|Year : 2017 | Volume
| Issue : 3 | Page : 538-543
Effect of various dental restorations on dose distribution of 6 MV photon beam
Mona Azizi1, Ali Asghar Mowlavi2, Mahdi Ghorbani3, David Davenport4
1 Department of Physics, School of Sciences, Hakim Sabzevari University, Sabzevar, Iran
2 Department of Physics, School of Sciences, Hakim Sabzevari University, Sabzevar, Iran; International Centre for Theoretical Physics, Trieste, Italy
3 Medical Physics Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
4 Comprehensive Cancer Centers of Nevada, Las Vegas, Nevada, USA
|Date of Web Publication||31-Aug-2017|
Ali Asghar Mowlavi
Department of Physics, School of Sciences, Hakim Sabzevari University, Tohid Sahar Campus, Sabzevar
Source of Support: None, Conflict of Interest: None
Aim: The purpose of this study is to evaluate the effect of various dental restoration materials on dose distribution in radiotherapy of head and neck cancer with 6 MV photon beam of a medical linac.
Setting and Design: The dental restorations include tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco. Dose perturbation due to the dental restorations on 6 MV beam of Siemens Primus linac was calculated by MCNPX Monte Carlo code. These dental materials were separately simulated in a cubic water phantom.
Materials and Methods: Photon percentage dose change in the presence of tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco was calculated at various depths on the central axis of the beam relative to the dose in water. In another evaluation, the absolute dose (cGy) for water, tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco was obtained by calculating 100 cGy dose at 0.75 cm depth in the water phantom.
Results: Based on the calculations performed, maximum percentage dose change due to backscattering was 11%, 8%, 6%, and 4% for amalgam, Ni-Cr alloy, Ceramco and tooth at depth of 0.75 cm, respectively. The maximum dose perturbation by amalgam is due to its higher mass density and atomic number.
Conclusion: Ignoring the effect of dental restoration compositions on dose distribution causes discrepancy in clinical treatment planning system calculations.
Keywords: Dental restoration, dose distribution, Monte Carlo simulation, radiotherapy
|How to cite this article:|
Azizi M, Mowlavi AA, Ghorbani M, Davenport D. Effect of various dental restorations on dose distribution of 6 MV photon beam. J Can Res Ther 2017;13:538-43
| > Introduction|| |
Head and neck cancer accounts for about 3% of all cancers in the United States. Cancers that are known collectively as head and neck cancers usually begin in the squamous cells that line the moist, mucosal surfaces inside the head and neck (for example, inside the mouth, nose, and throat). Among these patients treated by radiation therapy, most of them have nonremovable dental restorations. When these materials are in the path of a megavoltage gamma ray, a considerable amount of backscatter dose arises. This radiation backscattering is caused by the different dental materials during the radiotherapy for head and neck cancer and can cause damage to the patient's oral cavity and healthy tissues. There are scientific studies that offer the dentists information concerning the consequences related to radiation therapy to facilitate collaborations with the medical team.,,
Treatment planning for a cancerous patient depends on the exact location of the tumor, the stage of cancer, and the person's age and general health. During the treatment, the quality of the patient's life should be considered. More patients will present challenges to radiotherapy with high energy X-ray beams due to an increase in dental reconstruction complexity. In other words, many of the radiotherapy patients have nonremovable dental restorations and prostheses with high atomic numbers and densities. Oral cavity and teeth may lie in the radiation field and become a concern for treatment planning. The presence of these high-density materials on the path of beam causes perturbation in dose distribution.
The American Association of Physicists in Medicine recommends in Report number 81 that for high-density materials such as prostheses and dental restorations, in delivery of the relevant dose to the target organ, the beam setup should be adjusted to avoid entrance dose through these materials. In many radiotherapy patients, there are more than one or two prostheses that often are located very close to the target volume. It is difficult to avoid dental restorations because of the small area of the oral cavity. In a common treatment planning system (TPS), only the electron densities of inhomogeneities are considered, whereas the compositions and the densities of the other materials such prostheses and dental restorations are not taken into account. This ignorance leads to some inaccurate dose delivery to tumor sites. Therefore, calculations from TPSs have approximations due to ignoring the compositions of the prostheses and dental restorations. The restrict criteria adopted in the radiotherapy treatment planning for organ tolerance doses highlight the importance of the accurate calculation of organ doses near the target. Providing more information about dental restorations or prostheses including their mass densities, compositions, and their effects on dose distribution can reduce these discrepancies.
