|Year : 2022 | Volume
| Issue : 3 | Page : 301-308
Determination of the small-field output factor for 6 MV photon beam using EGSnrc Monte Carlo
KW Chuah1, MZ Abdul Aziz2, J Jayamani1
1 Medical Radiation Programme, School of Health Sciences, Health Campus, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan, Malaysia
2 Department of Biomedical Imaging, Advanced Medical and Dental Institute, Universiti Sains Malaysia, 13200 Kepala Batas, Pulau Pinang, Malaysia
|Date of Submission||16-May-2022|
|Date of Decision||04-Jul-2022|
|Date of Acceptance||20-Jul-2022|
|Date of Web Publication||8-Nov-2022|
Dr. J Jayamani
Medical Radiation Programme, School of Health Sciences, Universiti Sains Malaysia, Health Campus, 16150 Kubang Kerian, Kelantan
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.
Keywords: Collimator scatter factor, phantom scatter factor, linear accelerator
|How to cite this article:|
Chuah K W, Abdul Aziz M Z, Jayamani J. Determination of the small-field output factor for 6 MV photon beam using EGSnrc Monte Carlo. J Med Phys 2022;47:301-8
|How to cite this URL:|
Chuah K W, Abdul Aziz M Z, Jayamani J. Determination of the small-field output factor for 6 MV photon beam using EGSnrc Monte Carlo. J Med Phys [serial online] 2022 [cited 2022 Nov 29];47:301-8. Available from: https://www.jmp.org.in/text.asp?2022/47/3/301/360597
| Introduction|| |
Radiotherapy is a treatment used to cure patients with benign and malignant cancer by irradiating accurate radiation dose to the tumor while minimizing unnecessary dose to the surrounding healthy tissues and organs.,, According to the International Atomic Energy Agency publication, there was a 40% curable rate globally by radiotherapy alone or in the combination with surgery or chemotherapy.
The output factor (Scp) is one of the important parameters in the monitor unit (MU) calculation to correct the radiation dose when the field size was changed from the reference field size, 10 cm × 10 cm. The Scp consists of both collimator scatter factor (Sc) and phantom scatter factor (Sp), respectively, where the Sc was measured in air, while, the Sp was measured within a water medium.
The Scp measurement required the usage of an ionization chamber (IC). However, there are issues such as electron contamination and lack of lateral electron equilibrium (LEE) when using the IC for Scp measurement. The IC position at depth lesser than 10 cm during the Scp measurement with photon energy of 6 MV causes electron contamination in the detector volume of the IC.,, Meanwhile, LEE is not established while using the IC with a buildup cap that has an inadequate longitudinal and lateral thickness which is <2 cm during the Sc measurement.
Apart from that, the IC measurement in the small-field size, defined as smaller than 4 cm × 4 cm, is one of the major issues in dose calculation, especially for Scp measurement. Advanced treatment planning such as intensity-modulated radiotherapy, volumetric modulated radiation therapy, and stereotactic treatment requires smaller field sizes, and the inaccuracy of the Scp measurement for the small-field size will give a high impact on the MU calculation.,, There are issues such as volume averaging effect and partial source occlusion in small-field size dose measurement.,,
Due to the issues faced during the Scp measurement using the IC, Monte Carlo (MC) simulation can be an alternative way to calculate these values. MC is a numerical method which simulates random walk of radiation beam transport through random number of sampling. This MC simulation is a reliable tool used to model the LINAC treatment head based on the accurate and detailed information of the LINAC geometry and component materials. This MC simulation was suggested to be used in the radiotherapy dose calculation as it has the equal strength as IC to calculate the dose precisely by taking into account the loss of LEE and aspects of electron and photon beam transport, especially in the small-field size and heterogeneous situation.,
| Materials and Methods|| |
Output factor measurement
In this study, Elekta Synergy (Agility multileaf collimator [MLC]) (Elekta, United Kingdom) was used to measure the Scp using IC (PTW-Freiburg, Semiflex IC 31010, sensitive volume of 0.125 cm3). The Scp was measured for field sizes: 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm in a water phantom (30 cm × 30 cm × 30 cm). The Scp was measured using source-to-surface distance (SSD) of 90 cm on the water phantom and the IC was positioned at 10 cm depth in the water phantom, as shown in [Figure 1]a. The dose at arbitrary field sizes was normalized to the dose at the reference field size (10 cm × 10 cm) to obtain the Scp value, as shown in Equation 1:
|Figure 1: The water phantom and IC positioning for the (a) Scp in water phantom at SAD 100 cm and (b) Sc in air at SAD 100 cm. SAD: Source-to-axis distance, Sc: Collimator scatter factor, Scp: Scatter output factors, IC: Ionization chamber|
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where D(water, FS) is the dose in arbitrary field size and D(water, FSref) is the dose for the reference field size of 10 cm × 10 cm in a water phantom.
