Effect of the geometry of the intraoral position-indicating device on effective dose

Effect of the geometry of the intraoral position-indicating device on effective dose Robert A. Cederberg, MA, DDS, a Neil L. Frederiksen, DDS, PhD, b Byron W. Benson, DDS, MS, c and Thaddeus W. Sokolowski, MSc, J Dallas, Tex. TEXASA & M UNIVERSITYSYSTEM/BAYLORCOLLEGEOF DENTISTRYAND TEXASONCOLOGYATTHE SAMMONSCANCERCENTERAT BAYLORUNIVERSITYMEDICALCENTER

Objective. The objective of this study was to calculate and compare the effective dose and to estimate risk from the use of intraoral position-indicating devices of differing geometries.

Study design. Thermoluminescent dosimeters were placed at selected sites in the upper portion of a tissue-equivalent human phantom to record the equivalent dose to weighted tissues and organs. The phantom was exposed to simulated complete mouth surveys with either a long (29.8 cm) or short (19.6 cm) round open-end position-indicating device, a long (35.3 cm) or short (23.3 cm) rectangular open-end position-indicating device, or a pointed (29.6 cm) closed-end position-indicating device. Results. The effective dose was calculated as the sum of the equivalent doses to each organ or tissue multiplied by that organ or tissue's weighting factor. The salivary glands were included as part of the remainder. The effective dose ranged from 362 I.tSvfor the pointed position-indicating device, to 63/aSv for both the long and the short rectangular position-indicating devices. Conclusions. These effective doses were calculated to represent a probability for stochastic effects that range in magnitude from 26 x 10-6 to 4.6 x 10-6.

(Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;84:101-9)

Since the introduction of intraoral radiographic techniques nearly 100 years ago, there has been a need for an external aiming or position-indicating device (PID) to direct the x-ray beam. Over the years several shapes and lengths of PIDs have been used with dental x-ray units. Pointed PIDs were once favored because the conical shape allowed a less obstructed view of the film's relationship to teeth and provided a visual indication of the point of entry of the central ray.1 Shielded open-end PIDs, both round and rectangular, have become more widely used in the past 40 years because of the reported decrease in patient exposure that could be achieved. 2-4 All PID geometries used in dentistry today have been evaluated as to their influence on x-radiation exposure and subsequent radiobiologic effect. Greet 5 used thermoluminescent dosimetry to measure absorbed doses to various anatomic locations in a phantom when using five different PIDs and three operating potentials. He found that when a short pointed PID was used, the average dose to locations studied exceeded those when an

Received for publication Oct. 21, 1996; returned for revision Dec. 27, 1996;acceptedfor publication Feb. 5, 1997. a Assistant Professor, Department of Diagnostic Sciences. b Professor, Director of Radiology, Department of Diagnostic Sciences. c AssociateProfessor, Department of Diagnostic Sciences. d Texas Oncologyat the SammonsCancerCenter, BaylorUniversity Medical Center. Copyright© 1997 by Mosby-YearBook, Inc. 1079-2104/97/$5.00 + 0 7/16/81110

open-end PID was used by at least 10% and that the corneal dose was up to 63% greater when a short pointed PID was used. Weissman and Sobkowski4 studied two PIDs used with two paralleling instruments, as well as a pointed cone used with the bisecting angle technique. Their study measured absorbed doses to eight anatomic sites and concluded that a 16 inch shielded open-end PID, when used with a rectangular collimated film holder, provided the most acceptable intraoral periapical survey with the least patient risk. Such studies led Alcox6 to suggest methods that would minimize both patient and staff exposure in the dental office. He recommended the use of a long cone paralleling technique with rectangular collimation and discouraged the use of pointed plastic cones because of their tendency to scatter the beam. The Code of Federal Regulations (CFR) 7 contains no geometric requirement, other than length, for PIDs used on dental x-ray systems. Twenty-one CFR 1020.31 specifies that if x-ray equipment is capable of operation at or above 50 kVp, it shall be provided with a means to limit the source-skin distance to not less than 18 cm and if capable of operating below 50 kVp, 10 cm. The lack of a requirement for the use of open-end PIDs is in part the result of a 1975 analysis by the Food and Drug Administration (FDA). 8 This study reviewed the data that compared the effect of various PIDs on the physical characteristics of the x-ray beam and data that measured skin and organ doses delivered to patients or phantoms during oral radiographic examinations. The material studied included that of Ice et al.3; they showed 101

