[ RadSafe ] Myth or Fact? (Dental dose)

[Peter Thomas]

New Zealand's NRL produced a report 15 years ago based on TLD
measurements in a RANDO phantom to get the ratio of organ doses to the
entrance skin dose for a couple of dental exams.  The following is for a
single bitewing film at 60kV based on an entrance skin dose of 4.71 mGy
(471 mrad).  It's an old report but I'd expect similar results under
today's conditions given the same entrance skin dose.

Entrance skin  4.71 mGy
Ovaries 0.0000047 mGy
Testes 0.00044 mGy
Bone (in head/neck) 0.019 mGy
Breast 0.00196 mGy
Lungs 0.00106 mGy
Thyroid 0.0215 mGy
Brain 0.0094 mGy
Sinuses 0.426 mGy
Salivary Glands 0.275 mGy
Pharynx 0.327 mGy
Larynx 0.048 mGy
Liver 0.00011 mGy
Stomach 0.000052 mGy
Kidneys 0.000033 mGy
Small Intestine 0.000014 mGy
Eyeballs 0.021 mGy

The main contributors to the effective dose are the dose to the thyroid
and the dose to the salivary glands (0.025 weighting factor under ICRP60
rules where a remainder tissue has a higher does than any of the tissues
with individual weighting factors).  The skin dose drops by a factor of
600 due to the ratio of the irradiated area to that of the whole skin.
The bone dose gets mangled a bit by apportioning between bone surface
and bone marrow, enhancement factors and averaging over the whole body
and ends up contributing about 0.0005 mSv to the effective dose

Thyroid  0.05 x 0.02146 = 0.001 mSv
Salivary Glands 0.025 x 0.275 = 0.007 mSv

Effective Dose = 0.008 mSv (about 10 microSv or 1 mrem)

Apologies for writing a lot of zeros.  I didn't want to have to write
'micro' everywhere or cause confusion about what units I was using.
Hope I haven't made a transcription error.  In the original experiment
they set up an x-ray unit in its own room with a timer so they could
deliver the equivalent of about 10000 (ten thousand) exposures to poor
old RANDO.  The entrance skin dosimeters were removed part-way through
so the dose wouldn't be above 10 Gy and the final results were scaled
appropriately.  This is how they claim to be able to measure ratios down
to about 1 in a million (ovaries/entrance surface).

The original report is:
B D P Williamson, NRL 1990/6

End result (apologies for the bandwidth): 10 microSv or 1 mrem
(effective dose) is a reasonable estimate in my opinion of the dose from
a single dental x-ray.

Peter Thomas
Medical Physics Section
ARPANSA
 

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On
Behalf Of Otto Raabe
Sent: Friday, 25 August 2006 11:52 AM
To: Bev; radsafe
Subject: Re: [ RadSafe ] Myth or Fact?


At 02:34 PM 8/24/2006, Bev wrote:
>     I recently read a book called "Myths, Lies, and Downright 
> Stupidity"
> by John Stossel of 20/20 fame: "Every year, the average U.S. citizen
is 
> exposed to natural radiation equal to about 360 dental X-rays."
*************************************************************

Stossel doesn't understand the radiological  science.

The 360 mrem per year is the BEIR V average for people in the
continental 
U.S. for effective whole body dose-equivalent ( effective dose) from 
natural, medical, other man-made sources. The1 mrem for a dental X-ray
is 
only an estimate of the dose-equivalent to a small portion of the jaw.
The 
tissue weighting factor (Wt) for the jaw must be really small. If its 
0.001,  he should have said 360,000 dental X-rays.

Otto

**********************************************
Prof. Otto G. Raabe, Ph.D., CHP
Center for Health & the Environment
University of California
One Shields Avenue
Davis, CA 95616
E-Mail: ograabe at ucdavis.edu
Phone: (530) 752-7754   FAX: (530) 758-6140
*********************************************** 


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