- Email: [email protected]
Comparative radiation doses in dental radiography
dental radiology Editor: LINCOLN R. MANSON-HING, D.M.D., American Academy of Dental Radiology School of Dentistry, University 1919 Seventh Avenue South Birmingham, Alabama 35233
MS.
of Alabama
Comparativeradiation dosesin dental radiography Wah Lee, B.S.,* Rockville, Md. BUREATJ AND
OF ~DIOLOGICAL
DRUG
HEALTH,
ADMINISTRATION,
UNITED
DIVISION STATES
OF BrOLOGICAL PUBLIC
HEALTH
EFFECTS,
FOOD
SERVICE
Dental radiographs were taken with x-ray techniques of 44 to 91 KVP and cone lengths of 4, 8, and 16 inches at two filtrations and at a constant entrance-skin field size in a tissue-equivalent head phantom. Exposure times were adjusted for each x-ray technique to yield radiographs of similar diagnostic qualities. When a radiograph was taken wit,h a low-kilovoltage, short-cone technique instead of a highkilovoltage, long-cone technique, the integral dose in the irradiated tissue increased 8.2 times. The skin dose increased 7.5 times; the mandibular dose, 3.1 times; the thyroid dose, 4.3 times; and the eye dose, 10 times. Very little significant alteration in doses to the phantom was observed when the total filtration in the 70 to 90 KVP range was changed from 1.5 to 2.5 mm. of aluminum. The dose values reported in this study are in elose agreement with the findings reported by Aleox and Jameson in a recent study under clinical conditions.
T
he 1970 survey by the Food and Drug Administration* on population exposure to x-rays in the United States reveals that about 59 million persons were subjected to dental radiography in 1970 and about 278 million dental radiographs were taken. The average radiation exposure to the skin per film was estimated to be 910 mr. A recent study by Alcox and Jamesor? indicated that a satisfactory radiograph can be made routinely with about 200 to 300 mr to the skin. Thus, an excess of 600 to 700 mr to the patient would appear to occur in an average dental radiograph. Part of the apparent excessive radiation exposure may be due to the employment of low-kilovoltage, short-cone x-ray techniques,
*Research Physicist, and Drug Administration.
Division
of Biological
Effects,
Bureau
of Radiological
Health,
Food
Volume 37 Number 6
Comparative
radiation
doses
963
Fig. 1. Dose sites in the head phantom.
part may be due to the practice of overexposing the film in order to decrease film-developing time,3 and part may be due to the use of low-speed films. It is commonly accepted that low-kilovoltage dental x-ray techniques result in higher dose to the skin than that due to high-kilovoltage techniques. Whether this higher skin dose results in higher or lower integral dose and.doses to other organs of interest has not been established. In this study, the effects of KVP, cone length, and filtration on integral dose, doses to the skin, mandibular marrow, eyes, and thyroid were investigated by means of various dental x-ray techniques. MATERIALS
AND METHODS
The x-ray source was a GE 90-11 dental x-ray machine. The control panel KVP meter accuracy was checked with a Hewlett-Packard spectrometer employing- a high-purity germanium detector. The meter-indicated KVP’s were found to be low by 1 to 4 kv. in the 40 to 90 KVP range. X-ray techniques used in this study covered the range of 44 to 91 KVP with cone lengths of 4, 8, and 16
Oral Burg. June, 1974
964
Lee
Table
I. Comparative
doses in dental radiography X-ray
Technique
technique” Exposwe time lsecond)
Entran,ce skin
8 x
0.167 0.33
201 + 13.8: 250 5 10.1
1.5 1.5 1.5
4x 4 16
0.40 1.25 2.00 0.50
1.5 2.5 2.5
168 8
10.40 .oo 0.20
Filtration+ (mm. 81)
NO.
EVP
1 2
91 72
1.5 1.5
: 5 6
52 44 91
i 9
72 91
Cone length
*All techniques employed a single bitewing tTota1 filtration (inherent + added). $At the 96 per cent confidence limit.