Followed by the total scatter (Scp) measurement, the Sc was measured in air by placing the IC together with a 3 mm thickness of acrylic buildup cap at source-to-axis distance of 100 cm, as shown in [Figure 1]b. The dose at arbitrary field sizes was normalized to the dose at the reference field size, 10 cm × 10 cm to obtain the Sc value, as shown in Equation 2:
where D(air, FS) is the dose in arbitrary field size and D(air, FSref) is the dose for the reference field size of 10 cm × 10 cm in air. Meanwhile, the Sp was determined by dividing the measured Scp with the measured Sc, as shown in Equation 3:
The Scp, Sc, and Sp measurements were not performed for the small-field sizes due to limitation of the IC measurement in small field. These measured data were used to validate the simulation outcome.
Linear accelerator modeling using BEAMnrc/EGSnrc
A BEAMnrc/EGSnrc code was used to model the Elekta Synergy (Agility MLC) linac in this study. The LINAC treatment head was modeled based on the technical data and information provided by the manufacturer (Elekta, United Kingdom) with a nondisclosure agreement (NDA) before beginning this study. A total of 9 component modules were used for modeling the linac, and [Figure 2] shows the schematic diagram of the simulated linac model. The 700icru. pegsdat file (ASCII text) was used for this simulation, and this file consists of the mass density, atomic number, and electron density of all the materials used in the LINAC model. The field sizes required for the Scp calculation process including 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm were defined in the BEAMnrc/EGSnrc by adjusting the MLC and jaw position.
|Figure 2: Modeled linac in BEAMnrc/EGSnrc: (a) XZ view, (b) YZ view. linac: Linear accelerator|
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This simulation process was run using the initial particle histories of 1 × 109 particles, and the phase-space file was positioned at 100 cm from the source to equal the linac beam data measurement setup for the IC measurement. The global electron cutoff (ECUT) and global photon cutoff energy (PCUT) were 0.7 MeV and 0.01 MeV, respectively. The incident electron kinetic energy and FWHM applied were 6.4 MeV and 1 mm, respectively, referring to previous publication. The information about the particle histories including the particle's position, direction, and energy was stored in the phase-space file after the simulation process in BEAMnrc/EGSnrc was completed. Each field size produces a Phase-space file to analyze the Sc, Scp values.
The BEAMDP user code was used to analyze the simulated outcome by using this phase-space file. The particle distribution from the phase-space file of BEAMnrc/EGSnrc in this study was analyzed in X-Y scatter plot option in BEAMDP. The scatter plot was used to validate the field size opening in the modeled linac before proceeding with the dose calculation using DOSXYZnrc/EGSnrc. The spectral distribution from the phase-space file of BEAMnrc/EGSnrc in this study was analyzed in spectral distribution from phase-space data option in BEAMDP.
Dose calculation in DOSXYZnrc/EGSnrc
The phase-space file generated from the BEAMnrc/EGSnrc simulation was used as an input to calculate the dose distribution in a 3D voxelized water phantom in DOSXYZnrc/EGSnrc. The DOSXYZnrc/EGSnrc was used to calculate the 3D absorbed dose distribution for different field sizes in a Cartesian coordinate of virtual water phantom. The water phantom of 30 cm × 30 cm × 30 cm with each voxel size of 0.3 cm × 0.3 cm × 0.1 cm, density of 1 g/cm3, and 700icru. pegsdat file of H2O700ICRU was used in this study. The water phantom was positioned under the linac treatment head at the central axis with the SSD of 100 cm. The x-axis of the water phantom was used to define cross-plane profile, and the y-axis to define in-plane profile, respectively.
