Re: Low Positive Doses in Personnel Monitoring

["Sandy Perle" <sandyfl@ix.netcom.com>]

Sue refers to the nagging problem of "false positives", for which 
there may be many causes. The most difficult one to deal with is 
plain statistics. I'll provide a few common causes, and some that are 
just chance causes, ones that have no root cause to mitigate the 
issue:

1. The most important factor to deal with is the "type" of dosimeter 
used in the facility. Is it a film or TLD? Film has significantly 
more error associated with it, at the LLD than does the TLD.

2.  If a person's dosimeter reads 9.9 mrem (after the Control Badge
Background is subtracted) the dose assigned is 0. If the same badge
responds as 10.1 mrem, the dose assigned is 10 mrem. There is
essentially no difference in the two results. It is a statistical
calculation.

3. In some cases a dosimeter is exposed to a source unbeknownst to 
the customer. Accountability of the dosimeter, the security of the
dosimeter is an issue that must be constantly monitored.									 6.

4. Film is also susceptible to other external factors that result in 
what looks like an exposure to ionizing radiation. Heat for instance 
can fog a film making it look like a radiation dose.

 5. It is important to note that a majority of the individuals badged 
at your facility did receive a zero dose assigned. If there were a 
major processing problem for a specific wear period, it is logical to 
assume that all of the dosimeters would be affected in the same 
manner. 

6. The most critical component of any processing program, be it 
in-house or commercial processor, is the Quality Assurance Program. 
What dies the processor do to nsure that not only does the dosimeter 
report an accurate and precise dose, based on algorithm statistics, 
but also how does the dosimeter respond when there is in fact no 
radiation exposure.

7. The processor needs to include as many non-irradiated dosimeters 
in the process to ensure that the factor of "false positives" is 
minimized.

8. In TLD processing there are many factors for affecting accuracy. 
they are the TLD themselves, the element correction factors applied, 
the fade factor for the period of wear, the reader calibration, the 
irradiator used to irradiate the QC checks, the number of QC 
dosimeters used to normalize the filed dosimeters, etc.

9. In film there are the issues of the intallation of the film 
packet, the temperature of the chemicals and the state of their 
chemnical content, aging issues of chemicals, time of each step in 
the process from development to rinsing and, the most important 
factor, the film algorithm itself. Films are notoriously inprecise at 
low dose, especially when an LLD of 10 mrem is used.

10. The processor needs to use a significant number of non-irradiated 
films to derive a true "non-irradiated" exposure to minimize false 
positives.

If all else fails, and the facility is CONFIDENT beyond a reasonable 
doubt that the individual did NOT receive any exposure for the time 
period, then the processor should be asked to change the dose for the 
period to ND, and this should be done in writing.


Dosimetry is NOT a state of the art process. It takes a lot of data 
and documentation and expertise to evaluate a process and the 
resultant dose. Of you, the facility have a concern, then this should 
be discussed with the technical group responsible for the processing. 
Only through this positive dialogue can there be a better 
understanding of how a dosimeter is processed, and, if there is a 
need to change, this can be facilitated.


------------------
Sandy Perle
Technical Director
ICN Dosimetry Division
Costa Mesa, CA 92626
Office: (800) 548-5100 x2306 
Fax:    (714) 668-3149
  
mailto:sandyfl@ix.netcom.com
mailto:sperle@icnpharm.com

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