Study of dental prostheses influence in radiation therapy

Effects of modern aesthetic dental fillings on proton therapy

Background and purpose

High-density dental fillings pose a non-negligible impact on head and neck cancer treatment. For proton therapy, stopping power ratio (SPR) prediction will be significantly impaired by the associated image artifacts. Dose perturbation is also inevitable, compromising the treatment plan quality. While plenty of work has been done on metal or amalgam fillings, none has touched on composite resin (CR) and glass ionomer cement (GIC) which have seen an increasing usage. Hence, this work aims to provide a detailed characterisation of SPR and dose perturbation in proton therapy caused by CR and GIC.

Materials and methods

Four types of fillings were used: CR, Fuji Bulk (FB), Fuji II (FII) and Fuji IX (FIX). The latter three belong to GIC category. Measured SPR were compared with SPR predicted using single-energy computed tomography (SECT) and dual-energy computed tomography (DECT). Dose perturbation of proton beams with lower- and higher-energy levels was also quantified using Gafchromic films.

Results

The measured SPR for CR, FB, FII and FIX were 1.68, 1.77, 1.77 and 1.76, respectively. Overall, DECT could predict SPR better than SECT. The lowest percentage error achieved by DECT was 19.7 %, demonstrating the challenge in estimating SPR, even for fillings with relatively lower densities. For both proton beam energies and all four fillings of about 4.5 mm thickness, the maximum dose perturbation was 3 %.

Conclusion

This study showed that dose perturbation by CR and GIC was comparatively small. We have measured and recommended the SPR values for overriding the fillings in TPS.

Monte Carlo modeling of the Elekta Versa HD and patient dose calculation with EGSnrc/BEAMnrc

Introduction

Numerous studies have proven the Monte Carlo method to be an accurate means of dose calculation. Although there are several commercial Monte Carlo treatment planning systems (TPSs), some clinics may not have access to these resources. We present a method for routine, independent patient dose calculations from treatment plans generated in a commercial TPS with our own Monte Carlo model using free, open‐source software.

Materials and methods

A model of the Elekta Versa HD linear accelerator was developed using the EGSnrc codes. A MATLAB script was created to take clinical patient plans and convert the DICOM RTP files into a format usable by EGSnrc. Ten patients’ treatment plans were exported from the Monaco TPS to be recalculated using EGSnrc. Treatment simulations were done in BEAMnrc, and doses were calculated using Source 21 in DOSXYZnrc. Results were compared to patient plans calculated in the Monaco TPS and evaluated in Verisoft with a gamma criterion of 3%/2 mm.

Results

Our Monte Carlo model was validated within 1%/1‐mm accuracy of measured percent depth doses and profiles. Gamma passing rates ranged from 82.1% to 99.8%, with 7 out of 10 plans having a gamma pass rate over 95%. Lung and prostate patients showed the best agreement with doses calculated in Monaco. All statistical uncertainties in DOSXYZnrc were less than 3.0%.

Conclusion

A Monte Carlo model for routine patient dose calculation was successfully developed and tested. This model allows users to directly recalculate DICOM RP files containing patients’ plans that have been exported from a commercial TPS.

Metal artifact reduction using common dental materials

OBJECTIVES

To determine the effect of different dental lab materials on cone beam computed tomography (CBCT) metal artifact at different resolutions.

METHODS

A total of seven common dental lab materials were molded to a dental sextant of four extracted, restored teeth. In addition to base alone (control), each material was scanned using the Carestream 9600 CBCT unit at three resolutions - 0.3 mm, 0.15 mm, and 0.075 mm - at manufacturer established exposure parameters. A single, representative axial view of each trial was evaluated for metal artifact both quantitatively by histogram analysis and qualitatively by profile plot analysis in ImageJ.

RESULTS

No statistically significant differences between the control and the dental materials were found; however, post-hoc tests showed significance between Blu-mousse^® and polyvinyl siloxane with dental materials and control, predominantly in lower resolutions.

