Shielding for dental offices

["Wayne F Johnston" <wjohnsto@dhhs.state.nh.us>]

The issue is shielding for dental intraoral machines, podiatric units, and
other low-output or low workload units/facilities.  I have to review
shielding plans and offer comments on them.  If you have had occasion to do
such designs, have you any generalities or advice to offer?

There seems to be a rule of thumb that two thicknesses of 5/8 inch gypsum
wallboard are adequate shielding for most dental offices.  As far as I can
tell this idea was possibly useful when there was an implied dose limit of
500 mrem per year for members of the public.  My state has adopted an
annual public dose limit of 100 mrem.
* Is the rule of thumb no longer useful with a 100 mrem annual public dose
limit?

NCRP report 35 (1970) is entitled "Dental X-Ray Protection."  It makes some
recommendations in chapter 3 about shielding design.  In particular, it
suggests there be calculations for the walls, floor, and ceiling as both
primary and secondary barriers and that the use factor for primary barrier
calculations be taken as 1/16 for a default value.  (Two pages later, it
suggests the ceiling and the wall the patient faces have use factors of
zero.  Go figure.)  The report uses the formalism of NCRP report 34 which
was superseded by report 49.  As in NCRP report 49, report 35 has some
tables in the back for thicknesses of lead and concrete to be used as
shielding.  A problem is that most dental offices have partitions made with
gypsum wallboard and are not leaded.

I have been doing some NCRP 49-type calculations with various x-ray tube
workloads, distances, etc. and have come to the tentative conclusion that 2
sheets of gypsum wallboard is not sufficient for primary shielding in all
cases.  After calculating the various required attenuations, I have then
used the curves in S A Glaze et al, "Use of Gypsum Wallboard for Diagnostic
X-Ray protective Barriers," Health Physics Vol. 36 pgs. 587-593 to
calculate the thickness of wallboard needed to do the job.
* Is the NCRP 49 formalism acceptable for use in dental shielding
calculations or does it lead to overly conservative, hence overly
expensive, designs (or not conservative enough)?
* If the NCRP 49 formalism is not appropriate, what would you use?

This issue is probably of little interest to most members of the list, so
you can E-mail me privately if you wish.  Any and all contributions
accepted with many thanks.

Wayne Johnston
wjohnsto@dhhs.state.nh.us
Tel. 603-271-4842
Fax 603-225-2325


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