(inches)
projection,
550 394 +_ -c 43 21 1249 -c 71 166 + 4.1 199 2 225 192 t
10 Ma., 2.75 inch diameter
9.0 2.2 2.2 in entrance
inches (open-end and unlined) at a constant entrance-skin field size of 2.75 inches in diameter (Table I). These techniques covered the range of x-ray techniques presently used in dental radi0graphy.l A single bitewing technique was used to irradiate a Rando head phantom comprising a complete adult human skull with full dentation. The tissue equivalency of the phantom was established by Alderson. Lithium fluoride dosimeters (l/8 by J&sby 0.035 inch) in No. 5 gelatin capsules were used for exposure measurements in the phantom. The dosimeters were calibrated in air with a Victoreen Model 555 exposure integrator and a Model 1OLA low-energy probe, both of which were calibrated with a free-air ionization chamber employing x-ray spectra similar to those used in this study. The energy response of the LiF dosimeters was found to be similar (+ 1.5 per cent) for all the x-ray techniques used in this study. Therefore, a single calibration was used for all the KVP-filtration combinations. The precision of the dosimeter response was t 2 per cent at the 1 r exposure level and + 5 per cent at the 100 mr level. Five to six radiographs were made with different exposure times for each of the nine x-ray techniques, and the films were processed according to the manufacturer’s specifications, The films used in this study were Kodak UltraSpeed dental films (Class D) , A panel of five dentists from the staff of the Bureau of Radiological Health was asked to select one film from each of the nine x-ray techniques on the basis of similarity in diagnostic quality. The selections were carried out under doubleblind conditions. All five dentists selected the same films in five of the nine groups, and four of five dentists selected the same films in the remaining four groups. In the areas of disagreement, the difference in film exposure amounted to 25 per cent among the films selected. For each of the nine x-ray techniques, the phantom was exposed to 10 to 50 r (depending on the KVP used) in order to obtain a statistically significant readout for each of the twenty-five dosimeters in the phantom (Fig. 1). Five independent sets of measurements were made, and the average values were used for dose calculation. An exposure-to-dose conversion factor of 0.92 rad
Comparative
Volume 37 Number 6
radiation
doses
965
Dose fmrads J Integral 24.0 26.8 35.0 53.7 116 14.1 16.9 23.7 24.4
(g-v-ads) 2 3.6t 2 4.0 TX 5.8 + 9.7 + 19.4 + 2.4 2 2.7 2 3.9 ? 3.7
skin field size, and Kodak
(
Man&b&ar 15.2 81.2 100 96.2 198 63.1 67.2 79.6 77.3 Ultra-Speed
marrow 2 3.2i 2 5.5 I! 13.0 i: 5.6 t 5.7 t 5.6 2 8.1 + 9.8 5 10.7
(
Eyes
Thyroid 2.01 1.82 1.75 2.75 5.60 1.31 1.17 1.79 2.07
t 0.14i + 0.19 + 0.25 + 0.73 + 1.41 _+ 0.16 2 0.13 rt 0.13 2 0.20
1.07 1.02 0.94 3.71 6.70 0.80 0.67 1.19 1.37
2 It f + + 5 2 2 t
0.26$ 0.33 0.34 2.84 5.93 0.05 0.20 0.29 0.19
dental film.