CONCLUSIONS

The current study provides initial evidence on the influence of dental materials have on CBCT metal artifact as described by beam hardening, photon starvation, scatter, and noise, especially at lower resolutions. Blu-Mousse^® and polyvinyl siloxane reduced the perceived beam hardening and photon starvation artifact the greatest, relative to other materials, at all three resolutions and lower resolutions, respectively.

Investigating dose homogeneity in radiotherapy of oral cancers in the presence of a dental implant system: an in vitro phantom study

Background

Materials with high atomic numbers are part of the composition of dental implant systems. In radiotherapy of oral cavity cancers, an implant can cause dose perturbations that affect target definition, dose calculation, and dose distribution. In consequence, this may result in poor tumor control and higher complications. In this study, we evaluated dose homogeneity when a dental implant replaced a normal tooth. We also aimed to evaluate the concordance of dose calculations with dose measurements.

Materials and methods

In this study, 2 sets of planning CT scans of a phantom with a normal tooth and the same phantom with the tooth replaced by a Z1 TBR dental implant system were used. The implant system was composed of a porcelain-fused-to-metal crown and titanium with a zirconium collar. Three radiotherapy plans were designed when the density of the implant material was corrected to match their elements, or when all were set to the density of water, or when using the default density conversion. Gafchromic EBT-3 films at the level of isocenter and crowns were used for measurements.

Results

At the level of crowns, upstream and downstream dose calculations were reduced when metal kernels were applied (M-plan). Moreover, relatively measured dose distribution patterns were most similar to M-plan. At this level, relative to the non-implanted phantom, mean doses values were higher with the implant (215.93 vs. 192.25), also, new high-dose areas appeared around a low-dose streak forward to the implant (119% vs. 95%).

Conclusions

Implants can cause a high dose to the oral cavity in radiotherapy because of extra scattered radiation. Knowledge of the implant dimensions and defining their material enhances the accuracy of calculations.

Hyper-dense foreign bodies found in the bowel during IGRT following brachytherapy

A 28-year-old female with locally advanced adenocarcinoma of the cervix was undergoing treatment with external beam radiation therapy (EBRT), concurrent chemotherapy and high dose rate brachytherapy (BT). On-board imaging obtained prior to one of her external beam treatments revealed four radiopaque foreign bodies in her abdomen. The patient's treatment was delayed for further work-up of this new finding. Upon further investigation, it was discovered that the patient had recently started taking bismuth subsalicylate tablets (brand name: Pepto-Bismol, Procter & Gamble Co., Cincinnati, OH). A computed tomography (CT) scan of the tablets confirmed the size and Hounsfield Unit (HU) values coincided with the foreign object properties seen on the patient's scan. This unexpected finding is important to recognize as it consequently lead to a delay in treatment, additional imaging, and patient anxiety.

Dose perturbation due to dental amalgam in the head and neck radiotherapy: A phantom study

Dental amalgam, causes perturbation in photon dose distribution of head and neck (H&N) radiotherapy. The aim of this study was to evaluate the effects of dental amalgam on dose distribution of H&N radiotherapy and accuracy of dose calculations algorithm of commercial treatment planning system (TPS). In this study, the measurements were performed using a constructed H&N anthropomorphic. The sample of healthy teeth and teeth filled by amalgam inserted in the desired segment of the phantom in turn. After scanning and organs segmentation of phantom, intensity-modulated radiation therapy (IMRT) plan including 7 fields in the absence (plan 1) and presence (plan 2) of dental amalgam were created separately. Phantom was irradiated using 6 MV linear accelerator (SIMENS-ARTISTE, 5918). Assessment of the effects of dental amalgam on dose distribution and the accuracy of dose calculation algorithm of TPS was done by measurement and comparing of organ's received dose using thermoluminescent dosimeter (TLDs), placed on a phantom and TPS calculations. The scattering and attenuation due to the presence of dental amalgam led to an increase in parotid glands received dose (up to 24.38%) and a decrease in mean dose (up to -6.25%) PTV70. Results of this study revealed that discrepancies between the collapsed cone convolution (CCC) algorithm calculations Prowess Panther TPS and TLD measurements were -19.77% to 27.49% in presence of amalgam and -1.09% to 5.03% in presence of healthy teeth in phantoms. Attenuation and scattering due to amalgam in IMRT of H&N cancer may lead to a significant dose perturbation which is not predictable by dose calculation of TPS.

