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ORIGINAL ARTICLE
Year : 2022  |  Volume : 25  |  Issue : 4  |  Page : 516-523

Assessment of the effects of different dental restorative materials on radiotherapy dose distribution: A phantom study


1 Department of Radiation Oncology, Faculty of Medicine, Süleyman Demirel University, Isparta, Turkey
2 Department of Restorative Dentistry, Faculty of Dentistry, Süleyman Demirel University, Isparta, Turkey

Date of Submission19-Sep-2021
Date of Acceptance30-Dec-2021
Date of Web Publication19-Apr-2022

Correspondence Address:
Dr. Alper Ozseven
Department of Radiation Oncology, Suleyman Demirel University Isparta
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_1826_21

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   Abstract 


Background: One of the most specific effects of high-density dental restorative materials on head & neck cancer radiotherapy is generating variations on isodose distributions. These variations might have an impact on the accuracy and effectiveness of the radiation treatment. The aim of this study is investigating the possible dosimetric effect of six different restorative materials on isodose distributions in head & neck radiotherapy planning process. Materials and Methods: A special phantom was developed and twenty-one caries-free human third molars (a control group + six different restorative materials) were used for the measurements. After acquiring the computed tomography (CT) images, seven treatment plans were created. Hounsfield Unit (HU) numbers, horizontal line dose profile (HLDP) and vertical line dose profiles (VLDPs) were compared with the control group. Results: The amalgam sample deformed the HU numbers in CT images. The median HU value for the S4 material was considerably different than the other samples. The median values were quite close for the remaining samples. For the amalgam sample, the mean of the calculated median isodose values for HLDP and VLDP at 3.5 cm away from the isocenter line were lower than the mean of the control group 4.03% and 6.94%, respectively (for HLDP with tooth numbers of 36 and 38 P = 0.025 and P < 0.001, respectively; for VLDP P < 0.001). In C-S1 comparison results, the statistically significant differences were found for the measurement point at 1 cm away from the isocenter (P = 0.037, P = 0.002, and P = 0.018 for the tooth numbers 36, 37, and 38, respectively). In C-S2 and C-S6 comparisons, there was a statistically significant difference for tooth number 36 (P = 0.035 and P = 0.003, respectively). Conclusions: The findings of the present study showed that amalgam should not be used in head & neck cancer patients who are planned to have radiation therapy. A high viscosity glass ionomer cement (GIC) and a ceramic reinforced GIC sample can be used instead of amalgam to minimize the distorting effect on isodose distributions.

Keywords: Dental restorative materials, hounsfield unit, isodose distributions, radiotherapy treatment planning


How to cite this article:
Ozseven A, Ugurlu M. Assessment of the effects of different dental restorative materials on radiotherapy dose distribution: A phantom study. Niger J Clin Pract 2022;25:516-23

How to cite this URL:
Ozseven A, Ugurlu M. Assessment of the effects of different dental restorative materials on radiotherapy dose distribution: A phantom study. Niger J Clin Pract [serial online] 2022 [cited 2022 May 22];25:516-23. Available from: https://www.njcponline.com/text.asp?2022/25/4/516/343461




   Introduction Top


Within all cancer types, the incidence of head & neck cancers is about 5%, while the mortality rate is around 2%.[1],[2] Multidisciplinary handling is crucial for the treatment of head & neck cancer patients; radiotherapy, chemotherapy, and surgery are applied solely or together to cure cancer. The primary goal of using ionizing radiation in radiation therapy is to kill tumor cells, inevitably, the normal tissues located in the field of radiation are also relatively damaged.[3],[4],[5],[6] The radiotherapy of the head & neck region requires meticulous handling because it includes critical anatomical structures such as the eye, optic nerves, spinal cord, oral cavity, optic chiasm, larynx, and parotids. These anatomical structures in the field of radiotherapy are commonly described as organs at risk (OAR). Identifying OARs and target volumes are crucial for intensity-modulated radiation therapy (IMRT) planning.[7]