per roentgen was used for soft tissue5 and 1.01” for the mandibular marrow.6 The results were statistically analyzed at the 95 per cent confidence level, using the t values compiled by Person and Hartley7 and the method described by Kendle and Stuard* for variance analysis. The exposure rates of the x-ray machine were found to be linear in the range of 0.1 to 5.0 seconds of exposure duration. Therefore, dose values at each of the twenty-five sites were scaled down by the ratio of the predetermined exposure time required to produce the selected dental radiograph to the exposure time used in exposing the phantom. The integral dose was determined by the product of the average dose in the volume of tissue irradiated and the mass of the irradiated tissue. The volume of the irradiated tissue was determined by measuring the entrance and exit field sizes and the distance separating them. The mass of the irradiated volume was obtained by assuming the density of the irradiated tissue to be 1 gram per cubic centimeter. In computing the mean dose to the irradiated tissue, only those dose sites within the volume enclosed by the primary x-ray beam were considered. The mandibular marrow dose was obtained from a dose site in the mandible at the entrance side of the beam. The thyroid and eye doses are the average values for both the right and the left sides of the organs. RESULTS
A summary of doses to various organs of the head and the thyroid is presented in Table I. The highest doses are seen for technique No. 5 (44 KVP, 4-inch cone). The lowest doses (except for the thyroid and eyes) are seen for technique No. 6 (91 KVP, 16-inch cone). Techniques 1 to 5 indicate that as the KVP decreases, doses to the patient increase. These dose increases are especially pronounced at the 44 and 52 KVP levels. Technique pairs 3-4, l-6, and 2-7 show that shorter cone length techniques result in higher doses to the patient. “This conversion factor is for rib bone marrow. Since the rib and the mandible are similar in bone structure, no significant error is expected in using this value for the mandibular marrow.
40
60
PHOTONENERGY(keYI
Fig. 2. Uncorrected dental x-ray spectrum taken with 44 KVP, and a Hewlett-Packard spectrometer. Technique: Source: GE 90-11 dental x-ray machine.
a high-purity germanium detector 1.5 mm. aluminum total filtration.
Technique pairs 1-9 and 2-8 demonstrate that there is very little significant change in dose to the patient when the total filtration increases from 1.5 to 2.5 mm. of aluminum in the 70 to 90 KVP range. DISCUSSION Table I indicates that when a low-kilovoltage, short-cone x-ray technique is used instead of a high-kilovoltage, long-cone technique, not only is the expected skin dose substantially increased, the integral dose and the mandibular, eye, and thyroid doses are significantly increased also. The results of this study are in close agreement with those reported by Alcox and Jameson. Alcox and Jameson took their measurements on patients under clinical conditions employing x-ray techniques of 50 to 90 KVP with 4- to 16-inch cones and an entrance field size of 2.75 inches in diameter. They reported that 175 to 570 mr of exposure at the cone tip was required to obtain a satisfactory diagnostic film (Kodak Ultra-Speed) as compared to 180 to 598 mr of exposure to the skin with the same exposure parameters in this study. Their reported thyroid exposures ranged from 1.2 to 4.5 mr as compared to 1.3 to 3.0 mr, and their eye exposures were 0.8 to 12 mr as compared to 0.7 to 4.0 mr for this study. Relevant integral and mandibular dose data are not available for comparison. However, van Aken and van der Linden” reported a calculated average integral dose of approximately 65 g-rads per film for an x-ray technique employing 50 KVP, 2 mm. aluminum total filtration, 4-inch cone, and Ultra-Speed film for a sixteen-film full-mouth series. In a low-kilovoltage, short,-cone technique, such as technique No. 5, more
Volume 37 Number 6
Comparative
radiation
doses
967
