[ RadSafe ] Myth or Fact? (Dental dose)

[Yoshihiko Hayakawa]

Dear RADSAFE subscribers,

I think that the entrance skin dose of 4.71 mGy is slightly high.

There are at least three recent survey reports as follows;
UK survey: Napier, I.D. Reference doses for dental radiography.
     British Dental Journal, 1999; 186: 392-396.
US survey: Suleiman, O.H. et al. Radiographic trends of dental
     offices and dental schools.
     Journal of American Dental Association, 1999; 130: 1104-1110.
Spain survey: Gonzalez, L, et al. Reference doses in dental
     radiodiagnostic facilities.
     The British Journal of Radiology, 2001; 74: 153-156.

UK and US survey reports are cited by the UNSCEAR2000 Report.

UK survey: The average entrance dose was 3.3 mGy, which included
   the data at lower kV sets (45-55 kV). The average obtained at
   kV sets commonly used (60-70 kV) was 2.2 mGy. The report
   included much higher doses which I can not believe. The highest
   at 60-70 kV sets was 20.5 mGy…
   (Many years ago a professor in Sweden told me that the operation
    at less than 60 kV is illegal in Sweden.)
US survey: Mean entrance air kerma values ranged approx. between
   1 mGy and 2 mGy. Sometimes X-ray equipments were operated at
   higher kV settings in the United States.
Spain survey: The mean value of skin doses was 2.89 mGy.

If measurements are carried out at my school now, average entrance
doses distribute approx. between 1 mGy and 2 mGy at 60-70 kV
settings.

- - - - - -
The record of dental doses in the 20th century was described by
S.C. White, UCLA, in 1992 as follows;
White SC. 1992 assessment of radiation risk from dental
radiography.
Dentomaxillofacial Radiology, 1992 Aug; 21(3):118-26.

Based on White’s report, I can describe, “Doses with a single
dental exposure (bitewing/intraoral radiography) are equivalent
to between approx. 7 hours and 1 day background exposure.

- - - - - -
A reasonable estimate in my opinion of the effective dose from
a single dental (bitewing/intraoral) radiography ranges between
1 to 5 microSv (0.1 to 0.5 mrem). Another estimate in my opinion
of the effective dose from the full mouth survey ranges between
10 to 50 microSv (1 to 5 mrem).

I agree the statement that in Europe a skilled dentist (or his
technician) with state-of-the-art equipment will expose you to
about 0.01 mSv effective dose per examination. But in a recent
decade some sophisticated equipments have become world-wide
available.

Now it can be said in my opinion, "Every year, the average
globe citizen is exposed to natural radiation equal to about
720-3,600 dental X-rays."

Sincerely.
   Yoshihiko Hayakawa
   Tokyo Dental College
    mailto:yhayaka at tdc.ac.jp

Peter Thomas wrote:

> 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
>  
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