It has previously been reported that radiotherapy might cause complications, including xerostomia, osteoradionecrosis, candidiasis, radiation caries, and mucositis.[8],[9],[10] Moreover, multiple side effects might show up due to radiation damage, thus leading to serious impairment in quality of life.[11],[12],[13],[14] Most of the patients who are planned to receive head & neck radiotherapy have non-removable dental implants and restored teeth with different restorative materials. On account of their durableness and mechanical properties usually high atomic numbers (high-Z) materials are utilized in dental applications. When these restorative materials with high-Z are in the path of x-rays, plenty of noise and artefacts might arise during computed tomography (CT). Hereby, inaccurate electron density and Hounsfield Units (HU) might be generated in the resultant image, which leads to incorrect computation of the isodose distribution in the treatment planning process. Additionally, when the photon beam passes through high-Z dental materials, backscatter radiation doses are formed which leads to unwanted local dose enhancement at the boundary of the dental material and result in excess dose in the oral cavity.[15],[16],[17],[18] The interaction between ionizing radiation and the restorative materials might prompt possible two outcomes. One of those is the impact of ionizing radiation from high-energy X-rays on the physical and mechanical properties of restorative materials.[19],[20] The other one is that high-density restorative materials may modify isodose distributions in head neck cancer radiotherapy.[18],[21]

Commonly, radiation oncologists often advise dental examinations to head & neck cancer patients before radiotherapy to cure existing dental caries and facilitate replacement of metal-based dental alloys. The underlying reason for this common approach is that the radiation might cause some alterations in the physical properties of dental materials, such as surface roughness, hardness, diametric tensile strength, and chemical composition.[19],[20] On the other hand, experimental and Monte Carlo evaluations of the effect of high-density prostheses such as amalgam and titanium implants on dose distributions is widely studied in previous literature.[22],[23],[24] Previous studies have stated that considerable dose perturbations were recorded at the surfaces and surrounding tissues of dental restorative materials which can cause under or over treatment of target volume or radiation-related toxicities.[25],[26],[27],[28],[29]

In restorative dentistry, various restorative materials are employed for the restoration of teeth, such as resin composites, giomer, high viscosity glass ionomer cement (GIC), dental amalgam, ceramic reinforced GIC, and zirconia reinforced GIC. However, the dosimetric effects of most of these materials except amalgam, are unclear. Therefore, this study aimed to investigate the dosimetric effects of different restorative materials on isodose distributions in head & neck radiotherapy treatment planning. The null hypothesis of this study was that the dosimetric effects of different restorative materials on isodose distributions in head & neck radiotherapy treatment planning were different.


   Materials and Methods Top


Twenty-one caries-free human third molars were used. The teeth were disinfected in 0.5% chloramine-T, stored in distilled water, and used within 3 months after extraction. The teeth were randomly divided into seven groups. No preparation was performed at the teeth in the control group (C). In experimental groups, the standardized Class I cavities (length, width, and depth: 4 × 4 × 3 mm) were prepared on the teeth using a high-speed hand piece and diamond burs (MANI, Tochigi, Japan) under water cooling. The teeth were restored using the restorative materials according to manufacturer instructions; S1: a nanohybrid resin composite (reliaFIL LC; Advanced Healthcare Ltd, Tonbridge, UK), S2: a giomer (Beautifil II; Shofu, Kyoto, Japan), S3: a high viscosity GIC (Fuji IX GP Capsule; GC, Tokyo, Japan), S4: an amalgam (Cavex Non-Gamma 2; Cavex, Haarlem, The Netherlands), S5: a ceramic reinforced GIC (Amalgomer CR; Advanced Health Care Ltd, Tonbridge, UK), and S6: a zirconia reinforced GIC (Zirconomer; Shofu, Kyoto, Japan). A two-step self-etch adhesive (Clearfil SE Bond; Kuraray, Tokyo, Japan) was applied before the use of the resin composite and giomer based on manufacturer instructions. The adhesive, resin composite and giomer were polymerized using a LED light-curing unit (Smartlite Focus; Dentsply, Milford, DE, USA, 1000 mW/cm2) according to the manufacturer's instructions. All restored teeth were stored in distilled water at 37°C for 24 h before testing.