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July 1997 that when short and long unshielded and shielded openend PIDs were used, the average exposure to monitored sites was exceeded by up to 27% when a short closedend pointed PID was used. Ice et al. 3 also reported that when a short pointed PID was used the average corneal exposure was up to 35% less than when an open-end PID was used. These findings were found at that time to conflict with those that had been reported by Greer. 5 The FDA report offered several explanations for the discrepancies found in the studies of Ice et al. 3 and Greer 5 and reported reasons why an open-end PID was not recommended. First, it was felt that the geometric shape of the PID determined in part the proximity of its end to the patient's skin surface. Because of anatomic limitations of certain projections such as the maxillary anterior, canine, and posterior areas, the pointed PID could be placed closer to the skin surface than the openend PID. Because of the increased distance between the end of the PID and the skin surface, the area of the xray beam at the skin surface was thought to be as much as 25% greater when an open-end PID of the same length as a pointed PID was used for making films with the bisecting angle technique. This finding led to the second reason; it was assumed that most dentists used the bisection of the angle technique with a short PID and were unlikely to change their techniques. The FDA analysis concluded that an amendment to the CFR that would require the use of an open-end PID might in fact result in an increase, rather than a decrease in dental xray exposure in the general population. The previously cited investigators 2-5 measured absorbed dose to various tissues and organs in phantoms and patients in an attempt to quantify the effect of PID geometry on the relative risk of intraoral radiography. This method of assessment did not consider either the radiosensitivity of the tissues or organs, or their relative contribution to the overall well-being of the body. The International Commission on Radiological Protection in ICRP 26 (1977) 9 attempted to overcome this by defining the relationship between the probability of stochastic effects in certain defined organs and tissues, and a unit of radiation dose termed the effective dose equivalent (HE). Later this unit was modified to create the effective dose (E) in ICRP 60.1° E is currently defined as the sum of the equivalent doses (Hr) to each organ or tissue of interest multiplied by that organ or tissue's weighting factor (Wr). Absorbed dose considers only the amount of energy imparted to a particular tissue or organ, whereas the calculation of the E considers the relative contribution of each organ or tissue in terms of the total detriment. With today's emphasis on the use of faster films and screens and even with digital radiography to satisfy the

principles of ALARA (as low as reasonably achievable), it is important to consider the contribution that PID type has to the total radiation detriment. Presently there are no data comparing what effect typically used PID geometries have on a calculated E and the probability of stochastic effects associated with their use. It can be expected that rectangular collimation has more effect on patient dose reduction than does PID length. It was the aim of this study to calculate and compare the E and to estimate the risk from the use of short and long, round and rectangular open-end PIDs and a short pointed closed-end PID.

MATERIAL AND METHODS Thermoluminescent dosimeters (TLD) used in this study (TLD-100 lithium fluoride extruded rods, 1 x 1 x 6 mm, Solon Technologies, Inc., Solon, Ohio) were from a matched extrusion lot, and subjected to a selection process in which those dosimeters whose response to x-rays varied by more than 10% from the group's mean were discarded. Before use, all TLDs were annealed at 400°C for 1 hour followed by 2 hours at 100°C. Calibration factors were calculated for each TLD after exposure to 90 kVp x-rays (Model 1000, HVL, 3.2 m m AI equivalent, Gendex, Milwaukee, Wisc.). Radiation intensities were measured with a radical 1015C radiation monitor (MDH Industries, Inc., Monrovia, Calif.) using a 6 cm 3 air equivalent volume ionization chamber. Dosimeters were read 24 hours after exposure to allow for the decay of short half-life glow peaks. TLDs were read with a Harshaw/QS TLD System 5500 Automatic TL Reader (Solon Technologies, Inc., Solon, Ohio) operated according to manufacturer's directions. The identity of all the selected dosimeters was maintained throughout the study to permit the use of individual calibration factors for the conversion of readings obtained in nC to R. The phantom used for this study consisted of fifteen 2.5 cm horizontal sections (Alderson Research Laboratories, Stamford, Conn.). Sites were chosen to record skin exposure, as well as absorbed doses to bone marrow, the thyroid, pituitary, parotid, and submandibular glands, and the esophagus (Table 1).9 Three unexposed TLDs were used to record background. For each complete mouth survey, 64 TLDs were placed at 27 sites (Table I) representing weighted tissues or organs in the upper portion of a tissue equivalent human phantom. A 19 film complete mouth survey, consisting of 15 periapical and 4 bite-wing radiographs, was simulated with each PID studied (Table II): (1) long round shielded open-end (FSFD [in this study defined as the distance from target to PID end] 29.8 cm, beam field size