The particle histories of 6 × 107 particles were applied during this simulation process. The input parameter isource 2 (phase-space source incident from any direction) was applied in DOSXYZnrc/EGSnrc code. The ECUT and PCUT applied were 0.7 MeV and 0.01 MeV, respectively. Photon splitting of 100 and particle recycling of 3 were applied in this simulation and MC simulation for the PDD, beam profile and TPR20,10.
LINAC Model Validation
The dose distribution from XZ, YZ, and XY views was retrieved from the STATDOSE application (in-built code in EGSnrc) to plot profile along the simulated particles for each field sizes in water medium. The simulated LINAC model was validated by compare the measured IC readings for PDD, beam profile and TPR 20,10 for the field size 10 cm x 10 cm (TRS 398) and followed the same procedure to validate the field size of 3 cm x 3 cm , 5 cm x 5 cm, 15 cm x 15 cm and 20 cm x 20 cm , respectively. The deviation between the measured data using IC with the calculated data using MC simulation was calculated by using the following equation:
where DMC is the MC simulated dose and DIC is the dose measurement by the IC. The LINAC model was validated by calculate the percentage difference between the IC measured and MC simulation for the PDD, beam profile and TPR20,10. The AAPM TG -105 protocol proposed the tolerance range of < 2% difference between the IC and MC readings.
Calculation of output factor using DOSXYZnrc/EGSnrc
The validated linac model was used to calculate the Scp in the DOSXYZnrc/EGSnrc. The same parameters were used in BEAMnrc/EGSnrc except for the phase-space file position was changed to 90 cm from the source to match the Scp setup for the IC measurement. Furthermore, the same parameters were used in DOSXYZnrc/EGSnrc except for the distance from the phase-space file to isocenter was changed to 10 cm depth. The Scp for 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm field sizes was simulated in the DOSXYZnrc/EGSnrc and was calculated by using Equation 1.
Calculation of collimator scatter factor using DOSXYZnrc/EGSnrc
The validated linac model was used to perform the Sc calculation in DOSXYZnrc/EGSnrc. The same parameters were used in BEAMnrc/EGSnrc, and the phase-space file position remained at 100 cm from the source to equal the Sc setup for IC measurement. The same parameters were used in DOSXYZnrc/EGSnrc except for the water phantom that was replaced by air (density of 0.001 g/cm3 and 700icru. pegsdat file of AIR700ICRU). The Sc for 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm field sizes was simulated in the DOSXYZnrc/EGSnrc and was being calculated using Equation 2.
Calculation of phantom scatter factor using DOSXYZnrc/EGSnrc
The Sp was calculated after simulation for Scp and Sc by using Equation 3. The Sp for 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm field sizes were determined in the MC simulation.
Output factor validation in Monte Carlo simulation
The percentage difference between the measured data using IC and the calculated data using EGSnrc MC simulation for Scp, Sc, and Sp was calculated by using Equation 4. The percentage difference of Scp, Sc, and Sp for the field sizes 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm was calculated. A good agreement between the measured and calculated data within the tolerance range of ±2% needs to be achieved.
| Results and Discussions|| |
LINAC model validation
The field size of the modeled LINAC was validated by the spectral distribution and X-Y scatter plot in the BEAMDP code using the phase-space file for field size of 10 cm × 10 cm. [Figure 3]a shows the spectral distribution from the modeled linac where the number of fluence simulated in this work adequate to run the simulation for work for 6 MV photon beam. Meanwhile [Figure 3]b shows the scatter distribution of 6 MV photon beam for the field size of 10 cm x 10 cm at 100 cm from the target. The figure shows the maximum intensity of the scatter particles at range between −5 cm and +5 cm in both X-axis and Y-axis, respectively. This showed the BEAMnrc's phase-space file agreed well with the modeled linac treatment head defined at the field size 10 cm × 10 cm.
|Figure 3: (a) Spectral distribution of 6 MV photon beam at the water phantom surface. The maximum energy absorbed by the water phantom was 6.4 MeV. (b): X-Y scatter plot of 6 MV photon beam for the field size 10 cm × 10 cm at 100 cm distance from the linac's target to the surface of the water phantom. linac: Linear accelerator|
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The PDD, beam profile, and TPR20,10 of the simulation calculate data were further validated by comparing the results from the IC measurement. The PDD from both the IC measurement and MC simulation were superimposed and it was found that the percentage difference was within the tolerance range of ± 2% at depth 1.5 cm, 10 cm, and 20 cm for the simulated field sizes (3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm), as shown in [Figure 4].
