Strange occupational exposures, part II

["Sandy Perle" <sandyfl@earthlink.net>]

The following posting was submitted to MedPhysics. I have provided my 
thoughts and agreed to post this to Radsafe to further assist Brian 
with information. PLEASE contact Brian directly if you have 
infotrmation which may help him in his search for answers.

Strange occupational exposures, part II

From: Brent Colby 
Date: Mon, 20 Mar 2000 16:14:01 

I'm still investigating the strange occupational exposures I 
mentioned some weeks ago.  You may recall that we have an 
interventional radiologist whose reported exposures have tripled 
recently.  That jump in reported exposures coincided with my switch 
to a new type of dosimeter.  I have double monitored staff since the 
post (old style and new style dosimeters), but haven't received 
reports back yet.  

As I've probed, I've noticed that the shallow, deep and eye doses for 
low energy exposures are not infrequently reported to be the same 
value.  I find that surprising for low energy photons but don't have 
enough good data to be very confident.  

Since my original post, I've received a few responses from other 
Physicists reporting some issues with the new dosimeters, but nothing 
like what I'd expect if the new dosimetry system tended to be error 
prone.  I am puzzled. The vendor is silent.  

Would others expect to see an identical shallow, deep and eye dose 
for low energy photons?  Are others seeing it?  I'd genuinely welcome 
 
Brent Colby
Physicist
MeritCare Health System
727 Broadway
Fargo, ND 58123
(701) 234 5613
brentcolby@meritcare.com

==========

Re: Strange occupational exposures, part II

From: Sandy Perle
Date: Mon, 20 Mar 2000 21:47:50 

> Would others expect to see an identical shallow, deep and 
> eye dose for low energy photons?  

Brent,

Based on the specif energy of the low energy x-rays, one would
NOT expect to see identical Deep, Eye and Shallow Doses. To
validate this one only need look at the various Ck values for the
Deep and Shallow conversion factors based on the various NIST
Beam Codes used to determine final dose, for either NVLAP or
DOELAP required programs. You obviously would come under the
NVLAP program. Depending on the various low energy photons,
the Deep will be either lower than, and what some don't understand
or expect, the Deep can be actually be higher than the Shallow
dose. The basis is in the Ck values, penetration coefficients and
the backscatter effects from various dosimeters. This is all
explained in the NVLAP and NIST dose conversion tables.

What you haven't addresses yet is not the case of mono-energetic
low energy photons, but what about the case of mixtures? If you
see a problem with a single energy, watch out for how the
dosimeter responds in a mixed field of varying low energy photons,
and, what about low energy photons with beta, or also with some
high energy photons as well. There is always scatter and with it,
varying degrees of x-ray energy in the work place. Ask for the basis
of the dose determinations.

While I have these documents, I suggest you ask your current
vendor for them, and, to explain their basis as to how they
determine dose.

From: Sandy Perle
Date: Tue, 21 Mar 2000 01:29:33

Two other considerations if the Deep, Eye and Shallow doses
reported are equal:

(1) The Deep Dose is measured at a depth of 1.0 cm., Eye Dose at 0.3 
cm. and Shallow Dose at 0.007 cm. If the doses reported are equal, 
for low energy photons, or, for beta, or for mixtures of any of these 
combinations, the issue most likely is .... the dosimeter 
configuration is such that the dosimeter CAN'T measure dose at either 
the 0.3 cm or 0.007 cm. depth. In layman's terms, the filters are too 
thick, and the dosimeter is essentially only measuring the Deep Dose 
and is then used to report all three required depths.  

(2) The processor may be rounding off the doses, using several 
methodologies, especially is the doses are low. For example, if the 
processor rounds up or down using 5 mrem increments, a Deep Dose of 
10.5, Eye Dose of 16 and a Shallow Dose of 19 may be reported a 15 
mrem across the board. If the processor rounds up to the highest 10 
mrem increment, the same doses are all reported as 20 mrem across the 
board. This is the worse case in that the person with a Deep Dose of 
10.5 now reports 20, almost a 100% increase in dose. Multiply that by 
100 wearers, or a 1,000 and you now have a very large "false" 
cumulative man-rem for this one time period. Now multiply that by the 
number of months and it's even larger.  

I believe the first item above is the most serious issue, if there is 
an issue, in that a dose isn't reported since it can't be detected, 
and therefore, can't be calculated. The basis for dose is ANSI N13.11 
for whole body monitoring, and ANSI N13.32 for extremity monitoring. 
Each facility should be cognizant of the requirememts set-forth in 
each.  

------------------------------------------------------------------------
Sandy Perle					Tel:(714) 545-0100 / (800) 548-5100   				    	
Director, Technical				Extension 2306 				     	
ICN Worldwide Dosimetry Division		Fax:(714) 668-3149 	                   		    
ICN Biomedicals, Inc.				E-Mail: sandyfl@earthlink.net 				                           
ICN Plaza, 3300 Hyland Avenue  		E-Mail: sperle@icnpharm.com          	          
Costa Mesa, CA 92626                                      

Personal Website:  http://www.geocities.com/capecanaveral/1205
ICN Worldwide Dosimetry Website: http://www.dosimetry.com

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