A special phantom was developed and used for measurements to mimic the head & neck region of the body. In order to enhance the repeatability and consistency of the measurements, 3 molar teeth were used for each sample. The restored teeth were placed in the mandibular molar region of a thermoplastic maxillofacial dental apparatus. Other teeth in the apparatus were plastic. The dental apparatus was located in a beaker that was filled up with distilled water and surrounded by 0.5 cm tissue-equivalent bolus material [Figure 1]. The purpose of this study design was to mimic the head & neck region as much as possible. The HU number of the beaker, distilled water, and bolus material is fairly similar to the bone, soft tissues, and muscle, respectively. All the metallic components of the thermoplastic maxillofacial apparatus were removed to avoid any artefact formation. Before image acquisition, the teeth in each group were placed into the same three molar teeth region of the maxillofacial dental apparatus [Figure 2].
Figure 1: The designed special phantom that used to mimic the head & neck region of the body

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Figure 2: Preparation of molar teeth on the maxillofacial dental apparatus

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Image acquisition and radiotherapy treatment planning

Daily warm-up and fast calibration of the CT scanner were performed before acquiring images. Each time the phantom was placed exactly in the same location of the CT couch and positioned in the same way with the help of alignment lasers that coincide with the center of the phantom [Figure 3]. CT images were obtained with 120 kVp (kilo-voltage peak), 200 mAs (milliampere seconds), and 1.25 mm slice thickness, covering the entire length of the phantom via CT scanner (Bright Speed Excel Select, General Electric Medical Systems).
Figure 3: Alignment of phantom by lasers for scanning process with CT

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After acquiring the images, seven treatment plans (a control group + six different restorative materials) were created using Eclipse Treatment Planning System (ETPS) on Varian DBX linear accelerator with source-to-axis distance of 100 cm. A single field with a size of 3 cm × 5 cm at isocenter that covers the all three molar teeth was used and source-to-surface distance was 95 cm. 6 MV x-rays and AAA (Anisotropic Analytical Algorithm) dose calculation algorithm was used in the planning process and 100 monitor units (MU) were prescribed for the treatment plan [Figure 4]. In all seven scenarios, the same parameters were utilized in the treatment planning process. In addition to that, the obtained seven image set were evaluated in terms of HU numbers by using appropriate ETPS tools to investigate potential changes in HU numbers.
Figure 4: View of the treatment planning process in transversal, coronal and sagittal axis (colorful lines representing the isodose distributions)

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Precisely the same region of interest (ROI) was plotted (green boxes) covering the distilled water for the measurement of HU values. For each sample, the mean of three molar teeth, minimum, and maximum of the HU numbers were recorded [Figure 5]. On the other hand, for the whole seven set-ups, 4 cm-long horizontal line dose profile (HLDP) along the y-direction (yellow line) and 2 cm-long vertical line dose profiles (VLDP) at 1 cm and 3.5 cm away from the isocenter along the x-direction (red lines) were measured in the transversal axis at each molar tooth (for the tooth numbers: 36, 37 and 38) for the specific scenario. The aim of choosing 1 cm and 3.5 cm away from isocenter was to represent the location of the tongue region and the buccal mucosa in the oral cavity, respectively [Figure 6].
Figure 5: Representation of the measurements of HU numbers (ROI in green color, isocenter line in orange color)

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Figure 6: Dose line profiles in transversal axis (isocenter with orange dot, horizontal line dose profile in the x-direction with yellow line and vertical line dose profiles in the y-direction with red lines)

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Evaluation of HU numbers and isodose distributions

The HLDP and VLDPs measured from each of the six different samples were compared one-to-one with those in the control group. A total of 200 and 100 measurement points were analyzed in each comparison for horizontal and vertical isodose values, respectively. In this comparison, the data belonging to the same points obtained from the same numbered teeth for each sample were always compared with the data belonging to the same points obtained from the same numbered teeth in the control group. Median (min-max) isodose values were recorded for each tooth in all samples at the mentioned measurement points. Besides, the mean of the HU numbers around the three molar teeth for all samples were recorded and compared with control group.

Ethical considerations

The study was approved by the Scientific Research Ethics Committee of the Medical Faculty of Suleyman Demirel University (protocol code: 6/125-25.02.2021). All procedures were performed in accordance with the ethical standards of the institutional research committee in alliance with the 1975 Helsinki declaration and its later amendments. The need for informed consent was waived owing to the retrospective nature of the study.

Statistical analysis

The data were analyzed using the SPSS Program, version 22 (Statistical Package for the Social Sciences; SPSS, Chicago, IL, USA). The distribution patterns of collected data were evaluated by descriptive statistics. The frequency histograms were plotted to verify if the data were normally distributed. The significance of the difference was analyzed using the Mann-Whitney U test. A significance level of 0.05 was used for all tests.


