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J Am Dent Assoc, Vol 139, No 9, 1237-1243.
© 2008 American Dental Association |  |
RESEARCH |
The Impact of 2007 International Commission on Radiological Protection Recommendations Regarding Dose Calculation
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Methods. The authors used a tissue-equivalent head phantom to measure dose. They calculated effective doses by using both 1990 and revised 2007 ICRP recommendations. Effective dose is a calculation that takes into consideration the different sensitivities of organs to long-term effects from ionizing radiation. It is the preferred method for comparing doses between different types of exposures.
Results. Effective doses (per the 2007 ICRP) in microsieverts were as follows: full-mouth radiographs (FMX) with photo-stimulable phosphor (PSP) storage or F-speed film with rectangular collimation, 34.9 µSv; four-image posterior bitewings with PSP or F-speed film with rectangular collimation, 5.0 µSv; FMX using PSP or F-speed film with round collimation, 170.7 µSv; FMX with D-speed film and round collimation, 388 µSv; panoramic Orthophos XG (Sirona Group, Bensheim, Germany) with charge-coupled device (CCD), 14.2 µSv; panoramic ProMax (Planmeca, Helsinki, Finland) with CCD, 24.3 µSv; posteroanterior cephalogram with PSP, 5.1 µSv; and lateral cephalogram with PSP, 5.6 µSv. These values are 32 to 422 percent higher than those determined according to the 1990 ICRP guidelines.
Conclusions. Although radiographs are an indispensable diagnostic tool, the increased effective doses of common intraoral and extraoral imaging techniques are high enough to warrant reconsideration of means to reduce patients’ exposure.
Clinical Implications. Clinicians can reduce patients’ dose substantively by using digital receptors or F-speed film instead of D-speed film, rectangular collimation instead of round collimation and radiographic selection criteria.
Key Words: Dental radiography; radiation dosage; risk assessment
Abbreviations: ADA: American Dental Association. • BW: Bitewing. • CCD: Charge-coupled device. • E: Effective dose. • FMX: Full-mouth radiographs. • HT: Equivalent dose. • IRCP: International Commission on Radiological Protection. • MEACR: Mass energy absorption coefficient ratio. • PA: Posteroanterior. • PSP: Photo-stimulable phosphor. • TLD: Thermoluminescent dosimeter. • wT: Tissue-weighting factor.
In 1990 the International Commission on Radiological Protection (ICRP) determined that effective dose was the preferred unit of measure for comparing risk from different radiographic examinations.1 Effective dose was created to provide a dose quantity related to the probability of health detriment due to stochastic effects of exposure to low doses of ionizing radiation. Effective dose is derived from the weighted sum of doses to tissues that are known to be sensitive to radiation and so can be derived only by calculation. The tissue-weighting factors are derived from the extrapolation of epidemiologic evidence. Effective dose was intended for use in radiation protection, but it has found wide application in comparing risks of exposures involving only certain parts of the body.2
In 2007, the ICRP updated the method for calculating effective dose on the basis of the latest available scientific information on the biology and physics of radiation exposure.3 The 2007 method involves revised estimates of the radiosensitivity of tissues and their corresponding tissue-weighting factors. Brain tissue was given more weight and, for the first time, salivary glands, oral mucosa and extrathoracic airway tissues were included in the weighting scheme. These changes regarding tissues in the maxillofacial region have the potential to affect estimations of risk from dental examinations significantly. We hypothesize that use of the 2007 ICRP data will result in an upward reassessment of effective dose from common dental radiographic examinations and their associated detriment.
MATERIALS AND METHODS
TOP
ABSTRACT
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS
REFERENCES
We used a phantom (RANDO, The Phantom Laboratory, Salem, N.Y.) consisting of a small adult skull and surrounded by soft-tissue–equivalent material (Figure
4) to simulate a patient in the various radiographic examinations. We placed thermoluminescent dosimeter (TLD) chips (TLD 100, supplied and analyzed by Landauer, Glen-wood, Ill.) at 24 selected locations in the head and neck of the phantom to record the distribution of the absorbed radiation dose. (For detailed information regarding placement of the chips, see Appendix 1, which is available as supplemental data to the online version of this article [found at "http://jada.ada.org"].) We performed 10 replicate examinations for each technique to provide a more reliable measure of radiation in the dosimeters. To determine the exposure per examination for each dosimeter, we divided TLD doses by 10.
