

ORIGINAL ARTICLE 

Ahead of print publication 


Comparison of Monaco treatment planning system algorithms and Monte Carlo simulation for small fields in anthropomorphic RANDO phantom: The esophagus case
Taylan Tugrul
Department of Radiation Oncology, Medicine Faculty of Van Yüzüncü Yıl University, Van, Turkey
Date of Submission  09Aug2020 
Date of Decision  19Dec2020 
Date of Acceptance  29Dec2020 
Date of Web Publication  07May2021 
Correspondence Address: Taylan Tugrul, Department of Radiation Oncology, Medicine Faculty of Van Yüzüncü Yıl University, Van Turkey
Source of Support: None, Conflict of Interest: None DOI: 10.4103/jcrt.JCRT_1143_20
Background: In this study, the dose distributions obtained by the algorithms used in Monaco treatment planning system (TPS) and Monte Carlo (MC) simulation were compared for small fields in the anthropomorphic RANDO phantom, and then, the results were analyzed using the gamma analysis method. Materials and Methods: In the study, dose distributions obtained from the collapse cone algorithm, MC algorithm, and MC simulation were examined. The EGSnrc was utilized for MC simulation. Results: In radiation fields smaller than 3 cm × 3 cm, the doses calculated by the CC algorithm are particularly high in the region of lung/softtissue interfaces. In the region of softtissue/vertebral interfaces, the doses calculated by the CC algorithm and the MC algorithm are compatible with the MC simulation. For each algorithm, the main reason for the nonoverlapping dose curves in small fields compared to MC simulation is that the lateral electronic equilibrium loss is not taken into account by the algorithms. Conclusion: The doses calculated by the algorithms used in TPS may differ, especially in environments where density changes are sharp. Even if the radiation dose from different angles is calculated similarly in the target area by the algorithms, the calculated doses in the tissues in each radiation field path may be different. Therefore, to increase the quality of radiotherapy and to protect critical organs more accurately, the accuracy of the algorithms in TPS should be checked before treatment, especially in multifield treatments such as stereotactic body radiation therapy and intensitymodulated radiotherapy for tumors in the abdominal region.
Keywords: Anthropomorphic RANDO phantom, BEAMnrc, DOSXYZnrc, Monaco treatment planning system, Monte Carlo simulation
How to cite this URL: Tugrul T. Comparison of Monaco treatment planning system algorithms and Monte Carlo simulation for small fields in anthropomorphic RANDO phantom: The esophagus case. J Can Res Ther [Epub ahead of print] [cited 2021 Jun 22]. Available from: https://www.cancerjournal.net/preprintarticle.asp?id=315584 
> Introduction   
The treatment planning system (TPS) constitutes the most important step of radiotherapy treatment because the exact dose reached to the patient must be known correctly. Therefore, the algorithms used by TPS have become an important issue to be examined today. The algorithms must be reliable for dose accuracy, especially in inhomogeneous environments and small fields.
Many TPSs include the Pencil Beam (PB) algorithm, which calculates missing or overdose in inhomogeneous environments.^{[1],[2],[3]} In the PB algorithm, the dose is predicted by a field intensity fluence with kernel which defines the dose deposition around the original photon.^{[4]} The PB algorithm performs correction factor according to the density of the media in the inhomogeneous environment. However, the PB algorithm cannot correctly calculate dose in the presence of inhomogeneous due to the use of onedimensional density correction. The onedimensional density correction cannot reflect a precise dose in large density changes and small fields because it cannot precisely calculate the interactions of secondary electrons.^{[5],[6],[7]}
Some TPSs use convolution–superposition algorithms. The collapsed cone (CC) algorithm is one of the sconvolution–superposition algorithms. The CC algorithm takes into account the effect of photons and electrons formed by the primary photon. Each effect is regarding scatter kernel energy deposition and fluency. The kernels that have lateral scattering are calculated using electron density (ρe). The final dose involving the total energy deposited is acquired by the CC algorithm. The dose predicted by using such an algorithm is very similar to the correct dose that occurred in the medium.^{[8],[9],[10],[11]} Especially for small fields, as an overestimation of the primary dose shows up in the algorithm, where the electron transport simply is modeled, the CC algorithm overestimates dose in entry areas of low density.^{[12],[13],[14],[15]}
The Monaco TPS also includes Monte Carlo (MC) algorithm in addition to the PB algorithm and CC algorithm. Today, MC simulation calculates the dose very closely to reality, taking into account the contribution of secondary photons and electron scattering and dose absorption, especially in inhomogeneous environments. The MC algorithm calculates interaction possibilities for various physical events using random numbers. Since this algorithm utilizes appropriate distribution functions that possess each interaction of photons and electrons in environment, the one constitutes impeccable dose distribution. In this work, the EGSnrcbased BEAMnrc and DOSXYZnrc codes were used for MC simulation and DOSCTP was used to appropriately view and analyze the calculated doses after the simulation. This MC code was used by many authors to verify dose calculations.^{[16],[17],[18],[19],[20],[21]} This code takes into account photon and electron interactions, starting with the primary electron colliding with the target. However, the MC algorithm in Monaco TPS, which uses the XVMC ++ based MC algorithm, starts to calculate photon and electron interactions after the photon enters the environment.
