states that the older houysings had more 
>shielding comparec to the newer ones. Here is what he posted.. 
>better using his words than mine. 

>>Housing leakage is definitely measurable from dental tube heads.  
>>The FDA says that leakage radiation shall not exceed 100 millirem 
>>in one hour at a distance of 1 meter.

Actually - more precisely stated - it is 100 mR in any one hour at 1 meter
AT MAXIMUM CONTINUOUS KV AND MA!  It would be meaningless to state the
limit in terms of mR/hr (which it seems the person originating the message
would prefer!)

Certainly this DOES mean that the manufacturers COULD allow leakage rates
as calculated - but they don't!

I have had lots of experience measuring leakage from dental x-ray units -
but certainly not as much as a state inspector might.  AND when I do such
a measurement I remove the collimator and put a lead blocker on the port so
as to avoid measuring scatter.  In ALL the measurments I have ever done I
have NEVER seen an instantaneous leakage rate of anywhere near 100 mR/hr at
1 M at OPERATIONAL kV and mA - let alone at continuous (which it is
IMPOSSIBLE to do from accessible controls).  Even doing measurements with
scatter off a phantom, at 1 M - I seldom see more than a couple 100 mR/hr -
instantaneous rates.


Also - 0.5 second would a relatively long exposure for modern techniques.
More like 0.1!

A dental unit would most likely melt into a puddle before there could be
any sort of exposure of concern from leakage.

Oh yeh - and the distance to the chest is irrelevant - since the apron
required is a lap apron and not the normal apron worn in x-ray labs.
Certainly a normal apron could be used - but it's not required.


>>The use of  "in one hour"
>>instead of "per hour"  allows the manufacturer to stipulate a duty 
>>cycle for the machine.  One duty cycle I've seen is that the 
>>machine should only be operated for one second out of every 60.  
>>This would allow a instantaneous rate of 6000 mR/hr , or 1.7 mR/sec 
>>at 1 meter.  Typical exposure times depending on mA and cone length 
>>are on the order of 0.5 sec and if one assumes a distance of 0.25 
>>meters to the patients chest and it looks like you could get 3.4 mR 
>>per shot.  14 films or so in a full mouth series yields 47 mR at 
>>0.25 meters.  

Ted de Castro
tdc@ehssun.lbl.gov
University of California Lawrence Berkeley Laboratory
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Berkeley, CA 94720
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