RE: Wayne Johnson's message RE Dental X-ray Shielding

[Sharyn.Baker@UCHSC.edu]

Hello Peter,

Here is the post.
My compliments,
Sharyn Baker
Instructor
Health and Safety Division
UCHSC



> 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 was superseded by report 49.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
> 
> 
> 
> ----------
> From: 	Vernig, Peter G.
> Reply To: 	radsafe@romulus.ehs.uiuc.edu
> Sent: 	Tuesday, February 16, 1999 9:39 AM
> To: 	Multiple recipients of list
> Subject: 	Wayne Johnson's message RE Dental X-ray Shielding
> 
> My apologies to the group.  I must have miscoded the address or for what
> ever reason my message was not delivered.  Wayne if you'd send me a
> message
> at the address below I may have some info of interest.
> 
> Peter G. Vernig
> Radiation Safety Officer
> VA Medical Center, Denver
> 
> 1055 Clermont Street, Denver, CO 80220
> ATTN:  RSO, Mail Stop 115
> peter.vernig@med.va.gov
> 303-399-8020 ex 2447
> FAX 303-393-5026
> 
> Any opinions expressed in this message are solely my own and do not
> necessarily reflect the opinion or policy of the Denver VA Medical Center,
> The Dept. of Veterans Affairs, or the U.S. Government.
> 
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