- Email: [email protected]
Regarding “Influence of CBCT exposure conditions on radiation dose”
OOOOE Volume 106, Number 5
and therefore much thicker than the latter. As far as the volume of MTA is concerned, Shemesh et al.3 used a mold (2 mm deep with a diameter of 5 mm) to standardize the volume. In our study, we used MTA to repair furcal perforations (2 to 3 mm deep with a diameter of 1 mm). Therefore, the volume of MTA body in the study by Shemesh et al.3 is at least 16 times larger than ours. Furthermore, they placed the set MTA discs directly into 4 mL of glucose solution, exposing a relatively large surface area of the MTA discs to the glucose solution. However, in our study, only the surface in the pulp chamber and the micro-space between MTA and dentine wall could contact the glucose solution. Another important factor to consider is the solubility of MTA. It is well known that MTA is mainly composed of an insoluble matrix of silica that maintains its integrity even in contact with water,4 with a constant decrease of solubility throughout a long experimental period (78 days).5 Based on all the differences between the 2 experimental designs, further studies are needed to answer the important question of how much oxidation will take place in a 1 mol/L1 glucose solution when it contacts 1 mm2 of set MTA. It is important to know, but we believe the difference is not of any statistical difference. However, the models using penetration of dyes or other chemicals are thought to be of no objective value to endodontic science.6 The bacterial penetration model may be more clinically relevant, but the results are likely affected by the antibacterial property of the filling materials. Therefore, clinically relevant and reliable methods to assess hydraulic flow in endodontically treated teeth are still to be found. Ling Zou, DDS, PhD Jun Liu, DDS, PhD Shihai Yin, DDS, PhD State Key Laboratory of Oral Diseases West China College of Stomatology West China Hospital of Stomatology Sichuan University Chengdu, P.R. China
Letters to the Editor 627 bility and porosity with different water-to-powder ratios. J Endod 2003;29:814-7. 5. Fridland M, Rosado R. MTA solubility: a long-term study. J Endod 2005;31:376-9. 6. The Editorial Board of the Journal of Endodontics. Wanted: a base of evidence. J Endod 2007;33:1401-2. doi:10.1016/j.tripleo.2008.06.022
Regarding “Influence of CBCT exposure conditions on radiation dose” To the Editor: I congratulate Dr. Palomo and coauthors for their assessment of dose reduction strategies for maxillofacial examinations with the CB Mercuray cone-beam computerized tomography (CBCT) unit.1 However, a number of inaccuracies are in need of correction to place this work in the context of other dosimetry studies. Most important, it should be noted that the draft 2005 recommendations of the International Commission on Radiological Protection (ICRP), noted in reference 12 as accessed from the ICRP website in February 2006, is not the same document as ICRP Publication 103, which was released in December 2007.2 In fact, it is a significantly different document from the revised draft that was available from the ICRP website as of December 2007. The ICRP revision of the 1990 recommendations went through several iterations, including significant changes, between the 2005 draft and Publication 103. Whereas the 2005 draft retained weights of 0.05 for the thyroid gland and esophagus, the 2007 recommendations reduced the weight for these tissues to 0.04. It is clear from Table I that the authors are using the 2005 draft weights rather than the January 2007 weights which were adopted for Publication 103. It also appears that remainder tissues were not included in calculations of effective dose using either 2005 draft weights or Publication 103 weights. Remainder tissues cannot be overlooked without significant underestimation of effective dose. This is because the extrathoracic region (airway) and oral mucosa, as well as lymph nodes and muscle, are included in the remainder group. In addition, the remainder group accounts for a weight of 0.12 in the
REFERENCES 1. Zou L, Liu J, Yin SH, Li W, Xie J. In vitro evaluation of the sealing ability of MTA used for the repair of furcation perforations with and without the use of an internal matrix. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105:e61-5. 2. Xu Q, Fan MW, Fan B, Cheung GS, Hu HL. A new quantitative method using glucose for analysis of endodontic leakage. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;99:107-11. 3. Shemesh H, Souza EM, Wu MK, Wesselink PR. Glucose reactivity with filling materials as a limitation for using the glucoseleakage model. Int Endod J 2008. In press. 4. Fridland M, Rosado R. Mineral trioxide aggregate (MTA) solu-
Table I. Comparison of effective dose calculations from 2 studies 3
Ludlow et al. 2007 weights Palomo et al.2 2005 draft without remainder Ratio Ludlow/Palomo data, % FOV, Field of view.