Investigating the effect of dental implant materials with different densities on radiotherapy dose distribution using Monte-Carlo simulation and pencil beam convolution algorithm

OBJECTIVES

The aim of this study was to investigate the effect of dental implant materials with different physical densities on dose distribution for head and neck cancer radiotherapy planning.

METHODS

Titanium (Ti), Titanium alloy (Ti-6Al-4V), Zirconia (Y-TZP), Zirconium oxide (ZrO₂), Alumina (Al₂O₃) and polyetheretherketone (PEEK) dental implant materials were used for determination of implant material effect on dose distribution. Dental implant effect was investigated by using pencil beam convolution (PBC) algorithm of Eclipse treatment planning systems (TPS) and Monte Carlo (MC) simulation technique. 6 MV photon beam of the Varian 2300 C/D linear accelerator was simulated by EGSnrc-based BEAMnrc MC code system.

RESULTS

Reasonable consistency was determined for percentage depth dose (PDD) curves between MC simulation and water phantom measurements at 6.4 MeV initial electron energy. The consistency between modelled linear accelerator PDD curve calculations and water-phantom PDD measurements were compatible within 1 % range. The dose increase in front of the dental implant calculated by MC simulation is in the range of 0.4-20.2%. We found by MC and PBC calculations that the differences in dose increase in front of the dental implant materials is in the range of 0.1-17.2% and is dependent on the physical density of the dental implant.

CONCLUSIONS

Dose increase for Zirconia was noted to be maximum while PEEK implant dose increase was minimum among the whole dental implant materials studied. This study revealed that the Eclipse TPS PBC algorithm could not accurately estimate the backscatter radiation from dental implant materials.

Treatment planning dose accuracy improvement in the presence of dental implants

Streaking artifacts in computed tomography (CT) scans caused by metallic dental implants (MDIs) can lead to inaccuracies in dose calculations. This study quantifies and compares the effect of MDIs on dose distributions using the collapsed cone convolution superposition (CCCS) and Monte Carlo (MC) algorithms, with and without correcting for the density of the MDIs. Ion chamber measurements were taken to test the ability of the algorithms in Pinnacle³ and Monaco to calculate dose near high-Z materials. Nine previously treated patients with head and neck cancer were included in this study. The MDI and the streaking artifacts on the CT images were carefully contoured. For each patient, a plan was optimized and calculated using the Pinnacle³ treatment planning system (TPS). Two dose calculations were performed for each patient: one with overridden densities of the MDI and CT artifacts and one without overridden densities of the MDI and CT artifacts. The plans were then exported to the Monaco TPS and recalculated for the same number of monitor units (MUs) using its MC dose calculation algorithm. The changes in dose to the planning target volume (PTV) and surrounding healthy tissues were examined between all the plans using VelocityAI. For the ion chamber measurements, when correct density information was used, Monaco was within 3% of the measured values, whereas the doses calculated in Pinnacle³ varied up to 7%. The CCCS algorithm in Pinnacle³ calculated only a significant decrease in PTV coverage for 1 patient when the densities were overridden. The MC algorithm in Monaco was able to calculate a significant change in PTV coverage for five of the patients when the density was overridden. Additionally, when healthy tissues affected by streaking artifacts were assigned the correct density, cumulative (from all the fractions) point doses increased up to 46.2 Gy. Not properly accounting for MDIs can impact both the high-dose regions (PTVs) and surrounding healthy tissues. This study demonstrates that if MDIs and the artifacts are not appropriately accounted for by contouring and assigning to them the correct density, there is a potential risk of compromising the quality of the plan regarding PTV coverage and dose to healthy tissues.