Perturbation in dose distribution due to the presence of high-density materials has been studied previously.,,,,, Chin et al. investigated the effect of dental restoration and prostheses on dose distribution in 60Co and 10 MV photon beams. They found that this backscatter dose can be attenuated by a low-density material or 4 mm air gap. In another study, Beyzadeoglu et al. measured the scattered dose from titanium dental implants to evaluate the angle alteration effect on dose due to radiation backscattering near titanium implant. Friedrich et al. worked on the scattering effects of irradiation on the surrounding tissues of titanium dental implants. This variation can cause the significant radiation scattering and the risk of dose enhancement. Reitemeier et al. evaluated the effect of absorption and backscattering with 6 and 15 MV photon beams. Four dental materials such as gold alloy, pure titanium, amalgam, and a synthetic material were studied. They found out with an appropriate thickness of stent can decrease the damage of backscattering. De Conto et al. investigated the variation of dose distribution and electron contamination due to dental prostheses with 6 MV photon beam. In another study, Abdul Aziz et al. investigated high-density inhomogeneities interfaces with a 9 MeV electron beam. In this study, three types of phantom were used such as dental amalgam, dental only, and homogeneous phantom.
In the present study, three common dental restorations including amalgam, Ni-Cr alloy, and Ceramco used in dental clinics are investigated. In the previous studies, the variations of dose distribution of these common dentures have not been studied comprehensively. In addition, different sources of radiations were evaluated, whereas various radiotherapy machines have different photon spectra and dose distributions. One of the linac machines is Siemens Primus linac, and these days' patients are treated by it, in different countries. There is currently no published data on dose distribution with 6 MV photon beam of a Siemens Primus linac. In the present study, the variation of dose distribution of 6 MV photon beam of a Siemens Primus linac in the presence of different dentures such as amalgam, Ni-Cr alloy, and Ceramco in head and neck cancer is evaluated. The main goal is to compare the effect of tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco in 6 MV photon mode.
| > Materials and Methods|| |
Validation of linac simulation
For dosimetric calculations, a MCNPX simulation program of a Siemens Primus linac that has been validated by experimental data in a previous study was used. Herein, a summary of simulation validation of the previous study is presented; the percent depth dose was obtained for 6 cm × 6 cm, 10 cm × 10 cm, and 20 cm × 20 cm radiation fields. Dose profiles were calculated for 6 cm × 6 cm, 10 cm × 10 cm, and 20 cm × 20 cm at 5 cm depth in a water phantom. In this code, all Siemens Primus (6 MV photon beam) head components were modeled by the related geometric information of the head. This medical linac consists of target, primary collimator, flattening filter, photon dose chamber, mirror, and Y and X-jaws. Each component of this head is composed of different materials. The target of Siemens Primus linac consists of cylindrical layers of water, gold, graphite, air, and stainless steel with different thickness and radii. In a medical linear accelerator, the electrons emitted from a source strike the target and bremsstrahlung photons are produced. For experimental measurement of dose values, a water phantom (RFA-300; IBA Dosimetry GmbH, Schwarzenbruck, Germany) of 30 cm × 30 cm × 30 cm in dimensions was used at a source to surface distance (SSD) of 100 cm. The dosimetry data were measured by a Wellhofer-Scanditronix dosimetry system (Wellhöfer, Uppsala, Sweden), and a diode detector on a Siemens Primus linear accelerator in 6 MV photon mode at the Reza Radiation Oncology Centre (Mashhad, Iran). Finally, to validate the simulations; the Monte Carlo results were compared with the experimental data. The discrepancies between the obtained results of MCNPX code and measurements were <5%. The results by the two methods were in good agreement, and the accuracy of the linac modeling was confirmed. Two programs were used; first, the input file of the accelerator simulation was run for 2 × 109 source particle histories; and then a number of 2.89 × 108 photons and electrons were scored on a horizontal plane called using source surface write card. All programs were run by these numbers of particles. In the second program, the particles on this horizontal plane were read by source surface read card and the absorbed dose was scored in the water phantom.