   Results Top


HU numbers

The calculated median HU values for different restorative materials are presented in [Table 1]. It can be seen from the results that the median HU value for the S4 material were considerably different than the other samples. The median HU value for the S4 sample were -337, -160, and -337 for the tooth numbers 36, 37 and 38, respectively. On the other hand, the median values were quite close for the remaining samples. The resultant median values for all tooth numbers were in the order of 1 or 2 for the remaining samples in contradistinction to S4.
Table 1: Median HU values for the selected ROI on transversal images

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Isodose distributions

The calculated median (min-max) isodose values related to HLDP and VLDPs at both 1 cm and 3.5 cm away from the isocenter are shown in [Figure 7]. Resultant HLDP data for all tooth numbers of the S4 sample considerably varies from the other samples. Moreover, this deviancy was more noticeable when all samples were compared with the control group. Although no noticeable differences were found between the calculated median values of isodoses for 1 cm away from the isocenter for VLDP, on the contrary the median isodose values at 3.5 cm away from the isocenter were perceptible. On the other hand, the statistical analyses generated more complicated results. Detailed comparison of the samples with the control group in terms of both HLDP and VLDPs are listed in [Table 2].
Figure 7: Calculated median values for HLDP, VLDPs at 1 cm and 3.5 cm away from the isocenter on transversal images

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Table 2: Calculated P – values for HLDP and VLDPs

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The differences were statistically significant in the C-S4 comparison on HLDP for the tooth numbers of 36 and 38 (P = 0.025 and P < 0.001, respectively). Likewise, HLDP, the differences were found to be statistically significant in C-S4 comparisons from the point of VLDP at 3.5 cm away from the isocenter (P < 0.001). Besides, at 1 cm away from the isocenter, the differences were statistically significant in C-S4 comparison (P < 0.001 for tooth number 36, 37 and P = 0.023 for tooth number 38).

Apart from the C-S4 comparison, C-S1 comparison results were notable. The differences were found to be statistically significant for the measurement point at 1 cm away from the isocenter (P = 0.037, P = 0.002, and P = 0.018 for the tooth numbers 36, 37, and 38, respectively). Even though at that measurement location C-S2 and C-S6 comparisons were seemed to be statistically significant for tooth number 36 (P = 0.035 and P = 0.003, respectively), the data consistency couldn't be verified with the remaining teeth locations.


   Discussion Top


In this study, the dosimetric effects of six restorative materials in head & neck cancer radiotherapy were evaluated. The study was performed in two stages. At first, the effects of different dental restorative materials on HU numbers in CT images were explored. After that, the influence of these different restorative materials on isodose distributions was assessed by comparing them with the control group in terms of HLDP and VLDPs. For this study, the null hypothesis was accepted that the dosimetric effects of different restorative materials on isodose distributions in head & neck radiotherapy treatment planning were dissimilar especially with the amalgam sample.

The head & neck region are accepted as one of the most critical regions where inhomogeneous body structures are found throughout the whole body.[22] This inhomogeneity may cause dramatic changes in isodose distributions in radiotherapy treatment planning.[18] Besides, dental restorative materials may have negative influences on these abrupt dose variations.[21] Although radiotherapy is a local treatment method, it affects the life quality of cancer patients due to its side effects.[8],[9] The enhancement on the dose distributions around the teeth arising from the backscatter radiation caused by the different materials in the maxillofacial region may cause side effects, mainly mucositis, on healthy tissues.[12],[13],[14] Long-term, increased side effects and complications at the treatment site may degrade patients' quality of life.[3]

In commissioning process of a medical linear accelerators in radiation oncology centers, the data set of the measurements of water phantom are commonly used. The obtained percentage depth doses and dose profiles data are acquainted into the TPS to use in external beam radiotherapy planning. Generally, electron densities of inhomogeneous structures are taken into account, however the densities and compounds of restorative materials that used as dental fillings are not considered in former calculation algorithms.[25]

