Re: Multiple Dosimetry and Weighting Factors

[Bob Flood <bflood@SLAC.Stanford.EDU>]

At 11:38 AM 2/21/97 -0600, you wrote:
>The footnote for the Table of organ weighting factors  in 10CFR20 refers to
>the calculation of Total Effective Dose Equivalent (TEDE) which is the sum
>of internal EDE and external DDE (from perosnnel dosimeter reading). It is
>this TEDE that is limited to 5 rems a year, according to 10CFR20--an
>important issue that is often neglected by many.   A whole body weighting
>factor of one is recommended by NRC to weight the DDE.  The reason for NRC
>to "invent" this whole body weighting is that the DDE tends to overestimate
>the EDE as in the case of multi-badging and large angle exposure.   It opens
>the door for other smaller-than-one whole body weighting factors, provided
>you can justify it. 
>
This is quite correct. The NRC didn't want to have to inspect/assess each
licensee's home-invented technique for relating DDE measurements at various
locations on the body to EDE. It specified Wt=1.0 for general use, but left
the door open should the HPS develop a multibadging/EDE standard that the
NRC could endorse. In that sense, the ball is in the NRC's court - perhaps
a Reg Guide endorsement is in the works.

A method was mentioned in one of the postings on this topic describing the
possibility of tracking dose to multiple locations on the body and
reporting as the dose of record the maximum. I know of at least one place
that does this (it's not my fault, I swear it! I was just following
orders!) and I strongly recommend avoiding this. Conventionally, one can
sum the dose measured by a person's single chest badges with the maximum
dose from a set of multibadges to determine a value that can be legally
recorded for that person. This approach is the sum of the maxima - always
using the max dose from a multibadge set regardless of body location. This
is conservative and can demonstrate compliance with dose limits, but,
because a max dose that happened to the right thigh may be added to max
dose on another job where the max was received by the head, it can also
represent a total dose that no part of the body actually received.

By tracking the (typically) 8 multibadge receptor locations at all times,
assigning the doses measured by the single chest badge to each location and
assigning separate values when they are available from multibadge data, a
running total can be kept for each receptor, with the max at any one time
being the dose of record. This technique uses the maximum of the sums. This
is legal (I was given the task of setting up such a system at a
Federally-owned utility in the south which shall remain nameless), but the
software for dose tracking is very complicated and requires a lot more
storage space, and can dim the lights all over town when it recalculates
everyone's doses. It also can confuse workers. For example, Bubba's YTD
dose could be displayed on the computer screen as 211 mrem, but what is not
displayed is the fact that this is the dose to his head, while the next
highest dose is 150 mrem to his chest. If Bubba gets more dose (measured by
multibadges and electronic dosimeters) about 50 mrem to the chest but 10
mrem to the head, he remembers he got 50 more mrem but his total only went
from 211 to 221. And he doesn't understand the bookkeeping that goes into
it. I enthusiastically recommend avoiding this approach to dose tracking.


>For those who are interested in more information related to this topic,
>there is paper on HPJ, and I welcome any comment.
>
>Xu, X. G.; Reece, W. D.; Poston, J. W. A Study of the Angular Dependence
>Problem in Effective Dose Equivalent Assessment. Health Physics.
>68(2):214-224; 1995.
>
This is an excellent reference. These gentlement have developed a
2-dosimeter method for estimating EDE in non-uniform photon fields that is
far more practical than that given in the ANSI standard. I hope that the
regulators give this the same consideration they give to the standard.


Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.