12” FOV
9” FOV
6” FOV
1,073 761
569 680
407 603
141%
84%
67%
628
Letters to the Editor
calculation of effective dose. Weights for the 5 individually weighted tissues (1990) or 7 individually weighted tissues (2007) that are directly exposed in maxillofacial examinations sum to 0.24, but the remainder group, which is weighted at 0.05 (1990) increases to 0.12 (2007). To exclude the remainder group from the Publication 103 weights is to potentially ignore a third of the dose to the maxillofacial region. Decisions on numbers of dosimeters to use and their strategic placement are made necessary because of the cost and time involved in using full-body phantoms and distributing dosimeters uniformly throughout a phantom. Certainly in a study such as this, involving numerous dosimeter runs, the need to limit numbers of dosimeters and sites within the phantom is understandable. However, the absence of dosimeters in the calvarium makes it impossible to measure the reduction in dose to the bone marrow resulting from smaller fields of view. Bone marrow has a high weight of 0.12, and the calvarium constitutes 72% of the marrow in the head and neck area. In a study of CBCT dosimetry using ICRP 2007 tissue weights, the ratio of calvarial to cervical spine exposure for the CB Mercuray was 88% for the 12” field of view (FOV), and 52% and 9% for the 9” and 6” FOVs, respectively.3 This explains the direction of differences in ratios of effective dose calculation (Sv) for 120 kV and 15 mA for the 2 studies seen in Table I. John B. Ludlow, DDS, MS, FDSRCSEd Professor of Oral and Maxillofacial Radiology School of Dentistry, University of North Carolina Chapel Hill, NC REFERENCES 1. Palomo JM, Rao PS, Hans MG. Influence of CBCT exposure conditions on radiation dose. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105:773-82. 2. Valentin J. The 2007 recommendations of the International Commission on Radiological Protection. Publication 103. Ann ICRP 2007;37:1-332. 3. Ludlow JB, Ivanovic M. Comparative dosimetry of dental CBCT devices and 64 row CT for oral and maxillofacial radiology. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2007;106:930-8. doi:10.1016/j.tripleo.2008.06.031
In reply: We would like to thank Dr. Ludlow for his interest in our article. In his letter, Dr. Ludlow raises 3 objections, each of which warrants comment. 1. We used the ICRP 2005 guidelines rather than the version released in 2007 simply because the latter was not available at the time the project was prosecuted. It takes considerable time for an article to be published; during that interval, the appearance of updates are beyond the authors’ control. To avoid
OOOOE November 2008
confusion, we carefully pointed out in our references the day, month, and year of our access to the ICRP document. The other 2 observations have to do with the methodology for the calculation of the effective dose of the craniofacial complex. The calculation of an effective dose has to do with interpretation of an ICRP document and the estimation of the craniofacial percentage of such structures as skin and muscles relative to the whole body. Unfortunately, there is no standard for such a calculation. Different projects have used different numbers of dosimeters, ranging from 9 to 24, different dosimeter locations, and/or different percentage values.1-4 It may be a good idea for the American Academy of Oral and Maxillofacial Radiology (AAOMR) to put together a committee to standardize such values and to recommend the number of dosimeters and the methodology to be used. Given the lack of a standardized methodology, we based our study on earlier research projects as well our own interpretation of the ICRP guidelines. Furthermore, we chose to use an internal comparative method for our results. We did not want the message of our article to be that at a given specific setting, the effective dose was what we stated. We wanted to show the effects of varying the setting and collimation, and to compare them with traditional radiographs such as a full mouth series or a panoramic exposure. The values for both CBCT settings and traditional radiographs were given in our study, following the exact same methodology, thus providing a basis for comparison. We did not compare our numbers to others in the literature, because there could be little agreement owing to different methodologies and interpretations. A second important point we want to make is that a radiation dose project, in our opinion, should always be performed in conjunction with an image quality project. Otherwise, dosimetry measurements just become a numbers game with no clinical relevance. We know from physics that lower settings produce lower radiation dose but also a lower image quality. We need to determine the lowest settings for each procedure that still provide an image of diagnostic quality, and then look to manufacturers to adjust their machines accordingly or provide appropriate operator control. Currently there is no standard in the CBCT industry. We have commercially available scanners running on a 2 mA mode and others running at 15 mA, all marketed for use in the same clinical situation. In our study, we showed that the same scanner is able to provide a wide range of radiation dose values; therefore, it is not the scanner or brand that provides more or less radiation dose,