Dose distribution in the presence of dental restorations
This study was performed by MCNPX 2.6.0 software to simulate 6 MV X-ray beam from a Siemens Primus medical linear accelerator (Siemens AG, Erlangen, Germany). The radiation field was 10 cm × 10 cm and the SSD was 100 cm. MCNPX is a Monte Carlo radiation transport computer code that transports and tracks different particles at various energies. This code has different applications including in the field of design and shielding of accelerators, radiation therapy, dosimetry, and nuclear physics.
Three types of frequent dental restorations such as amalgam, Ni-Cr alloy, and Ceramco are evaluated in this study. An amalgam allows filling a cavity caused by tooth decay and repairs a deteriorated tooth. Sometimes, it is used to cover a dental implant when a tooth is missing. Ni-Cr alloy is one of the common restorations used in full coverage of dental crowns. Ceramic restorations are used for cosmetic reasons. Indeed, they closely reconstruct the appearance of dental enamel. Four-tooth samples were simulated independently: A tooth, a tooth restored with amalgam, a tooth with Ni-Cr alloy, and a tooth restored with Ceramco. The mass densities and compositions for various dental restoration materials are listed in [Table 1]. A healthy tooth consists of root and crown. In this study, the tooth was modeled as a rectangular cube including a crown with dimensions of 0.8 cm × 0.5 cm × 0.8 cm. The crown was defined including 20.0% enamel and 80.0% dentine. A block of 0.8 cm × 0.5 cm × 0.8 cm was embedded as the dental root on the inferior of the crown. Three cubical teeth were embedded in a water phantom with dimension of 30 cm × 30 cm × 30 cm. These two healthy teeth were stabilized, and the middle tooth was filled with the dental restoration. A schematic diagram of phantom configuration is presented in [Figure 1]. The distance between the top surface of the water phantom and the teeth is 1 cm. The crown of the middle tooth was partially filled with amalgam. This tooth was designed such a way that its volume should be consisted of 50% root, of 20% crown, and 30% amalgam. This geometry was the same for Ni-Cr alloy and Ceramco restorations. The photon beam direction was along the Z-axis. The middle tooth was placed in the center of a 10 cm × 10 cm field.
|Table 1: Effective mass densities and compositions of dentine, enamel, root, amalgam, Ni-Cr alloy, and Ceramco defined in the Monte Carlo simulations|
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|Figure 1: Geometry of the dental phantom used in the simulation. The distance between the water phantom and the top of the dental surface is 1 cm. The coordinate of the reference point are (0,0,-100.75) cm|
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For evaluation of the photon percentage dose change, five depths were considered before and after the middle tooth. The percentage photon dose change with and without the presence of tooth with various restoration materials was calculated using the following formula:
Where D1 is the photon dose in the absence of the tooth at a certain depth (open field) and D2 is the dose at the same depth with the presence of tooth with dental restoration material. Positive or negative percentage dose changes indicate that there are increased or decreased dose in the presence of the sample compared to the open field case. Five circular cylinders with 0.25 cm radius and 0.65 cm height were defined to score the dose. These cylindrical cells were placed upward and beyond the dental restoration part. *F8 tally was used to score the energy deposition of 6 MV photon beam with a unit of MeV. Except for energy cutoff of 10 keV for electrons and photons and cell importance, no other variance reduction method was utilized. Generally, cell importance's can assign to each cell in the geometry. It means the cell importance to the interesting tally should be proportional to the number of particles sampled in each cell. The cells near the tally have a greater importance than cells placed far away. Because of the large amount of statistical uncertainties and to reduce the computing time, ten separate input files with various random seed numbers were applied for each restoration material. Finally, the errors of ten programs combined each other to reach the total acceptable error. Finally, the average dose was calculated based on the dose in of the ten programs was obtained and reported as dose. The maximum Type A uncertainty of Monte Carlo calculations was <2.56%.