When we use the small field size, the maximum lateral range of secondary electrons is small than the field size. This range is less in the charged particle equilibrium. The collimator part also partially closes the target from the measuring point. As a result, this trouble generates a hard reduction on the output dose and the penumbras of two opposed jaws overlap.^{[22],[23],[24]} In the presence inhomogeneous of low density, the electron's tracks are extended and the charged particle equilibrium decreases.^{[4],[22],[23],[24]} Since the influence of electron transport increases with the reduction of the environment ρe for small fields, it is important to analyze the effect of ρe on dose calculation accuracy using algorithms. The algorithm studies on the water equivalent phantom were examined by many authors, but the ones in an environment similar to the human body are limited, especially about Monaco TPS. This reason prompted us to carry out the present work. In the present article, we compared the dose distribution obtained by Monaco TPS and MC simulation for small fields in anthropomorphic RANDO phantom like an esophagus case and examined the results by the gamma analysis method. The part studied on the anthropomorphic RANDO phantom is the region where the density changes are sharp.
> Materials and Methods   
Doses calculated by Monaco TPS
The anthropomorphic RANDO phantom was scanned by a Siemens SOMATOM Sensation CT (Siemens Healthineers, Germany) using 3mm slice thickness, and the Digital Imaging and Communications in Medicine (DICOM) images were sent to Monaco TPS (v. 5.10.04).^{[25]} The Monaco TPS comprises three different algorithms, these algorithms are PB, CC, and MC, which uses XVMC++ based MC algorithm. Since, the PB algorithm is not preferred in treatments such as stereotactic body radiation therapy (SBRT) and intensitymodulated radiotherapy (IMRT) and there are many studies about PB comparison, the PB algorithm is not used in this study. Six megavoltage photon energy was implemented for CC algorithm and MC algorithm and the different field sizes, which are 1 cm × 1 cm, 2 cm × 2 cm, 3 cm × 3 cm, 4 cm × 4 cm, and 5 cm × 5 cm, were used for each algorithm. The four gantry angles were used for each field, 0°, 90°, 180°, and 270°. The dose in the anthropomorphic RANDO phantom was obtained by giving the same monitor unit (MU) values (50 MU) from each gantry angle. The dose distributions obtained after the calculation were taken from Monaco TPS (as *.ALL file format) for examination on PTW VeriSoft (PTW, Freiburg, Germany). Sample dose distributions on the Monaco TPS are shown in [Figure 1].  Figure 1: Sample dose distributions with Monte Carlo algorithm on the Monaco treatment planning system in the anthropomorphic RANDO phantom. (a) 1 cm × 1 cm, (b) 2 cm × 2 cm, (c) 3 cm × 3 cm, (d) 4 cm × 4 cm, (e) 5 cm × 5 cm
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Monte Carlo simulation
In the verification of the MC simulation, we need to use the measurement results from the water phantom. The percent depth dose (PDD) and lateral dose profile data were obtained in water by a farmer ion chamber (PTW, Freiburg Germany) at 10 cm × 10 cm and sourcetoskin distance (SSD) = 100 cm. The lateral dose profile was taken at maximum dose depth (d_{max}). PDD and lateral dose profile results were used to validate the MC simulation.