Dosimetric impact of dental metallic crown on intensity-modulated radiotherapy and volumetric-modulated arc therapy for head and neck cancer

Metal dental restoration materials cause dose enhancement upstream and dose disturbance downstream of the high‐density inhomogeneous regions in which these materials are used. In this study, we evaluated the impact of a dental metallic crown (DMC) on intensity‐modulated radiotherapy (IMRT) and volumetric‐modulated arc therapy (VMAT) for head and neck cancer. Additionally, the possibility of sparing the oral mucosa from dose enhancement using an individual intraoral mouthpiece was evaluated. An experimental oral phantom was designed to verify the dosimetric impact of a DMC. We evaluated the effect on single beam, parallel opposing beam, arc beam, IMRT, and VMAT treatment plans. To evaluate the utility of a 3‐mm‐thick intraoral mouthpiece, the doses across the mouthpiece were measured. For single beam irradiation, the measured doses at the entrance and exit planes of the DMC were 51% higher and 21% lower than the calculated dose by the treatment planning system, respectively. The maximum dose enhancements were 22% and 46% for parallel opposing beams and the 90° arc rotation beam, respectively. For IMRT and VMAT, the measured doses adjacent to the DMC were 12.2%±6.3% (mean±1.96 SD) and 12.7%±2.5% higher than the calculated doses, respectively. With regard to the performance of the intraoral mouthpiece for the IMRT and VMAT cases, the disagreement between measured and calculated doses at the outermost surface of the mouthpieces were −2.0%, and 2.0%, respectively. Dose enhancements caused by DMC‐mediated radiation scattering occurred during IMRT and VMAT. Because it is difficult to accurately estimate the dose perturbations, careful consideration is necessary when planning head and neck cancer treatments in patients with DMCs. To spare the oral mucosa from dose enhancement, the use of an individual intraoral mouthpiece should be considered.

PACS numbers: 87.55.km, 87.55.N‐, 87.55.Qr

High-density dental implants and radiotherapy planning: evaluation of effects on dose distribution using pencil beam convolution algorithm and Monte Carlo method

High atomic number and density of dental implants leads to major problems at providing an accurate dose distribution in radiotherapy and contouring tumors and organs caused by the artifact in head and neck tumors. The limits and deficiencies of the algorithms using in the treatment planning systems can lead to large errors in dose calculation, and this may adversely affect the patient's treatment. In the present study, four commercial dental implants were used: pure titanium, titanium alloy (Ti-6Al-4V), amalgam, and crown. The effects of dental implants on dose distribution are determined with two methods: pencil beam convolution (PBC) algorithm and Monte Carlo code for 6 MV photon beam. The central axis depth doses were calculated on the phantom for a source-skin distance (SSD) of 100 cm and a 10 × 10 cm2 field using both of algorithms. The results of Monte Carlo method and Eclipse TPS were compared to each other and to those previously reported. In the present study, dose increases in tissue at a distance of 2 mm in front of the dental implants were seen due to the backscatter of electrons for dental implants at 6 MV using the Monte Carlo method. The Eclipse treatment planning system (TPS) couldn't precisely account for the backscatter radiation caused by the dental prostheses. TPS underestimated the back scatter dose and overestimated the dose after the dental implants. The large errors found for TPS in this study are due to the limits and deficiencies of the algorithms. The accuracy of the PBC algorithm of Eclipse TPS was evaluated in comparison to Monte Carlo calculations in consideration of the recommendations of the American Association of Physicists in Medicine Radiation Therapy Committee Task Group 65. From the comparisons of the TPS and Monte Carlo calculations, it is verified that the Monte Carlo simulation is a good approach to derive the dose distribution in heterogeneous media.