than half of the photons in the spectrum are below 27 kev. (Fig. 2). These photons are relatively inefficient in producing radiographic image, yet they are readily absorbed by the intervening tissues between the skin and the film.1° For example, 20 kev. and 40 kev. photons have approximately the same absorption coefficient in silver; yet the absorption coefficient of the 20 kev. photon is eight times higher in soft tissues and twelve times higher in bone than that of the 40 kev. photon. However, the amount of exposure required at the film to produce a satisfactory dental radiograph is approximately the same for both the highand low-kilovoltage techniques (20 to 30 mr) .z Furthermore, the inverse-square fall-off is much more rapid for the short-cone technique than that for the longcone technique. All these factors contribute to the higher skin dose from a lowkilovoltage, short-cone technique. The increase in integral dose from the low-kilovoltage, short-cone x-ray technique over that from the high-kilovoltage, long-cone technique is due to a larger volume of tissue irradiated and a higher average dose to the irradiated tissue. The larger volume of irradiated tissue from the short-cone technique is due to its greater beam divergence resulting from the shorter anode-skin distance. The higher average dose from the low-kilovoltage, short-cone technique can be explained as follows: The skin dose is approximately seven times higher for the low-kilovoltage, short-cone technique, and the amount of exposure required at the film for both cases is approximately the same. In both techniques, more than 90 per cent of the skin dose is absorbed by the intervening tissues and the lead foil on the back of the film pack.* Therefore, the increase in integral dose clue to the higher depth dose distribution from the high-kilovoltage, longcone technique is more than compensated by the much larger skin dose and the larger volume of irradiated tissue from the low-kilovoltage, short-cone technique. The increase in mandibular marrow dose from the low-kilovoltage technique is not as pronounced as that in the skin and integral doses because of the proximity of the mandible to the film for the same reasons cited in the previous paragraph. The increases in thyroid and eye doses are due mainly to the proximity of these organs to the edge of the primary beam from the lowkilovoltage, short-cone technique. CONCLUSION The findings in this study indicate that when a low-kilovoltage, short-cone x-ray technique is used in dental radiography instead of a high-kilovoltage, long-cone technique, not only is the expected skin dose substantially increased, but the integral dose and mandibular marrow, eye, and thyroid doses are significantly increased also. Change of total filtration from 1.5 to 2.5 mm. of aluminum in the 70 to 90 KVP range results in very little significant change in doses to the patient. No conclusion should be drawn from these data alone that one of the x-ray techniques is the technique of choice in dental radiography. The author wishes to thank Mr. Harry Youmans, Jr., for his technical advice and encouragement during the course of the study. The author is also indebted to Miss Lynda Kramer, Mr. Earl Denny, Drs. Wayne Jameson, C. Larry Crabtree, Orlen Johnson, Keith
968
Oral Surg.
Lee
June, 1974
Winkler, and Loren F. Mills for their technical assistance. by Mr. Thomas R. Fewell and Dr. Tommie J. Morgan.
The x-ray
spectrum
was provided
REFERENCES 1. Population Exposure to X-rays: U.S. 1970, FDA 73-8047, November, 1973. 2. Alcox, R. W., and Jameson, W. R.: Patient Exposures From Intraoral Radiographic Examinations, J. Am. Dent. Assoc. 88: 568-579, 1974. 3. Jameson, W. R., Division of Training and Medical Applications, Bureau of Radiological Health: Private communication, November, 1973. 4. Alderson, S. W.: The Role of Phantoms in Radiology, Cathode Press 23: 2, 1966. 5. NBS Handbook 85, Washington, D. C., 1964, National Bureau of Standards. Report on the Determination of Dose in Bone Marrow From 6. Spiers, F. W.: Interim Radiological Procedures, Br. J. Radiol. 36: 238, 1963. 7. Person, E. S., and Hartley, H. E. (editors) : Biometrika Tables for Statisticians, ed 5. London, 1966, Cambridge University Press, vol. 1. 8. Kendle, M. A., and Stuard, A.: The Advanced Theory of Statistics, ed 2, New York, 1963, Hafner Publishing Company, vol. 1. 9. van Aken, J., and van der Linden, L. W. J.: The Integral Absorbed Doses in Conventional and Panoramic Complete Mouth Examination, Oral Roentgenol. 22: 603, 1966. Source for Odontological Roentgenology, 10. Henrickson, C. 0.: Iodine-125 as a Radiation Acta Radiol., Supp. 269, 1967. Reprint
requests
to:
Wah Lee U. S. Department of Health, Education, Public Health Service Food and Drug Administration Bureau of Radiological Health Division of Biological Effects 5600 Fishers Lane Rockville, Md. 20852
and Welfare
MS.