In this study, it was observed that the mean HU numbers in the CT images of the amalgam sample were prominently different from the HU numbers that should be in the control group. The evident artefacts arose from the amalgam sample and the median HU value seems to be different for tooth 37 compared to tooth 36 and 38. This could be mainly caused by the interaction mechanism of the photon with the high-density restorative material. In addition to the fact that tooth 37 is located in the middle of tooth 36 and tooth 38, and because of the randomness of the interaction of the x-ray with the amalgam sample for the selected ROI, the median value of tooth 37 differed from those of tooth 36 and tooth 38. One of the most negative impact of image artifacts can be on OAR delineation during the contouring process of the treatment planning. Given the high dose gradients, accurate target and normal tissue contours are at the heart of a high quality treatment plan. Therefore, the use of the modification method can be practical to compensate for the variations in the HU numbers of CT images caused by restorative materials. Previous studies have assessed different methods to rectify the dose perturbations stimulated by restorative materials in the oral cavity.[18],[29] The accuracy of the HU numbers of CT images utilized in radiotherapy treatment planning is quite determinant on isodose distributions. The relationship of the HU number with the electron density can directly affect these isodose distributions. Moreover, Compton scattering is the most dominant photon-tissue interaction in radiation therapy and heavily dependent on electron density.[30]

It can be inferred from the both HLDP and VLDPs, the discrepancies of the amalgam sample were significant. Remarkably, the calculated mean of the median isodose values for HLDP at isocenter line for the amalgam sample was 4.03% lower than the mean of the control group. In addition to that, the computed mean of the median isodose values for VLDP at 3.5 cm away from the isocenter line was 6.94% lower than the mean of the control group. On the other hand, the calculated mean of the median isodose values for VLDP at 1 cm away from the isocenter line was almost identical with the mean of the control group. Statistical results were verified these findings that C-S4 comparison were found to be statically significant for VLDPs.

Apart from the evaluation of C-S4, C-S1 comparison produced noticeable results. Statistical results indicated that significant differences were found between the calculated isodose distributions of a nanohybrid resin composite sample and control group. Nonetheless, this inference was not confirmed by both the VLDP at 3.5 cm away from the isocenter and HLDP. Nevertheless, after amalgam, the most deviated results were gathered with nanohybrid resin composite sample.

The result was distinct for amalgam which may be due to chemical content of the material. On the other hand, the obtained result for both giomer and zirconia reinforced GIC were controversial. The comparison results of C-S2 and C-S6 were quite similar. Particularly, VLDPs of both giomer and zirconia reinforced GIC at 1 cm away from the isocenter line for Tooth No: 36 affirmed the discrepancy. On the other hand, this local dosimetric difference wasn't supported by the results at other measurement points.

Last but not least, the remarkable inference can be made from the attained data of both high viscosity GIC and ceramic reinforced GIC. In present study, the mentioned samples were seemed to be the stand out restorative materials in the studied group. The comparison results of both C-S3 and C-S5 indicated that no deviation was recorded at any measurement point. That give us an opinion about the possibility of being the better dental restorative materials for head & neck cancer patients.

The measurement points at 3.5 cm away from the isocenter line was critical to investigate the possible backscatter radiation arising from the dental restorative materials. Azizi et al.[15] compared experimental findings of the dose distributions at various depths for different restorative materials by utilizing MCNPX simulation codes.[25] Unlike our results, they declared dose increase in backscatter region and attenuation in doses beyond the amalgam sample. Similarly, Spirydovich et al.[24] recorded an about 50% backscatter dose enhancement at the adjacent side of dental amalgam. On the other hand, De Conto et al.[27] reported comparable results with the findings that presented in our study. Moreover, it has been stated that in the sample of tooth with amalgam, no differences were observed on dose distribution at backscatter region. These consequences were associated with the composition and configuration of the tooth sample. Furthermore, contrary to experimental results, backscatter radiation was reported in Monte Carlo simulations inside the tooth. On the other hand, only Monte Carlo simulations showed a decrease beyond the amalgam sample. However, these generalizations weren't verified with the TPS calculations and experimental measurements.[27]


   Conclusion Top


In this study the dosimetric effects of different restorative materials in head & neck cancer radiotherapy treatment planning process were investigated. The significant perturbations of HU numbers were obtained from the amalgam samples on CT images. Moreover, amalgam dramatically deteriorated the CT images, therefore the vertical and horizontal isodose distributions deviated from the expected results. Based on the results of the present study, it can be said that amalgam should not be used as a restorative material in head & neck cancer patients who are going to have radiation therapy. Instead of amalgam, a high viscosity GIC or a ceramic reinforced GIC might be used to minimize the distorting effect on isodose distributions. Further research is needed to verify the findings by carrying out experimental measurements with the same set-up conditions by using an ionization chamber.

Acknowledgements

The authors received no financial support for the research and/or authorship of this article.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2]



 

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