|Figure 4: The PDD versus depth for field sizes (a) 3 cm × 3 cm, (b) 5 cm × 5 cm, (c) 10 cm × 10 cm, (d) 15 cm × 15 cm, and (e) 20 cm × 20 cm between the IC measurement and MC simulation for Elekta Synergy (Agility MLC) linac. IC: Ionization chamber, MC: Monte Carlo, PDD: Percentage depth dose, linac: Linear accelerator, MLC: Multileaf collimator|
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For the beam profile, the relative radiation dose was decreased as the depth of measurement was increased [Figure 5]. Besides, the beam flatness was improved as the depth of measurement was increased. Moreover, the percentage difference between the IC measurement and MC simulation of beam flatness in cross-plane and in-plane directions achieved a good agreement where the percentage difference was within the tolerance range of ± 2% for the simulated field sizes (3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm) at the depth 10 cm, as shown in [Figure 5]. However, several beam flatness at 10 cm depth with the percentage difference of more than the tolerance range of ±2% between the calculated and measured data. This may be due to the electron contamination that occurred at the depth smaller than 10 cm. The results did not affect the validation of the linac model as the comparison of the measured and calculated beam profiles was focused at 10 cm water depth according to the IEC protocol., Furthermore, in this study, the Scp simulation was executed at 10 cm depth; thus, the calculated Scp was not affected by the beam quality.
|Figure 5: The relative dose in beam profile between IC measurement and MC simulation: (a) cross-plane and (b) in-plane for field size 3 cm × 3 cm; (c) cross-plane and (d) in-plane for field size 5 cm × 5 cm; (e) cross-plane and (f) in-plane for field size 10 cm × 10 cm; (g) cross-plane and (h) in-plane for field size 15 cm × 15 cm; (i) cross-plane and (j) in-plane for field size 20 cm × 20 cm. IC: Ionization chamber, MC: Monte Carlo|
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Apart from that, the TPR20,10 difference between the IC measurement and MC simulation was 1.32% and it was in a good agreement within the tolerance range of ±2%, as shown in [Table 1]. The PDD, beam profile, and TPR20,10 comparison between the IC measurement and MC simulation show that the linac was modeled accurately in the MC simulation for all field sizes.
|Table 1: The beam quality, tissue–phantom ratio, TPR20,10 comparison between the ionization chamber measurement and Monte Carlo simulation for 6 MV photon beam in Elekta Synergy linear accelerator|
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Output factor calculation in Monte Carlo simulation
[Figure 6]a shows the Scp of the IC measurement and MC simulation for the field sizes 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm, respectively. The figure shows that the Scp was increased as the field size was increased. The calculated Scp in the MC simulation was superimposed on the IC measurement with the error bar difference of ± 2% which was in a good agreement within the tolerance range of ± 2%. The highest difference was 0.95% for field size 15 cm × 15 cm. In a previous study, Yani et al. found that the calculated Scp in the EGSnrc MC simulation for field size 3 cm × 3 cm, 5 cm × 5 cm, and 10 cm × 10 cm was in the range between 0.86 and 1.00 and the percentage difference between the measured and calculated Scp were within 2%. The Scp measurement and calculation were done at 10 cm water depth in this study to avoid the effect of electron contamination from the Elekta linac treatment head, and the results had proved a good agreement between the measured and calculated Scp., Thus, the measured Scp using the IC avoided the issue of electron contamination when the IC was placed at 10 cm depth. Apart from that, the MC simulation acted as an alternative method to calculate the Scp as it can overcome the issue of electron contamination during the simulation process.
|Figure 6: (a) The Scp, (b) Sc, and (c) Sp of IC measurement and MC simulation for field size 3 cm × 3 cm, 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm, respectively. The error bar indicated ± 2% difference. Sc: Collimator scatter factor, Sp: Phantom scatter factor, Scp: Scatter output factor, IC: Ionization chamber, MC: Monte Carlo|
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[Figure 6]b shows the Sc of the IC measurement and MC simulation for field sizes 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm, meanwhile the Sc value for the field size 3 cm × 3 cm (shaded area) from the MC simulation alone as there was no IC measurement due to IC limitation. The figure shows that the Sc increased as the field size was increased. The calculated Sc in the MC simulation was compared against the IC measurement with the error bar difference of ±2% which was in a good agreement within the tolerance range of ±2%. The highest difference was 0.68% for field size 20 cm × 20 cm.