and therefore much thicker than the latter. As far as the volume of MTA is concerned, Shemesh et al.3 used a mold (2 mm deep with a diameter of 5 mm) to standardize the volume. In our study, we used MTA to repair furcal perforations (2 to 3 mm deep with a diameter of 1 mm). Therefore, the volume of MTA body in the study by Shemesh et al.3 is at least 16 times larger than ours. Furthermore, they placed the set MTA discs directly into 4 mL of glucose solution, exposing a relatively large surface area of the MTA discs to the glucose solution. However, in our study, only the surface in the pulp chamber and the micro-space between MTA and dentine wall could contact the glucose solution. Another important factor to consider is the solubility of MTA. It is well known that MTA is mainly composed of an insoluble matrix of silica that maintains its integrity even in contact with water,4 with a constant decrease of solubility throughout a long experimental period (78 days).5 Based on all the differences between the 2 experimental designs, further studies are needed to answer the important question of how much oxidation will take place in a 1 mol/L1 glucose solution when it contacts 1 mm2 of set MTA. It is important to know, but we believe the difference is not of any statistical difference. However, the models using penetration of dyes or other chemicals are thought to be of no objective value to endodontic science.6 The bacterial penetration model may be more clinically relevant, but the results are likely affected by the antibacterial property of the filling materials. Therefore, clinically relevant and reliable methods to assess hydraulic flow in endodontically treated teeth are still to be found. Ling Zou, DDS, PhD Jun Liu, DDS, PhD Shihai Yin, DDS, PhD State Key Laboratory of Oral Diseases West China College of Stomatology West China Hospital of Stomatology Sichuan University Chengdu, P.R. China
Letters to the Editor 627 bility and porosity with different water-to-powder ratios. J Endod 2003;29:814-7. 5. Fridland M, Rosado R. MTA solubility: a long-term study. J Endod 2005;31:376-9. 6. The Editorial Board of the Journal of Endodontics. Wanted: a base of evidence. J Endod 2007;33:1401-2. doi:10.1016/j.tripleo.2008.06.022
Regarding “Influence of CBCT exposure conditions on radiation dose” To the Editor: I congratulate Dr. Palomo and coauthors for their assessment of dose reduction strategies for maxillofacial examinations with the CB Mercuray cone-beam computerized tomography (CBCT) unit.1 However, a number of inaccuracies are in need of correction to place this work in the context of other dosimetry studies. Most important, it should be noted that the draft 2005 recommendations of the International Commission on Radiological Protection (ICRP), noted in reference 12 as accessed from the ICRP website in February 2006, is not the same document as ICRP Publication 103, which was released in December 2007.2 In fact, it is a significantly different document from the revised draft that was available from the ICRP website as of December 2007. The ICRP revision of the 1990 recommendations went through several iterations, including significant changes, between the 2005 draft and Publication 103. Whereas the 2005 draft retained weights of 0.05 for the thyroid gland and esophagus, the 2007 recommendations reduced the weight for these tissues to 0.04. It is clear from Table I that the authors are using the 2005 draft weights rather than the January 2007 weights which were adopted for Publication 103. It also appears that remainder tissues were not included in calculations of effective dose using either 2005 draft weights or Publication 103 weights. Remainder tissues cannot be overlooked without significant underestimation of effective dose. This is because the extrathoracic region (airway) and oral mucosa, as well as lymph nodes and muscle, are included in the remainder group. In addition, the remainder group accounts for a weight of 0.12 in the
REFERENCES 1. Zou L, Liu J, Yin SH, Li W, Xie J. In vitro evaluation of the sealing ability of MTA used for the repair of furcation perforations with and without the use of an internal matrix. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105:e61-5. 2. Xu Q, Fan MW, Fan B, Cheung GS, Hu HL. A new quantitative method using glucose for analysis of endodontic leakage. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;99:107-11. 3. Shemesh H, Souza EM, Wu MK, Wesselink PR. Glucose reactivity with filling materials as a limitation for using the glucoseleakage model. Int Endod J 2008. In press. 4. Fridland M, Rosado R. Mineral trioxide aggregate (MTA) solu-
Table I. Comparison of effective dose calculations from 2 studies 3
Ludlow et al. 2007 weights Palomo et al.2 2005 draft without remainder Ratio Ludlow/Palomo data, % FOV, Field of view.