of Alabama
Comparativeradiation dosesin dental radiography Wah Lee, B.S.,* Rockville, Md. BUREATJ AND
OF ~DIOLOGICAL
DRUG
HEALTH,
ADMINISTRATION,
UNITED
DIVISION STATES
OF BrOLOGICAL PUBLIC
HEALTH
EFFECTS,
FOOD
SERVICE
Dental radiographs were taken with x-ray techniques of 44 to 91 KVP and cone lengths of 4, 8, and 16 inches at two filtrations and at a constant entrance-skin field size in a tissue-equivalent head phantom. Exposure times were adjusted for each x-ray technique to yield radiographs of similar diagnostic qualities. When a radiograph was taken wit,h a low-kilovoltage, short-cone technique instead of a highkilovoltage, long-cone technique, the integral dose in the irradiated tissue increased 8.2 times. The skin dose increased 7.5 times; the mandibular dose, 3.1 times; the thyroid dose, 4.3 times; and the eye dose, 10 times. Very little significant alteration in doses to the phantom was observed when the total filtration in the 70 to 90 KVP range was changed from 1.5 to 2.5 mm. of aluminum. The dose values reported in this study are in elose agreement with the findings reported by Aleox and Jameson in a recent study under clinical conditions.
T
he 1970 survey by the Food and Drug Administration* on population exposure to x-rays in the United States reveals that about 59 million persons were subjected to dental radiography in 1970 and about 278 million dental radiographs were taken. The average radiation exposure to the skin per film was estimated to be 910 mr. A recent study by Alcox and Jamesor? indicated that a satisfactory radiograph can be made routinely with about 200 to 300 mr to the skin. Thus, an excess of 600 to 700 mr to the patient would appear to occur in an average dental radiograph. Part of the apparent excessive radiation exposure may be due to the employment of low-kilovoltage, short-cone x-ray techniques,
*Research Physicist, and Drug Administration.
Division
of Biological
Effects,
Bureau
of Radiological
Health,
Food
Volume 37 Number 6
Comparative
radiation
doses
963
Fig. 1. Dose sites in the head phantom.
part may be due to the practice of overexposing the film in order to decrease film-developing time,3 and part may be due to the use of low-speed films. It is commonly accepted that low-kilovoltage dental x-ray techniques result in higher dose to the skin than that due to high-kilovoltage techniques. Whether this higher skin dose results in higher or lower integral dose and.doses to other organs of interest has not been established. In this study, the effects of KVP, cone length, and filtration on integral dose, doses to the skin, mandibular marrow, eyes, and thyroid were investigated by means of various dental x-ray techniques. MATERIALS
AND METHODS
The x-ray source was a GE 90-11 dental x-ray machine. The control panel KVP meter accuracy was checked with a Hewlett-Packard spectrometer employing- a high-purity germanium detector. The meter-indicated KVP’s were found to be low by 1 to 4 kv. in the 40 to 90 KVP range. X-ray techniques used in this study covered the range of 44 to 91 KVP with cone lengths of 4, 8, and 16
Oral Burg. June, 1974
964
Lee
Table
I. Comparative
doses in dental radiography X-ray
Technique
technique” Exposwe time lsecond)
Entran,ce skin
8 x
0.167 0.33
201 + 13.8: 250 5 10.1
1.5 1.5 1.5
4x 4 16
0.40 1.25 2.00 0.50
1.5 2.5 2.5
168 8
10.40 .oo 0.20
Filtration+ (mm. 81)
NO.
EVP
1 2
91 72
1.5 1.5
: 5 6
52 44 91
i 9
72 91
Cone length
*All techniques employed a single bitewing tTota1 filtration (inherent + added). $At the 96 per cent confidence limit.