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Table I. Equivalent dose/dosimeter location/TLD number Equivalent Dose (pSv) Organ~tissue Thyroid Salivary glands Parotid (right) Parotid (left) Submandibular (fight) Submandibular (left) Bone marrow Third molar (fight) Third molar (left) First molar (right) First molar (left) First premolar (right) First premolar (left) Mandibular symphysis Second cervical vertebra Sixth cervical vertebra Lateral calvarium (fight) Lateral calvarium (left) Posterior calvarium Anterior calvarium Esophagus Skin Philtmm Occipital Vertex Preauricular (right) Preauricular (left) Posterior neck Chin Brain (pituitary)

LRO

LRtO

SRO

SRtO

PC

Phantom level*

Number of TLDs per site

904

180

2645

678

1921

9

3

4915 5346 4193 4767

552 648 634 430

15351 13878 13528 12276

1655 1414 2311 1785

9544 12620 9862 10668

6 6 7 7

3 3 3 3

5589 6546 7165 7403 7184 7593 7112 1143 157 60 61 52 201 351

2634 2965 3961 4174 4096 3836 1882 295 <1 112 60 66 137 146

11170 13147 14525 15566 15622 15990 15259 3263 642 230 108 494 488 928

5587 6254 7186 8022 7609 8451 6470 623 80 102 132 27 255 301

7865 8907 10196 11526 11428 11376 10478 2894 475 295 212 177 664 831

6 6 6 6 6 6 7 6 9 2 2 2 2 10

3 2 2 2 3 2 3 2 2 2 2 2 2 3

5425 59 50 991 1236 288 8731 437

867 114 100 170 187 <1 1599 170

11533 69 244 5440 1632 355 17845 1268

4011 63 30 561 338 111 6587 416

14520 59 79 3467 4016 287 17276 1829

5 2 0 3 3 8 7 3

2 2 2 2 2 2 2 3

LRO, Long round open-end PID, equivalent dose = mean of 4 determinations. LRtO, Long rectangular open-end PID. SRO, Short round open-end PID, equivalent dose = mean of 2 determinations. SRtO, Short rectangular open-endPID. PC, Pointed closed-end PID, equivalent dose = mean of 2 determinations. *Level number of tissue-equivalenthuman phantom (AldersonResearch Laboratories, Stamford, Conn.).

[BFS] 34.92 cm2); (2) long rectangular shielded openend (FSFD 35.3 cm, BFS 15.53 cm2); (3) short round shielded open-end (FSFD 19.6 cm, BFS 34.92 cm2); (4) short rectangular shielded open-end (FSFD 23.3 cm, BFS 15.53 cm2); (5) pointed closed-end (FSFD 29.6 cm, BFS 76.01 cm2). Standardized vertical angulation of the x-ray beam for each projection was according to the bisecting angle technique. 11 Because there are no published angles for predetermined film positions with a parallel technique, and it was not possible to use a film-holding instrument with the phantom, the bisecting angle technique was chosen for reproducibility. Horizontal angulation of the beam was determined by alignment of the end of the PID parallel to a tangent of the external surface entry point for each projection. The pointed cone was positioned in the same way by estimating the tangent of the cone end tip that would most

Table II. PID specifications P1D type

Long round Short round Long rectangular Short rectangular Short pointed

Length (FSFD) (cm)

Beam field size (cm2)

29.8 19.6 35.3 23.3 29.6

34.92 34.92 15.53 15.53 76.01

Measured radiation intensity ( C/kg) 3.66 7.04 2.22 5.73 4.82

x 104 x 10-4 × 10-4 × 10-4 N 10-4

Exposure Factor

0.52 1.65 0.64 0.76

closely parallel the tangent of the external surface entry point. For each intraoral film simulation, the phantom including the TLDs was exposed to five imaging cycles with 90 kVp x-rays and mAs factors (anterior projection, 3.8; posterior and bite-wing projection, 4.5; and

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July 1997 "[able III. Relative equivalent dose Equivalent dose (pSv) Organ~tissue