Based on this simulation work, the calculated Sc for the field size 3 cm × 3 cm was 0.955. There was a lack of data in small-field size using IC as the application of 3 mm thick buildup cap in the IC during the Sc measurement might not guarantee a LEE condition. A study by Fogliata et al. found that the percentage difference of measured Sc by using IC with a 2 mm thickness buildup cap calculated Sc by using the PENELOPE MC code for field size were within 1% difference and proved that the MC code able to provide accurate values within the tolerance range of 2% difference. They also found that the application of an adequate thickness of buildup cap with a minimum 2 cm thickness in the IC was important to avoid the lack of LEE that occurred in the IC during the Sc measurement., With the help of MC simulation, such issue can be avoided as the MC simulation can overcome the lack of LEE when calculating the Sc. Thus, the MC simulation acts as an alternative to calculate Sc when the thickness of buildup cap of the IC is not adequate to measure the Sc.
[Figure 6]c shows the Sp of the IC measurement and MC simulation for field sizes 5 cm × 5 cm, 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm, and for field size 3 cm x 3 cm, the SP value from the MC simulation only. The figure shows that the Sp increased as the field size was increased. The calculated Sp in the MC simulation was compared against the IC measurement with the error bar difference of ± 2% which was in a good agreement within the tolerance range of ± 2%. The highest difference was 1.39% for field size 5 cm × 5 cm. Based on this simulation work, the calculated Sp for the field size 3 cm × 3 cm was 0.884. In a previous study, Davoudi et al. had showed a good agreement between measured Sp using the IC and calculated Sp using the EGSnrc MC simulation for field sizes 10 cm × 10 cm, 15 cm × 15 cm, and 20 cm × 20 cm with the maximum percentage difference of 0.83% where the percentage difference was within the tolerance range of ± 2%. MC simulation was suggested to calculate the Sp directly to avoid the inaccuracy in measurement by using the IC.
In small-field dosimetry, there was lacking of values from the IC measurement from our center. Since Sc and Sp for field size 3 cm × 3 cm were not measurable, they were calculated from the MC simulation in this study and the values were 0.955 and 0.884, respectively. The results shown were valid because previous studies mentioned that the MC simulation was often being used as a benchmark in dose calculation algorithm, especially in small-field dosimetry due to the ability of MC simulation to eliminate the uncertainties of the IC measurement such as lack of LEE, volume averaging effect, and partial source occlusion during simulation process.,, A study by Yani et al. proved that the calculated Scp by the EGSnrc MC simulation was in good agreement with the IC-measured Scp in small-field size 3 cm × 3 cm with the percentage difference between the measured and calculated Scp of 1.8% which was within the tolerance range of 2%. Apart from that, Aitelcadi et al. proved that the calculated Sc by the MC simulation was in a good agreement with the measured Sc by the IC in small-field size 3 cm × 3 cm with the percentage difference of 0.6%. A study by Fogliata et al. and Davoudi et al.validated the calculated Sc for the field size of 4 cm × 4 cm within ± 1% difference., These studies proved that the MC simulation can act as an alternative way to calculate the Scp in small-field dosimetry.
| Conclusion|| |
The aim of this study was to determine the small-field Scp of the Elekta Synergy (Agility MLC) linac by using the EGSnrc MC. The calculated Scp in MC simulation was able to be determined accurately despite external factors that affected the IC measurement such as IC position, buildup cap thickness, and small-field dosimetry, still showing a good agreement between the measured and calculated Scp with the variation within the tolerance range of ±2%. The calculated Sc and Sp for field size 3 cm × 3 cm in this study were 0.955 and 0.884, respectively. Thus, the study showed that the determination of Scp for small- and large-field sizes can be performed efficiently by using the EGSnrc MC simulation.
We would like to thank Elekta Limited from the UK and Madam Rosemary for assisting in providing detailed information on the Elekta Synergy Agility used in the Monte Carlo simulation work. This work was also supported by the Universiti Sains Malaysia [304/PPSK/6315497].
Financial support and sponsorship
This work was supported by the Universiti Sains Malaysia [304/PPSK/6315497].
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]