(inches)
projection,
550 394 +_ -c 43 21 1249 -c 71 166 + 4.1 199 2 225 192 t
10 Ma., 2.75 inch diameter
9.0 2.2 2.2 in entrance
inches (open-end and unlined) at a constant entrance-skin field size of 2.75 inches in diameter (Table I). These techniques covered the range of x-ray techniques presently used in dental radi0graphy.l A single bitewing technique was used to irradiate a Rando head phantom comprising a complete adult human skull with full dentation. The tissue equivalency of the phantom was established by Alderson. Lithium fluoride dosimeters (l/8 by J&sby 0.035 inch) in No. 5 gelatin capsules were used for exposure measurements in the phantom. The dosimeters were calibrated in air with a Victoreen Model 555 exposure integrator and a Model 1OLA low-energy probe, both of which were calibrated with a free-air ionization chamber employing x-ray spectra similar to those used in this study. The energy response of the LiF dosimeters was found to be similar (+ 1.5 per cent) for all the x-ray techniques used in this study. Therefore, a single calibration was used for all the KVP-filtration combinations. The precision of the dosimeter response was t 2 per cent at the 1 r exposure level and + 5 per cent at the 100 mr level. Five to six radiographs were made with different exposure times for each of the nine x-ray techniques, and the films were processed according to the manufacturer’s specifications, The films used in this study were Kodak UltraSpeed dental films (Class D) , A panel of five dentists from the staff of the Bureau of Radiological Health was asked to select one film from each of the nine x-ray techniques on the basis of similarity in diagnostic quality. The selections were carried out under doubleblind conditions. All five dentists selected the same films in five of the nine groups, and four of five dentists selected the same films in the remaining four groups. In the areas of disagreement, the difference in film exposure amounted to 25 per cent among the films selected. For each of the nine x-ray techniques, the phantom was exposed to 10 to 50 r (depending on the KVP used) in order to obtain a statistically significant readout for each of the twenty-five dosimeters in the phantom (Fig. 1). Five independent sets of measurements were made, and the average values were used for dose calculation. An exposure-to-dose conversion factor of 0.92 rad
Comparative
Volume 37 Number 6
radiation
doses
965
Dose fmrads J Integral 24.0 26.8 35.0 53.7 116 14.1 16.9 23.7 24.4
(g-v-ads) 2 3.6t 2 4.0 TX 5.8 + 9.7 + 19.4 + 2.4 2 2.7 2 3.9 ? 3.7
skin field size, and Kodak
(
Man&b&ar 15.2 81.2 100 96.2 198 63.1 67.2 79.6 77.3 Ultra-Speed
marrow 2 3.2i 2 5.5 I! 13.0 i: 5.6 t 5.7 t 5.6 2 8.1 + 9.8 5 10.7
(
Eyes
Thyroid 2.01 1.82 1.75 2.75 5.60 1.31 1.17 1.79 2.07
t 0.14i + 0.19 + 0.25 + 0.73 + 1.41 _+ 0.16 2 0.13 rt 0.13 2 0.20
1.07 1.02 0.94 3.71 6.70 0.80 0.67 1.19 1.37
2 It f + + 5 2 2 t
0.26$ 0.33 0.34 2.84 5.93 0.05 0.20 0.29 0.19
dental film.