BEAMnrc and DOSXYZnrc were utilized for simulation and dose calculations, respectively. The head geometry of Siemens Artiste linear accelerator, which produced 6 MV photon beams, was simulated using BEAMnrc and the workflow described in previous studies was used.^{[18]} The simulation includes components such as the exit window, target, primary collimator, flattening filter, monitor chamber, Y jaws, and X multileaf collimator (MLC). The material data in the components were acquired from “700icru.pegs4” data file in EGSnrc. This data file includes physical density and crosssection data for particles and all materials.^{[18]} The number of histories used for simulation in BEAMnrc was 6 × 10^{8} particles. For all simulations, ISOURC 19 (Elliptical Beam with Gaussian distribution in X and Y) and directional bremsstrahlung splitting (as the variance reduction parameters) (DBS) were used. In ISOURC 19, the monoenergetic beam value is 6.3 MeV and the full width of half maximum (FWHM) value is 0.3 cm.^{[18]} The five different fields which are the 1 cm × 1 cm, 2 cm × 2 cm, 3 cm × 3 cm, 4 cm × 4 cm and 5 cm × 5 cm, 10 cm × 10 cm sizes was created with MLC and jaws. The electron cutoff energy and the photon cutoff energy parameters for the BEAMnrc and DOSXYZnrc were set as 0.7 MeV and 0.01 MeV, respectively.^{[26],[27],[28],[29],[30],[31]} The output file of BEAMnrc is called phasespace data and this file is used for dose calculation in DOSXYZnrc. 6 × 10^{8} histories were run for simulation in DOSXYZnrc. The DOSXYZnrc calculated the dose distributions for all fields in the anthropomorphic RANDO phantom. The dose distribution formed by the photons coming from a single axis can be easily analyzed by the DOSXYZnrc. Due to limited support in DOSXYZnrc to analyze and combine doses on DICOM images, a graphical user interface (GUI) called DOSCTP was used to calculate threedimensional dose distribution after MC simulation.^{[26],[32],[33]} In addition, DOSXYZnrc simulation parameters can also be set in the DOSCTP GUI. For MC simulation verification, the PDD and lateral dose profile values obtained from DOSXYZnrc for 10 cm × 10 cm at 100 cm SSD and d_{max} were compared with results obtained from water phantom. The dose distributions obtained by DOSCTP in the anthropomorphic RANDO phantom are demonstrated in [Figure 2].  Figure 2: The dose distributions obtained through DOSCTP in the anthropomorphic RANDO phantom. (a) 1 cm × 1 cm, (b) 2 cm × 2 cm, (c) 3 cm × 3 cm, (d) 4 cm × 4 cm, (e) 5 cm × 5 cm
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> Results   
The verification of the simulations
For MC simulation and measurement results, we obtained values at 10 cm × 10 cm field size and SSD = 100 cm. We have to examine the PDD and lateral dose profile for verification of MC simulation. Therefore, we should compare the MC simulation results with measurement results. We utilized the PDD results to examine the quality index (TPR_{20,10}).
Since we take measurements at SSD = 100 cm, we have to use the TPR_{20,10} converter formula to calculate the TPR_{20,10} value. The formula of TPR_{20,10} is:
where D_{20} and D_{10} are the doses at a depth of 20 cm and the dose at a depth of 10 cm, respectively.^{[34],[35]} D_{20} and D_{10} for MC simulation are 38.60 and 67.16, respectively. For measurement result, D_{20} and D_{10} are 38.7 and 67.32, respectively. When we calculate the TPR_{20,10} through equality (1), we achieve 0.6681 and 0.6683 for MC simulation and measurement results, respectively. Comparison of lateral dose profiles and PDDs obtained from MC simulation and water phantom for verification is shown in [Figure 3] and [Figure 4], respectively. Consequently, when we examine the results, we can see that the PDD and lateral dose profiles prove the accuracy of the MC simulation.  Figure 3: For lateral dose profiles, comparison of Monte Carlo simulation and water phantom for verification
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 Figure 4: For percent depth dose, comparison of Monte Carlo simulation and water phantom for verification
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Comparison of Monte Carlo simulation with Monaco treatment planning system
After verification of simulation, we compared the dose distributions obtaining from MC simulation and TPS for small fields in the anthropomorphic RANDO phantom.
For the five field sizes, the dose distributions were created in Monaco TPS using CC algorithm. In the same conditions, the doses were recalculated using the MC algorithm. The dose weight for each gantry angle and the field sizes were subsequently created on DOSCTP, and the doses were constituted on the anthropomorphic RANDO phantom with the help of MC simulation. A plan file on the dose data in “*.ALL” format and a dose file having dose distribution in “*.3ddose” format were taken from Monaco TPS and DOSCTP, respectively. The file “* .3ddose” from DOSCTP was converted to “* .dcm” file to open it in PTW VeriSoft. All files were imported into PTW VeriSoft for examination with gamma analysis. We used gamma analysis values of 3 mm for the position and <3% for the calculated dose. The doses obtained in the anthropomorphic RANDO phantom were normalized to the maximum dose in PTW VeriSoft. The gamma analysis results that occur after comparing the results obtained from the Monaco TPS with the MC simulation are shown in [Table 1].