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We averaged doses at different positions within a tissue or organ. The products of these values and the percentage of a tissue or organ irradiated in a radiographic examination were used to calculate the equivalent dose in microsieverts (Appendix 3, available as supplemental data to the online version of this article [found at "http://jada.ada.org"]). Appendix 4 (available as supplemental data to the online version of this article [found at "http://jada.ada.org"]) provides the details for the calculation of equivalent dose for bone marrow, bone, skin and extrathoracic airway.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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wT x HT and by using 2007 ICRP calculations,3 where effective dose (E) is the sum of the products of the tissue-weighting factors (wT), the relative contribution of each organ or tissue to the overall risk (Table 1) and the equivalent doses (HT) (Table 2). Salivary glands and oral mucosa consistently received the highest equivalent doses of all tissues from common dental radiographic examinations. Of these, an FMX with D-speed film and round collimation resulted in the largest effective dose (Table 3, page 1242). The effective doses for examinations computed with the 2007 ICRP method are increased from 32 to 422 percent above the corresponding values computed with the 1990 ICRP method.
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, page 1242).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The 2007 ICRP tissue and tissue-weighting factors reflect newly available cancer incidence and mortality data, whereas the 1990 ICRP guidelines were based only on mortality data. Incidence data provides a more complete description of cancer burden than does mortality data alone, particularly for cancers that have a high survival rate. Many of the cancer incidence data come from the Life Span Study of Japanese atomic bomb survivors, which has been updated through 1998, and have been corrected by means of DS86 bomb dosimetry.3 Increasing evidence of cancer risk in salivary glands and brain caused these organs to be given increased weight in 2007. Indeed, an increased incidence of cancers in these regions has been linked to dental radiography.7–9 The 2007 ICRP recommendations also reduced the weight for the thyroid gland and esophagus to 0.04 from 0.05.
The increases in effective dose for panoramic imaging measured in this study reflect primarily the addition of salivary glands as a target tissue. Inclusion of salivary glands is particularly important because in panoramic scanning, the location of the posterior rotational centers coincides with the parotid and submandibular glands, and the anterior rotational center coincides with the sublingual glands. Although much of the scanned anatomy is exposed to radiation only transiently, structures at the rotational centers are exposed continuously. Accordingly, effective doses arising from dental panoramic imaging are larger than those associated with common imaging procedures that produce a more uniform distribution of absorbed energy within the scanned volume.
Dental radiography is indispensable, because it contributes enormous diagnostic benefits. However, study of the Japanese survivors of the World War II atomic bombings and numerous other cohorts exposed to ionizing radiation has shown that radiation can cause cancers.3 The difficulty comes in estimating the risks from low-level exposures such as those used in dentistry. In this dose range, epidemiologic studies fail because of statistical limitations, thus necessitating the use of mathematical risk models. Most prominent among these is the linear nonthreshold dose-response model. This model extrapolates the risk associated with high doses where there are known effects to the low-dose range and where direct measurement of harm, if any, is not possible because of limitations in statistical analysis. In the absence of persuasive evidence of a threshold dose, below which there are no adverse effects, use of the linear nonthreshold dose-response model is widely considered to be a prudent and conservative basis for estimating risk and establishing policies for radiation protection. If there is a threshold dose, then the actual risk attributable to dental radiography may be much lower than that currently estimated, or even zero. Thus, given the increased possibility of the risk as measured in this study, we are compelled to reconsider means of reducing all unnecessary exposure.
The results of this study reinforce previous American Dental Association (ADA) recommendations10 regarding simple and effective means to reduce patient exposure substantively. The most widely used FMX technique in the United States includes use of a round aiming cylinder and D-speed film. This technique carries more than a 20-per-million risk of death. A state-of-the-art FMX, made via rectangular collimation and a high-speed film or digital sensor, results in a two-per-million increased risk of fatal cancer. This 10-fold reduction in risk comes from making minor changes in radiographic technique and underscores the validity of these ADA recommendations:
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CONCLUSIONS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Until we have clear evidence for a threshold dose below which our patients are not at risk, we must assume that radiography involves a small, but real, risk to our patients. When examining a patient and considering his or her radiographic needs, each clinician should ask the simple question, "How is this exposure likely to benefit my patient?" When the clinician can identify a reasonable indication for the exposure, then it is most likely that for this patient the benefit will far exceed any risk, and the clinician will satisfy the exhortation of Hippocrates to "do no harm."
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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