Re: Dental X-rays

[julian.gibbs@mcmail.vanderbilt.edu]

     You raised several questions regarding safety and efficacy of dental x 
     ray examinations.  Since I have a dental background and have spent the 
     last 35 years full-time in academic radiology, I believe I can provide 
     accurate and useful information to you.  In inverse order:
     
     First, there is no accepted frequency for repeat exams (full-mouth or 
     otherwise) in asymptomatic patients.  Instead, high-yield criteria 
     have been developed that help the practitioner base his prescription 
     for radiographs on history and physical findings.  These criteria are 
     accepted by essentially all dental organizations.  They are rather 
     similar to the appropriateness criteria of the American College of 
     Radiology for medical radiologic exams.  Of course, symptoms alter 
     these considerations, and exams are tailored to investigate the 
     symptoms.  The central issue is tailoring the radiologic procedures to 
     what the patient brings to the office.  In typical healthy 
     asymptomatic adults returning for 6-month checkup the most likely 
     decision is no radiographs necessary.
     
     Patient dose is now best expressed as effective dose (or effective 
     dose equivalent); because of the rather large uncertainty in these 
     calculations I tend to use the data interchangeably--although I 
     realize such is not exactly correct.  At present, the typical dentist 
     uses a 70-kVp half-wave rectified x-ray machine, with appropriate 
     filtration and a beam collimated to a 7-cm circle at the plane of skin 
     entry.  ANSI Speed Group D films now account for more than 80% of the 
     US market.  Under these conditions the effective dose equivalent for a 
     full-mouth exam (about 20 films) is about 0.3 to 0.4 mGy.  Switching 
     to Speed Group E film reduces this by a factor of about 2, to 0.15 to 
     0.25 mGy.  This change is free of cost; the purchase price of D and E 
     films is identical.  Further, beam-film alignment devices are now 
     commercially available that enable rectangular collimation of the beam 
     to the size of the film; these further reduce effective dose 
     equivalent by a factor of 4-5, to 0.03-0.05 mGy.  The cost to the 
     dentist to make this change is a few hundred dollars--i.e., 
     negligible.  Recently-developed direct digital intraoral image 
     receptors allow for a bit more dose reduction.  If we accept the NCRP 
     estimate of 3 mSv/y from natural environmental radiation, then a 
     standard dental exam by current practice is equivalent to some 45 days 
     background.  State-of-the-art technology reduces this to about 4.5 
     days.
     
     I have taken these data from the results of Monte Carlo simulations we 
     performed some 10 years ago.  They are published in Dentomaxillofacial 
     Radiology 16:67 (1987) and 17:15 (1988).
     
     I hope I have answered your questions.  If you need more, please feel 
     free to get back to me, either via radsafe or privately.
     

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     S. Julian Gibbs, DDS, PhD                           Voice 615-322-3190
     Professor of Radiology                                FAX 615-322-3764
     Vanderbilt University Medical Center
     Nashville TN 37232-2675    Internet julian.gibbs@mcmail.vanderbilt.edu
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