LRO

LRtO

SRO

SRtO

PC

Thyroid Salivary glands Parotid (fight) Parotid (left) Submandibular (fight) Submandihular (left) Bone marrow Third molar (right) Third molar (left) First molar (fight) First molar (left) First premolar (fight) First premolar (left) Mandibular symphysis Second cervical vertebra Sixth cervical vertebra Lateral calvarium (right) Lateral calvafium (left) Posterior calvafium Anterior calvarium Esophagus Skin Philtrum Occipital Vertex Preauricular (right) Preauricular (left) Posterior neck Chin Brain (pituitary)

904

297

1308

434

1460

4915 5346 4193 4767

911 1069 1046 710

7983 7217 7035 6384

1059 905 1479 1142

7253 9591 7495 8108

5589 6546 7165 7403 7184 7593 7112 1143 157 60 61 52 201 351

4346 4892 6536 6887 6758 6329 3105 487 <1 185 99 109 226 241

5808 6836 7553 8094 8123 8315 7935 1697 334 120 56 257 254 483

3576 4003 4599 5134 4870 5409 4141 399 51 65 84 17 163 195

5977 6769 7673 8760 8685 8646 7963 2199 361 224 161 135 505 632

5425 59 50 991 1236 288 8731 437

1431 188 165 281 309 <1 2638 281

5997 36 127 2829 849 185 9279 659

2567 40 19 359 216 71 4216 266

11035 45 60 2635 3052 218 13130 1390

LRO, Long round open-end PID. LRtO, Long rectangular open-end PID. SRO, Short round open-end PID. SRtO, Short rectangul~ open-end PID. PC, Pointed closed-end PID.

maxillary molar projection, 5.3) established for average adult techniques with D-speed film. These exposure techniques and film speeds were chosen in order to remain consistent with established techniques used at this facility. The net (dosimeter values in C x 10-9 minus background) reading of each TLD in nC units was divided by 5, converted to R using the TLD-specific calibration factor, and converted to Gy using the factor 8.69 x 10-3.12 For this study, the mean absorbed dose recorded by two to three dosimeters was equated with equivalent dose. For all calculations, the marrow content of the mandible, calvarium, and cervical spine was considered to be 16.5% of total body marrow (11.8% calvarium, 1.3% mandible, 3.4% cervical vertebra), 13 the bone surface of the skull and cervical spine 15.5% of total bone surface (14.1% skull and 1.4% cervical vertebra), 14 and

the skin of the head and neck 9.0% of the total skin surface of the body. 15 Absorbed dose in cortical bone was calculated by multiplying the mean marrow dose by 4.63, which is the ratio of f factors or the factors for the conversion of exposure to absorbed dose for bone and soft tissue. 10 E was calculated as the sum of the equivalent doses to each organ or tissue multiplied by that organ or tissue's weighting factor. 1° Tissue-weighting factors represent the relative contribution of that organ or tissue to the total detriment resulting from a uniform whole body irradiation. Weighted organs not included in this study were the gonads, colon, lung, stomach, bladder, breast, and liver because of their small contribution to the effective dose equivalent. 16 For the same reason, the brain, as represented by the pituitary gland, was the only remainder organ included. All others were

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Cederberg et al. 105

Table IV. Long open-end PIDs effective dose (gSv) Collimation Rectangular

Round* wrt

Hr

0.120 0.050 0.050 0.010 0.010 0.050 0.025 0.025

123 351 904 216 2343

15 17 45 2 23

4805 44§

120

Site Bone marrow Esophagus Thyroid

Skin Bone surface Remainder

Salivary glands Other remainder

(wrllr)

Hr

(wItIr)

60 146 180 39 1141 67*

7 7 9 <1 11 4

1

223

Effective dose

38 63

Relative effective dose *Equivalent doses and effective dose represent the mean of 4 determinations.

tICRP Publication 60. Radiation protection. 1990 Recommendations of the International Commission on Radiological Protection. Oxford: PergamonPress; 1990. ~Hrto the salivary glands was 566 gSv and to the brain 170 gSv. This value (67 gSv) represents the mean of 11 remainder organs. §HTto the brain was 437 gSv. This value (44 gSv) represents the mean of 10 remainder organs.