| > Results|| |
[Table 2] lists the percentage dose change in the case of tooth only, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco at different depths. The calculated absolute dose (cGy) in the phantom for the four mentioned materials is represented in [Table 3].
|Table 2: Percentage dose change in presence of tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco|
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|Table 3: Absolute dose (cGy) in water and in the presence of tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco|
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In [Figure 2], the percentage dose change at various depths for tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco is plotted. [Figure 3] illustrates the absolute dose (cGy) versus depth in water, tooth, and tooth with other mentioned dental materials.
|Figure 2: Percentage dose change versus depth (cm) in the presence of tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco, relative to dose in water|
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|Figure 3: Absolute dose (cGy) versus depth (cm) in water phantom and in the presence of tooth, tooth with amalgam, tooth with Ni-Cr alloy, and tooth with Ceramco|
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| > Discussion|| |
In the present study, the effect of tooth and dental restorations (amalgam, Ni-Cr alloys, and Ceramco) on dose distribution of a 6 MV photon beam was evaluated by MCNPX version 2.6.0 code. As it can be seen from [Table 2] and [Figure 2], the percentage dose change varies with depth in the presence of the mentioned materials. By evaluating this plot, the percentage dose change starts with an increasing dose at depths before the tooth for all the four materials. At depth of 0.75 cm before the various dentures, the major percentage dose changes of 11% and 8% were observed when 6 MV photon passes through amalgam and Ni-Cr alloy, respectively. In this region, the lowest changes in the dose, with amounts of 4% and 6%, are related to the tooth and tooth with Ceramco, respectively.
According to the data of [Figure 2], behind the water-tooth interface, the maximum and minimum photon percentage dose changes are related to amalgam with 8.0 g/cm3 density and tooth with 2.33 g/cm3 density. As a result, in this region, the backscatter phenomenon has dominated. Therefore, this backscattering is increased by the effective atomic number of dental materials. This difference in dose distribution is due to the differences in densities and compositions of dental restorations. The radiation backscattered in the few millimeters before the samples can damage the healthy tissues which are located before the tooth.
Beyond the tooth sample, simulation data for tooth, amalgam, Ni-Cr alloy, and Ceramco start with a negative percentage dose change at 2.10 cm depth. In this region, the dose attenuation of amalgam is greatest, and the underdosage is 19.4% which means the calculated percentage dose change for amalgam is − 19.4% relative to the open field dose. Therefore, after passing the photon beam through these dentures, the maximum percentage dose change at 2.10 cm is related to amalgam, Ni-Cr alloy, tooth, and tooth with Ceramco with values of − 19.40%, −18.20%, −12.80%, and − 11.60%, respectively. After this depth, these four materials follow increasing trends of the percentage dose change up to 4.0 cm depth. At depth of 4.0 cm, the maximum percentage dose change is for tooth, tooth with Ceramco, tooth with Ni-cr alloy and tooth with amalgam with values of 1%, −0.20%, −7.30%, and − 8.40%, respectively. All the data related to the before and after the samples indicate that the percentage dose change for a special denture varies with depth. These data depend on the compositions of the materials.
Another interesting quantity is the absolute dose (cGy/100 monitor unit [MU]) indicates the dose differences due to various dental restorations in 6 MV photon beam of a Siemens Primus linac. These data which were calculated by MCNPX code are listed in [Table 3]. These data were calculated in such a way that the amount of absolute dose for water was considered 100 cGy (100 MU) as reference dose at 0.75 cm depth. According to the [Figure 3], it can be observed that there are some differences in the absolute dose for water and tooth, amalgam, Ni-Cr alloy, and Ceramco materials. In depth of 0.25 cm from the phantom surface, the absolute dose for these four materials was approximately 70 cGy and equal to absolute dose in water. At depth of 0.75 cm, these discrepancies can be considerable, the evaluated absolute dose are 103.75, 105.64, 108, and 111.19 cGy for tooth, Ceramco, Ni-Cr alloy, and amalgam, respectively. By considering these data, tooth restored with amalgam and tooth resulted in dose enhancement of 11 cGy and about 4 cGy relative to water, respectively.