per roentgen was used for soft tissue5 and 1.01” for the mandibular marrow.6 The results were statistically analyzed at the 95 per cent confidence level, using the t values compiled by Person and Hartley7 and the method described by Kendle and Stuard* for variance analysis. The exposure rates of the x-ray machine were found to be linear in the range of 0.1 to 5.0 seconds of exposure duration. Therefore, dose values at each of the twenty-five sites were scaled down by the ratio of the predetermined exposure time required to produce the selected dental radiograph to the exposure time used in exposing the phantom. The integral dose was determined by the product of the average dose in the volume of tissue irradiated and the mass of the irradiated tissue. The volume of the irradiated tissue was determined by measuring the entrance and exit field sizes and the distance separating them. The mass of the irradiated volume was obtained by assuming the density of the irradiated tissue to be 1 gram per cubic centimeter. In computing the mean dose to the irradiated tissue, only those dose sites within the volume enclosed by the primary x-ray beam were considered. The mandibular marrow dose was obtained from a dose site in the mandible at the entrance side of the beam. The thyroid and eye doses are the average values for both the right and the left sides of the organs. RESULTS
A summary of doses to various organs of the head and the thyroid is presented in Table I. The highest doses are seen for technique No. 5 (44 KVP, 4-inch cone). The lowest doses (except for the thyroid and eyes) are seen for technique No. 6 (91 KVP, 16-inch cone). Techniques 1 to 5 indicate that as the KVP decreases, doses to the patient increase. These dose increases are especially pronounced at the 44 and 52 KVP levels. Technique pairs 3-4, l-6, and 2-7 show that shorter cone length techniques result in higher doses to the patient. “This conversion factor is for rib bone marrow. Since the rib and the mandible are similar in bone structure, no significant error is expected in using this value for the mandibular marrow.
40
60
PHOTONENERGY(keYI
Fig. 2. Uncorrected dental x-ray spectrum taken with 44 KVP, and a Hewlett-Packard spectrometer. Technique: Source: GE 90-11 dental x-ray machine.
a high-purity germanium detector 1.5 mm. aluminum total filtration.
Technique pairs 1-9 and 2-8 demonstrate that there is very little significant change in dose to the patient when the total filtration increases from 1.5 to 2.5 mm. of aluminum in the 70 to 90 KVP range. DISCUSSION Table I indicates that when a low-kilovoltage, short-cone x-ray technique is used instead of a high-kilovoltage, long-cone technique, not only is the expected skin dose substantially increased, the integral dose and the mandibular, eye, and thyroid doses are significantly increased also. The results of this study are in close agreement with those reported by Alcox and Jameson. Alcox and Jameson took their measurements on patients under clinical conditions employing x-ray techniques of 50 to 90 KVP with 4- to 16-inch cones and an entrance field size of 2.75 inches in diameter. They reported that 175 to 570 mr of exposure at the cone tip was required to obtain a satisfactory diagnostic film (Kodak Ultra-Speed) as compared to 180 to 598 mr of exposure to the skin with the same exposure parameters in this study. Their reported thyroid exposures ranged from 1.2 to 4.5 mr as compared to 1.3 to 3.0 mr, and their eye exposures were 0.8 to 12 mr as compared to 0.7 to 4.0 mr for this study. Relevant integral and mandibular dose data are not available for comparison. However, van Aken and van der Linden” reported a calculated average integral dose of approximately 65 g-rads per film for an x-ray technique employing 50 KVP, 2 mm. aluminum total filtration, 4-inch cone, and Ultra-Speed film for a sixteen-film full-mouth series. In a low-kilovoltage, short,-cone technique, such as technique No. 5, more
Volume 37 Number 6
Comparative
radiation
doses
967