Table Vo Short open-end PIDS effective dose (gSv) Collimation

Round* Site Bone marrow Esophagus Thyroid

Skin Bone surface Remmnder S~ivaryglands Other remainder

Rectangular

wrt

HT

(wrHr)

Hr

(wrttr)

0.120 0.050 0.050 0.010 0.010 0.050 0.025 0.025

237 928 2645 477 4858

29 47 133 5 49

119 301 678 150 2382 201'

14 15 34 2 24 10

13758 127§

344 3 610 317

Effective dose R d a t i v e effective dose

99 63

*Equivalent doses and effective dose represent the mean of 2 determinations.

*ICRP Publication 60. Radiation protection. 1990 Recommendations of the International Commission on Radiological Protection. Oxford: PergamonPress, 1990. *Hr to the salivary glands was I791 gSv and to the brain 416 gSv. This value (201 gSv) represents the mean of 11 remainder organs. 4Hrto the brain was 1268 gSv. This value (127 gSv) represents the mean of 10 remainder organs.

assumed to receive an equivalent dose of zero. With the addition of the salivary glands as part of the remainder, the total number of remainder organs became 11 for purposes of calculation. In those cases in which the salivary glands received an equivalent dose in excess of the highest dose to any of the measured weighted organs, a weighting factor of 0.025 was assigned to the salivary glands and a weighting factor of 0.025 applied to the average dose of the other 10 remainder organs. Limitations imposed by the x-ray unit's timer did not allow for a constant exposure to be achieved at the surface of the phantom with the several focal spot to film distances of the PIDs (Table II). To account for this, equivalent doses relative to that delivered to the phan-

tom in the long round open-end PID study were calculated for each PID and used in calculations of E. The relative equivalent dose was derived as the product of the ratio of radiation intensities (exposure factor, Table II) measured at constant exposure techniques and the PID specific equivalent dose.

RESULTS Measured equivalent doses resulting from the use of the several different PIDs are summarized in Table I. To account for differing FSFDs among PIDs and the resulting differences in radiation intensities at the end of the PID, relative equivalent doses were calculated. These relative equivalent doses (Table III) were calculated by

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ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY July 1997

Table Vl. Pointed closed-end PID effective dose (gSv)

Table VII. Probability of stochastic effects by PID*

Round collimation* Site Bone marrow Esophagus Thyroid Skin Bone surface Remainder Salivary glands Other remainder Effective dose Relative effective dose

PID type

wf

HT

(WrHT)

0.120 0.050 0.050 0.010 0.010

207 831 1921 511 3525

25 42 96 5 36

0.025 0.025

10673 183*

267 5 476 362

*Equivalent doses and effective dose represent the mean of 2 determinations. tlCRP Publication 60. Radiation protection. 1990 Recommendations of the International Commission on Radiological Protection. Oxfordi Pergamon Press; 1990. ;H r to the brain was 1829 gSv. This value (183 gSv) represents the mean of 10 remainder organs.

multiplying each mean dose by the exposure factor for each PID (Table II). This exposure factor was calculated as the ratio of the measured radiation intensity for each PID and the long round PID. The highest relative equivalent doses were recorded at the skin of the chin and the philtrum, 13130 gSv and 11035 gSv respectively for the pointed PID. The lowest relative equivalent doses were recorded in the area of the sixth cervical vertebra and posterior neck for the long rectangular PID (<1 gSv) and the posterior calvarium for the short rectangular PID (17 gSv). The relative equivalent dose calculated for the thyroid gland when long rectangular collimation was used was 297 gSv. This value was only 20% to 70% that calculated for any of the other PIDs. Right and left relative equivalent doses for seven different anatomic sites were recorded for each PID. These included the parotid and submandibular glands, the first and third molars and the first premolar, the lateral calvarium, and the preauricular skin (Table III). Percent differential between the right and left side sites ranged from an average of 9.3% for the long round PID to 23.7% for the short round PID. E was calculated for each PID with the relative equivalent doses. The salivary glands received an equivalent dose that exceeded that of any of the measured weighted tissues or organs when round open-end PIDs were used (Tables IV and V). Because of this finding, they were assigned a weighting factor of 0.025 or one half of the remainder for calculation of effective dose. In contrast, the salivary glands received an equivalent dose that was less than that of any of the measured weighted tissues or organs when rectangular PIDs were used. In these cases, they were included in the remainder for the

Long round cone Short round cone Long rectangular cone Short rectangular cone Pointed cone