12” FOV
9” FOV
6” FOV
1,073 761
569 680
407 603
141%
84%
67%
628
Letters to the Editor
calculation of effective dose. Weights for the 5 individually weighted tissues (1990) or 7 individually weighted tissues (2007) that are directly exposed in maxillofacial examinations sum to 0.24, but the remainder group, which is weighted at 0.05 (1990) increases to 0.12 (2007). To exclude the remainder group from the Publication 103 weights is to potentially ignore a third of the dose to the maxillofacial region. Decisions on numbers of dosimeters to use and their strategic placement are made necessary because of the cost and time involved in using full-body phantoms and distributing dosimeters uniformly throughout a phantom. Certainly in a study such as this, involving numerous dosimeter runs, the need to limit numbers of dosimeters and sites within the phantom is understandable. However, the absence of dosimeters in the calvarium makes it impossible to measure the reduction in dose to the bone marrow resulting from smaller fields of view. Bone marrow has a high weight of 0.12, and the calvarium constitutes 72% of the marrow in the head and neck area. In a study of CBCT dosimetry using ICRP 2007 tissue weights, the ratio of calvarial to cervical spine exposure for the CB Mercuray was 88% for the 12” field of view (FOV), and 52% and 9% for the 9” and 6” FOVs, respectively.3 This explains the direction of differences in ratios of effective dose calculation (Sv) for 120 kV and 15 mA for the 2 studies seen in Table I. John B. Ludlow, DDS, MS, FDSRCSEd Professor of Oral and Maxillofacial Radiology School of Dentistry, University of North Carolina Chapel Hill, NC REFERENCES 1. Palomo JM, Rao PS, Hans MG. Influence of CBCT exposure conditions on radiation dose. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;105:773-82. 2. Valentin J. The 2007 recommendations of the International Commission on Radiological Protection. Publication 103. Ann ICRP 2007;37:1-332. 3. Ludlow JB, Ivanovic M. Comparative dosimetry of dental CBCT devices and 64 row CT for oral and maxillofacial radiology. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2007;106:930-8. doi:10.1016/j.tripleo.2008.06.031
In reply: We would like to thank Dr. Ludlow for his interest in our article. In his letter, Dr. Ludlow raises 3 objections, each of which warrants comment. 1. We used the ICRP 2005 guidelines rather than the version released in 2007 simply because the latter was not available at the time the project was prosecuted. It takes considerable time for an article to be published; during that interval, the appearance of updates are beyond the authors’ control. To avoid
OOOOE November 2008
confusion, we carefully pointed out in our references the day, month, and year of our access to the ICRP document. The other 2 observations have to do with the methodology for the calculation of the effective dose of the craniofacial complex. The calculation of an effective dose has to do with interpretation of an ICRP document and the estimation of the craniofacial percentage of such structures as skin and muscles relative to the whole body. Unfortunately, there is no standard for such a calculation. Different projects have used different numbers of dosimeters, ranging from 9 to 24, different dosimeter locations, and/or different percentage values.1-4 It may be a good idea for the American Academy of Oral and Maxillofacial Radiology (AAOMR) to put together a committee to standardize such values and to recommend the number of dosimeters and the methodology to be used. Given the lack of a standardized methodology, we based our study on earlier research projects as well our own interpretation of the ICRP guidelines. Furthermore, we chose to use an internal comparative method for our results. We did not want the message of our article to be that at a given specific setting, the effective dose was what we stated. We wanted to show the effects of varying the setting and collimation, and to compare them with traditional radiographs such as a full mouth series or a panoramic exposure. The values for both CBCT settings and traditional radiographs were given in our study, following the exact same methodology, thus providing a basis for comparison. We did not compare our numbers to others in the literature, because there could be little agreement owing to different methodologies and interpretations. A second important point we want to make is that a radiation dose project, in our opinion, should always be performed in conjunction with an image quality project. Otherwise, dosimetry measurements just become a numbers game with no clinical relevance. We know from physics that lower settings produce lower radiation dose but also a lower image quality. We need to determine the lowest settings for each procedure that still provide an image of diagnostic quality, and then look to manufacturers to adjust their machines accordingly or provide appropriate operator control. Currently there is no standard in the CBCT industry. We have commercially available scanners running on a 2 mA mode and others running at 15 mA, all marketed for use in the same clinical situation. In our study, we showed that the same scanner is able to provide a wide range of radiation dose values; therefore, it is not the scanner or brand that provides more or less radiation dose,