than half of the photons in the spectrum are below 27 kev. (Fig. 2). These photons are relatively inefficient in producing radiographic image, yet they are readily absorbed by the intervening tissues between the skin and the film.1° For example, 20 kev. and 40 kev. photons have approximately the same absorption coefficient in silver; yet the absorption coefficient of the 20 kev. photon is eight times higher in soft tissues and twelve times higher in bone than that of the 40 kev. photon. However, the amount of exposure required at the film to produce a satisfactory dental radiograph is approximately the same for both the highand low-kilovoltage techniques (20 to 30 mr) .z Furthermore, the inverse-square fall-off is much more rapid for the short-cone technique than that for the longcone technique. All these factors contribute to the higher skin dose from a lowkilovoltage, short-cone technique. The increase in integral dose from the low-kilovoltage, short-cone x-ray technique over that from the high-kilovoltage, long-cone technique is due to a larger volume of tissue irradiated and a higher average dose to the irradiated tissue. The larger volume of irradiated tissue from the short-cone technique is due to its greater beam divergence resulting from the shorter anode-skin distance. The higher average dose from the low-kilovoltage, short-cone technique can be explained as follows: The skin dose is approximately seven times higher for the low-kilovoltage, short-cone technique, and the amount of exposure required at the film for both cases is approximately the same. In both techniques, more than 90 per cent of the skin dose is absorbed by the intervening tissues and the lead foil on the back of the film pack.* Therefore, the increase in integral dose clue to the higher depth dose distribution from the high-kilovoltage, longcone technique is more than compensated by the much larger skin dose and the larger volume of irradiated tissue from the low-kilovoltage, short-cone technique. The increase in mandibular marrow dose from the low-kilovoltage technique is not as pronounced as that in the skin and integral doses because of the proximity of the mandible to the film for the same reasons cited in the previous paragraph. The increases in thyroid and eye doses are due mainly to the proximity of these organs to the edge of the primary beam from the lowkilovoltage, short-cone technique. CONCLUSION The findings in this study indicate that when a low-kilovoltage, short-cone x-ray technique is used in dental radiography instead of a high-kilovoltage, long-cone technique, not only is the expected skin dose substantially increased, but the integral dose and mandibular marrow, eye, and thyroid doses are significantly increased also. Change of total filtration from 1.5 to 2.5 mm. of aluminum in the 70 to 90 KVP range results in very little significant change in doses to the patient. No conclusion should be drawn from these data alone that one of the x-ray techniques is the technique of choice in dental radiography. The author wishes to thank Mr. Harry Youmans, Jr., for his technical advice and encouragement during the course of the study. The author is also indebted to Miss Lynda Kramer, Mr. Earl Denny, Drs. Wayne Jameson, C. Larry Crabtree, Orlen Johnson, Keith
968
Oral Surg.
Lee
June, 1974
Winkler, and Loren F. Mills for their technical assistance. by Mr. Thomas R. Fewell and Dr. Tommie J. Morgan.
The x-ray
spectrum
was provided
REFERENCES 1. Population Exposure to X-rays: U.S. 1970, FDA 73-8047, November, 1973. 2. Alcox, R. W., and Jameson, W. R.: Patient Exposures From Intraoral Radiographic Examinations, J. Am. Dent. Assoc. 88: 568-579, 1974. 3. Jameson, W. R., Division of Training and Medical Applications, Bureau of Radiological Health: Private communication, November, 1973. 4. Alderson, S. W.: The Role of Phantoms in Radiology, Cathode Press 23: 2, 1966. 5. NBS Handbook 85, Washington, D. C., 1964, National Bureau of Standards. Report on the Determination of Dose in Bone Marrow From 6. Spiers, F. W.: Interim Radiological Procedures, Br. J. Radiol. 36: 238, 1963. 7. Person, E. S., and Hartley, H. E. (editors) : Biometrika Tables for Statisticians, ed 5. London, 1966, Cambridge University Press, vol. 1. 8. Kendle, M. A., and Stuard, A.: The Advanced Theory of Statistics, ed 2, New York, 1963, Hafner Publishing Company, vol. 1. 9. van Aken, J., and van der Linden, L. W. J.: The Integral Absorbed Doses in Conventional and Panoramic Complete Mouth Examination, Oral Roentgenol. 22: 603, 1966. Source for Odontological Roentgenology, 10. Henrickson, C. 0.: Iodine-125 as a Radiation Acta Radiol., Supp. 269, 1967. Reprint
requests
to:
Wah Lee U. S. Department of Health, Education, Public Health Service Food and Drug Administration Bureau of Radiological Health Division of Biological Effects 5600 Fishers Lane Rockville, Md. 20852
and Welfare