Probability (× 10 -6) 16 23 4.6 4.6 26

*The whole population probability coefficient is 7.3 × 10"2Sv- 1

purposes of calculation. Both long and short rectangular collimation were calculated to result in the lowest E (63 gSv), a value 3.5 to 5.0 times less than that calculated for round collimation depending on the length of the round PID. The use of the pointed PID resulted in an E of 362 gSv, almost six times greater than that calculated for either the long or short rectangular PID (Table VI). The reproducibility of the method used in this study for calculation of effective dose can be seen by comparison of the four determinations of E for the long round PID. The mean for these four determinations was 234 gSv, the calculated mean of 223 gSv (Table IV) was computed by the addition of equivalent dose means. The standard deviation of the four long round determinations was 8.76 with a coefficient of variance of 3.74%. The probability of stochastic effects by PID type resulting from complete mouth surveys are summarized in Table VII. Use of the pointed PID was calculated to result in risk similar to that of the short round PID (26 x 10 -6 and 23 × 10-6, respectively), 1.6 times that of the long round PID (16 x 10-6) and 5.6 times that of either the long or short rectangular PIDs (4.6 x 10-6).

DISCUSSION The objective of this study was to calculate and compare the E and estimated risk associated with the use of short and long, round and rectangular open-end PIDs, and a short pointed PID for intraoral radiography. Each simulated complete mouth survey made with the several PIDs was exposed with average adult techniques established for the long, round open-end PID and Dspeed film. This method of exposure required a mathematical standardization of the radiation intensity at the surface of the phantom and a calculation of E relative to that delivered by the long, round open-end PID because of several lengths of PIDs used in this study and lack of an infinitely variable timer on the x-ray unit. Several studies have reported on the effect of PID type on measured absorbed doses for simulated complete mouth surveys when factors such as exposure techniques, film speed, beam field size and radiograph-

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ic positioning techniques were comparatively equivalent. The method used for calculating E in the present study was modified from that described in ICRP 60 l° to include the salivary glands as previously described. 17 A calculation for effective dose using this method was done for five referenced studies and the computed Es are summarized in Table VIII. Winkler 2 compared a 16 inch lead-lined round PID to the same PID used with a rectangular shielded film holder. Dose comparison between the round beam (6.67 cm in diameter) and the rectangular beam (4.45 cm x 3.49 cm) showed that the integral absorbed dose for the rectangular beam was about one third that of the round. A reduction of 72% to 80% in E, depending on the FSFD, was found when round and rectangular PIDs were compared in this study. Ice et al.3 compared absorbed doses to different tissues and organs when various PIDs were used. They found that there was no significant difference between absorbed doses recorded at the parotid, thyroid, and cornea for each PID type when using 90 kVp techniques established for D-speed film. On the other hand, Greer 5 found that the pointed PID demonstrated the highest absorbed dose for the parotid, thyroid, and cornea and that there was a significant difference between absorbed doses at the three sites for each PID type. Weissman and Sobkowski 4 compared a lead-lined PID, unlined 40.6 cm PID, and a pointed plastic PID. When the shielded PID was used with an unshielded film holder, the absorbed dose to the thyroid was reduced 80% as compared with the pointed plastic PID. Avendanio et a l l 8 reported an E for a complete mouth survey using a long round open-end PID with 90 kVp and D-speed film techniques to be 150 gSv. The average E of 230 gSv (Table VIII) for the five referenced studies compares favorably with the average E for the long round PID of 223 gSv (Table IV) reported in this

study. As previously described, 17 the method used for calculation of E in this study resulted in a higher estimation for E. If the salivary glands were not included in the calculation of E for the long round PID, the E would be 102 gSv and closer to the average E of 84 gSv reported by White. 19 In addition, the salivary glands were found to contribute 57% of the E calculated from the use of either the short round PID or the short pointed PID. Comparison of the effect of collimation on effective dose can be seen in Table IV and V. Rectangular collimation provided approximately a fourfold decrease in the equivalent and effective doses for each tissue type studied. Underhill et al. 2° found a similar reduction when comparing complete mouth surveys made with round and rectangular collimation. Stenstrom et al. 21 also demonstrated that a rectangular field and a long

Table VIII. Effective dose comparison* (gSv) References

Bone marrow Esophagus Thyroid Skin Bone surface Remainder Effective dose

2

3

6

219 . 46 10 80 98 453

56

83 . 9 34 30 90 246

.