The Monte Carlo data demonstrate that absolute dose is attenuated after the tooth-water interface. These absolute doses are 93.84, 95.21, 101.60, and 102.96 cGy for amalgam, Ni-Cr alloy, tooth, and Ceramco compared to the absolute dose of 116.42 cGy for water phantom, respectively. Similarly, an increased trend is observed for depth of 3.0 cm. In addition, the results in [Figure 3] illustrate a smooth decreased trend for all the four mentioned dentures at 4.0 cm depth. The delivery of excess or reduced dose versus the reference dose of 100 cGy to the healthy tissue before and after the tooth denture can indicate the importance of considering the compositions of high-density materials in treatment planning.
In routine dosimetry of a medical linac, water phantom data are used as a reference. Results are introduced into the TPS as the standard reference in external beam radiotherapy. In a common TPS, only the electron densities of inhomogeneities are considered, whereas the compositions and densities of some other materials such as prosthesis and dental restorations are not taken into account. This ignorance leads to some discrepancies in accurate dose delivery to the tumor, especially in radiotherapy of head and neck cancer where many patients have dental restorations. The International Commission on Radiation Units and Measurements Report number 24 recommended that the uncertainty in radiation dose delivery in radiotherapy should be in the range of ± 5%. One acute side effect of head and neck radiation therapy is the alteration in the structure of the dental substances. Therefore, it highlights the importance of the study in this field to improve the calculation accuracy of dosimetric systems. By introduction of more detailed information about these dental restorations or prostheses, these errors can be reduced. The discrepancy is due to uncertainties in densities, compositions, positions, and differences caused by the backscattering and attenuation effects from various radiotherapies with different energies and for various dental restorations. [Figure 2] and [Figure 3] illustrate that the backscattered radiation before the sample and the underdosage after that occurs within <4 mm from the sample surface. Some studies have recommended the use of low-Z materials before the tooth when the tooth is placed in the field treatment. The results of this study are consistent with other recommendations. Chin et al. found that 3 mm of water-density material can protect the oral mucosa from the excess dose, but it presents difficulties for patients. In other studies, Reitemeier et al. and Farahani et al. recommended a protective stent with a low-Z shield approximately with 0.30 g/cm2 thickness to reduce the radiation overdose.,, However, evaluation of dose distribution in the presence of other dental restorations based on Monte Carlo approach and comparison of the results with experimental data for higher energies of photon mode can be subjects for future studies.
| > Conclusion|| |
In radiotherapy TPSs, calculation of dose distribution in head and neck cancer is computed by only considering the electron densities of inhomogeneities. Moreover, they do not consider accurate densities and compositions of various prostheses and dental restorations. The ignorance of their compositions leads to discrepancies in the TPS calculations.
Based on the results of the present study, there is a perturbation of dose due to different dental restorations. Therefore, the dose distribution due to the presence of these dental restoration materials can be affected. To upgrade the accuracy of dosimetric calculation, the compositions of dental restorations have to be taken into account in TPSs. There are studies that demonstrate the dose enhancement due to backscattering leads to mucositis. Insertion of an appropriate thickness of a low-Z material (water equivalent) before the tooth can protect the adjacent healthy tissues during radiation therapy. This method was proposed where the tooth placed in the field of treatment, but it is not practical. They also proposed to avoid passing beam through the dental restorations. In this study, our results confirm that the protection of healthy tissues among the irradiated head and neck cancer patients is consequential and reduces the considerable backscatter radiation. By decreasing the backscatter dose due to the dental restorations, the risk of developing oral diseases among these patients can be reduced.
The authors are thankful to Mashhad University of Medical Sciences for financial support of this work. This study was carried out as part of a Ph. D project at Hakim Sabzevari University.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]