The sample result compared with the gamma analysis method and the lateral axis region examined on the dose distribution are shown on PTW VeriSoft in [Figure 5]. As a result of the gamma analysis, the relative dose curves obtained along the lateral axis in the center are shown in [Figure 6].  Figure 5: The sample result compared with the gamma analysis method and the lateral axis region examined on the dose distribution on PTW VeriSoft
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 Figure 6: The relative dose curves obtained along the lateral axis in the center for CC algorithm, Monte Carlo algorithm and Monte Carlo simulation. (a and b) for 1 cm × 1 cm, (c and d) for 2 cm × 2 cm, (e and f) for 3 cm × 3 cm, (g and h) for 4 cm × 4 cm, (i and j) for 5 cm × 5 cm
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When the photon passes from the soft tissue to the lung environment, we can see from [Figure 6] that CC algorithm and MC algorithm overestimate the dose at the lung, for fields smaller than 3 cm × 3 cm. For 1 cm × 1 cm, both the algorithms could not calculate the dose similar to MC simulation in the lung region. However, these algorithms have reached almost the same dose as MC simulation in the central region. Compared with CC algorithm, the MC algorithm achieved a dose value closer to the MC simulation result in the other regions. For all fields, on the central axis, both the algorithms calculated the values close to the MC simulation results. We can also say that as the photon goes ahead, the dose difference between the results obtained from TPS and MC simulation decreases. For each algorithm, the gamma analysis results obtained from [Figure 6] demonstrate that when the field size increases, the dose calculated with CC algorithm starts to resemble the MC simulation results. Other studies on CC algorithm support our conclusion.^{[7],[12],[26],[31],[36]} However, we cannot compare MC algorithm results because there is no study on the MC algorithm in Monaco TPS. This study is also the first to compare MC simulation and MC algorithm in Monaco TPS in the anthropomorphic RANDO phantom.
For all fields, as the photon crosses from the softtissue regions to the vertebra environment, CC algorithm and MC algorithm calculate a similar dose to MC simulation results. As seen from the gamma result in [Table 1], the dose difference is less for MC algorithm. We can observe in [Figure 6] that in the transition from the softtissue environment to the lung environment or from the lung environment to the softtissue environment, the results of the MC algorithm are very close to MC simulation results. In [Table 1], it is easily understood that the gamma analysis pass rates in comparison of the MC simulation with CC algorithm were lower than those with the MC algorithm in Monaco TPS.
The CC algorithm is not similar to MC simulation results like MC algorithm (especially for in lowdensity inhomogeneous medium and small fields) can be attributed that CC algorithm is not taken into account the lateral electronic equilibrium like MC algorithm. The dose differences in gamma analysis occurred especially in the lung environment for each algorithm. These differences are more pronounced for field sizes less than or equal to 3 cm × 3 cm.
> Conclusion   
The dose distributions in the anthropomorphic RANDO phantom were calculated under the same conditions using MC simulation and algorithms in Monaco TPS then we compared results obtained from the MC simulation with algorithms in Monaco TPS (CC algorithm and MC algorithm) in this phantom like an esophagus case. The results of the CC algorithm and MC algorithm were found to agree pretty well with MC simulation for 3 cm × 3 cm or larger area sizes. However, the CC algorithm particularly overestimated doses in the region of lung/soft tissue interfaces when the fields smaller than 3 cm × 3 cm are used. In the region of softtissue/vertebra interfaces, the CC algorithm and MC algorithm are in harmony with MC simulation. For each algorithm, in small fields, the main reason for nonoverlapping dose curves compared with MC simulation is that the loss of lateral electronic equilibrium is not taken into account by the algorithms. The algorithms used in TPS have important effects on dose distribution, especially in environments where density changes are sharp. Even if the algorithms calculate the radiation dose in the target region created by the photon beams coming from different angles similarly, there may be differences in the doses taken by critical organs such as the lung and heart. Therefore, the accuracy of the algorithms in TPS should be checked for multifield treatments such as SBRT and IMRT, especially for tumors in the abdominal region, before treatment is applied to the patient.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1]