. 11 <1 21 <1 88

4

21

Average

108

13 8 25 2 20 82 150

96 8 19 12 38 57 230

6 15 39 14 182

*A simulated complete mouth survey using a long round open-end PID and 90 kVp techniques established for D-speed film,

FSFD used with a paralleling technique could significantly reduce absorbed doses to radiosensitive tissues. Focal spot-to-film distance was found to have an effect on E in this study. A 30% increase in the calculated E for the short round open-end PID (317 gSv) was seen" when compared with the long round PID (223 gSv). This finding compares favorably with Gibbs et al. 22 who reported a 38% decrease in thyroid dose with a long round PID as compared with a short round PID when 90 kVp x-rays and either D or E speed film techniques were used. The effect of scatter radiation on the equivalent and effective dose may be seen by comparison of the long round PID with the pointed PID. The FSFDs of the two PIDs were equivalent (Table II). The PID-end radiation intensity of the pointed PID was 24.1% greater than the long round PID, and the beam field size of the pointed PID was 32.2% greater than the long round PID. However, when a comparison of E was made, it was found that the pointed PID was 38.4% (362 gSv, Table VI) greater than the long round PID (223 p.Sv, Table IV). These data would suggest by comparison that the larger beam field size alone does not account for the higher relative E seen, but that scatter produced by the pointed closed-end PID may contribute to this increase. The quantity of radiation delivered to the individual sites is dependent in large part on the alignment of the x-ray beam. Potter et al. 23 found that when a paralleling instrument was used for bite-wing projections, vertical and horizontal angulation errors could be produced. Because paralleling instruments were not used in this study, the ability to routinely duplicate vertical and horizontal positioning between projections may also be questioned. In addition, equivalent and effective doses are dependent, in part, on the location of some organs in relationship to the image plane, as defined by the location of the dosimeters. Velders et al. 24 reported that absorbed dose was dependent on the beam energy and

108 Cederberg et al.

also influenced by the distance the recording site was from the primary beam. It is apparent that small changes in beam angulation, which are typical for a dental practice, can cause considerable changes in absorbed dose. Analysis of the data in this study showed that not all sites that were located bilaterally received the same quantity of radiation. W h e n doses delivered to contralateral sites by the same PID type were compared, a range of differences between right and left side sites were found (Table III). The percent differential between relative equivalent doses to the right and left sides for seven sites in which both the right and left sides were measured was used to assess these differences. Regardless of PID type, the two sites with the least percent differential were the first molar (average 7%) and the first premolar (average 5%). These sites lie near the path of the x-ray beam during most complete mouth radiographic projections and would most likely receive equivalent amounts of radiation. Sites more remote from the average entrance points would be less likely to receive equivalent amounts of radiation and more likely to receive random exposures by the periphery of the beam. This was found regardless of PID type at measured sites farthest from the oral cavity. The average percent differentials by PID type were: long round (9.3%), long rectangular (17.7%), short round (23.7%), short rectangular (18.9%), and pointed (14.1%). When PID types were compared, use of the short round PID was found to result in the highest percent differential (70%) in the preauricular area. Possibly because of the more divergent nature of the beam, the short round PID had the highest average percent differential (23.7%) of all the PIDs. Beam alignment, angulation, field size, and possibly the quantity of scatter radiation combined to increase the relative equivalent doses for b o t h the short round PID and the pointed PID. When all of the factors that might potentially affect the quantity of equivalent doses to measured sites were taken into consideration, and a comparison of E made between similar radiographic techniques, it was felt that if the effective doses were within a factor of two or three of each other, they may be considered to be in the same subset of Es. The calculated probability of radiation-induced fatal cancer from a complete mouth survey reported here compared favorably with those of other studies. 17,18 Underhill et al. 2° reported a probability of stochastic effects for a long round PID to be 17 x 10-6; however, this was calculated for E-speed film techniques. This study demonstrated a probability of stochastic effects that ranged from 16 x 10-6 for the long round PID to 26 x 10-6 for the pointed P1D (Table VII). The difference between the two studies may be attributed to the film

ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY July 1997 speed simulated and the way in which the salivary glands were treated in the calculation of E. With the advent of improved intraoral long cone techniques and faster film speeds, the necessity for the use of a PID with a pointed closed-end geometry for intraoral radiography should be almost nonexistent. The results of this study suggest that the use of a pointed PID equates to a risk of 5.6 times that of a long rectangular PID. REFERENCES 1. Barr J, Stephens R. Dental radiology. Philadelphia: WB Saunders; 1980. p. 35. 2. Winkler K. Influence of rectangular collimation and intraoral shielding on radiation dose in dental radiography. J Am Dent Assoc 1968;77:95-101. 3. Ice RD, UpdegraveWJ, Bogucki EL. Influenceof dental radiographic cones on radiation exposure. J Am Dent Assoc 1971;83:1297-302. 4. Weissman D, Sobkowski E Comparative thermoluminescent dosimetryof intraoralradiographywith specialemphasison collimator dimensions.Swed Dent J 1986;10:59-71. 5. Greer DE Determinationand analysisof absorbed doses resulting from various intraoral radiographic techniques. Oral Surg Oral Med Oral Pathol 1972;34:146-62. 6. Alcox RW. Biological effects and radiation protection in the dental office. Dent Clin North Am 1978;22:517-32. 7. Office of the Federal Register, National Archives and Records Administration,Food and Drugs.A codificationof documentsof general applicability and future effect. Code of Federal Regulations. 1994. 8. Departmentof Health, Education,and Welfare, Food and Drug Administration,Bureauof RadiologicalHealth.Analysisof suggested amendmentto the performancestandardfor diagnosticxray systems and their major components (21 CFR 1020.30 1020.32): to requireprovisionof open-ended,shielded,positionindicatingdevices of dental intraoralx-ray equipment. 1975. 9. Radiation protection. Recommendations of the International Commission on Radiological Protection. Oxford: Pergamon Press, 1977:[ICRP publication26]. 10. Radiation protection. 1990 Recommendations of the InternationalCommissionon Radiological Protection. Oxford: Pergamon Press, 1990:[ICRPpublication60]. 11. Goaz PW, White SC. Oral radiology:principles and interpretation. 3rd ed. St. Louis: Mosby-YearBook; 1994. 12. Radiationdosimetry: x-rays generated at potentialsof 5 to 150 kV. Washington[DC]: InternationalCommissionon Radiation Units and Measurements, 1970:[ICRU report 17]. 13. Ellis RE. The distributionof active bone marrow in the adult. Phys Med Biol 1961;5:255-8. 14. BoswickJA. The art and science of burn care. Rockville[MD]: Aspen Publishers; 1987. 15. Johns HE, CunninghamJR. The physics of radiology. 4th ed. Springfield [IL]: Charles C. Thomas; 1983. 16. Gibbs SJ. Influenceof organs in the ICRP's remainderon effective dose equivalent computed for diagnostic radiation exposures. Health Phys 1989;56:515-20. 17. FrederiksenNL, Benson BW, Sokolowski TW. Effective dose and risk assessment from film tomography used for dental implantdiagnostics.DentomaxillofacRadiol 1993;23:123-7. 18. AvendanioB, Frederiksen NL, Benson BW, Sokolowski TW. Effective dose and risk assessmentfrom detailed narrow beam radiography. Oral Surg Oral Med Oral Pathol Oral RadiolEndod 1996;82:713-9. 19. White SC. 1992 assessmentof radiation risk from dental radiography. DentomaxillofacRadiol 1992;21:118-26. 20. UnderhillTE, ChilvaquerI, Kiurura K, Langlais RP, McDavid

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WD, Preece JW, et al. Radiobiologic risk estimation from dental radiology. Part 1. Absorbed doses to critical organs. Oral Surg Oral Med Oral Pathol 1988;66:111-20. 21. Strenstrom B, Henrikson C, Karlsson L, Sarby B. Effective dose equivalent from intraoral radiography. Swed Dent J 1987;11:717. 22. Gibbs S J, Pujol A, Chen T-S, James A. Patient risk from intraoral dental radiography. Dentomaxiilofac Radiol 1988;17:15-23. 23. Potter B, Shrout M, Harrell J. Reproducibility of beam alignment using different bite-wing radiographic techniques. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1995;79:532-5.

24. Velders XL, Aken J, van der Stelt PE Risk assessment from bitewing radiography. Dentomaxillofac Radiol 1991;20:209-13.

Reprint requests: Robert A. Cederberg, MA, DDS Texas A & M University System/Baylor College of Dentistry Department of Diagnostic Sciences EO. Box 660677 Dallas, Texas 75266-0677