Dosimetric Testing of Two Incident Electron Parameters for ... … · Keywords: Monte Carlo;...

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AJMP 2019, Volume 2, Number 2 48

FAMPO Federation of African

Medical Physics

organizations

Research Paper

AAffrriiccaann JJoouurrnnaall ooff MMeeddiiccaall PPhhyyssiiccss

2019; 2(2): 48-56. doi: 10.

Dosimetric Testing of Two Incident Electron

Parameters for Photon Beam Monte Carlo Model of an

Elekta Precise Linac

Oluwaseyi M. Oderinde 1,, Michael O. Akpochafor2, Rachel I. Obed3 and Ramotallah D. Jubril2

1. Department of Physics, University of Witwatersrand, Johannesburg, Gauteng, South Africa.

2. Department of Radiation Biology, Radiotherapy, and Radiodiagnosis, College of Medicine, University of Lagos, Idi-Araba, Lagos, Nigeria.

3. Department of Physics, Faculty of Science, University of Ibadan, Ibadan, Oyo State, Nigeria.

Corresponding author: Dr. Oluwaseyi M. Oderinde, Department of Physics, University of Witwatersrand, Johannesburg, Gauteng, South Africa. Tel: +27617452634; E-mail: [email protected]

© AJMP is the official journal of the Federation of African Medical Physics Organizations (FAMPO). This is registered under Nigerian company number (CAC/IT/No 54182). See http://fampo-africa.org/ ISSN 2643-5977

Received: 2019.12.03; Accepted: 2020.03.16; Published: 2020.05.02

Abstract

BEAMnrc/DOSXYZnrc Monte Carlo code is widely used for accurate dose calculation. This study

simulated and tested two incident electron-source parameters on dosimetric characteristics of photon

beam for an Elekta Precise linear accelerator (linac) model. The linac model of a 6 MV photon beam for

10 × 10 cm2 field was used to investigate the sensitivity of the two-incident electron sources. Optimal

source parameter was achieved by varying the parallel and mean angular spread (2D Gaussian

distribution) circular beam sources. In a parallel incident electron source, the beam radius (r) parameter

was varied while the sigma (σ) parameter in the mean angular spread beam source was varied. The

accuracy of this source model was evaluated by calculating the dose distribution in a homogeneous

water phantom. The simulated data were benchmarked with measurements for percentage depth doses

(PDDs) and lateral dose profiles using 2%/2mm and 3%/3mm gamma (γ) criteria. This study showed that

variations of the two incident beam sources parameter have an influence on dose distribution

characteristic apart from the depth-dose curves which are unaffected. The most accurate source

models(r = 0.01 mm and σ = 0.05 mm) discrepancies fell within 2%/2mm and 3%/3mm γ criteria for the

symmetry field considered. Parallel source beam radii of 0.01 – 4 mm generated the pass rates of 97.46

– 96.61% and 98.31 – 96.61% for 2%/2mm and 3%/3mm, respectively, while σ variations (0.05-2 mm) in

mean angular spread beam source resulted into pass rates of 97.46 – 96.61% and 98.31- 97.46%,

respectively. The two incident electron sources are suitable for dose calculation if tuned precisely.

However, lateral dose profiles are insensitive to minimal variation of incident electron source

parameters. When it comes to the choice of source parameter, preference can be given to the mean

angular spread beam source based on clinical consideration.

Keywords: Monte Carlo; virtual source model; incident electron source; Dosimetry

Introduction

Monte Carlo (MC) simulation of particle transport is an accurate computational method for evaluating dose distribution in radiotherapy. It is suitable for accurate dose calculation where physical measurement is impossible. Over the decades, users of

MC codes in medical physics have exponentially increased, whereby different research groups and institutions have contributed immensely to the development of several MC algorithms for radiotherapy application. Popular MC codes applied in Medical Physics research are; EGSnrc, GEANT4, PENELOPE, FLUKA, and MCNP Monte Carlo codes [1-5].



Accurate dose calculation depends on exact information of the linear accelerator (linac) head and geometry of the incident electron source (electron energy, radial intensity and angular spread of the electron beam). While linac head information is provided by the manufacturing company, we can only achieve accurate incident parameter through several iterations. Previous articles on MC techniques in radiotherapy physics have developed accurate source models for different products of linac and energy beams, and as well calculated the dose distributions in a homogenous and realistic patient phantom. Grevillot et al. developed an accurate source beam for Elekta Precise linac at 6MV x-ray beam using the GEANT4-based GATE MC code[6]. The study validated the source model for 5 × 5 -30 × 30 cm side of a square field with physical measurements. It was concluded that the source model is accurate with slight discrepancies of about 1 to 2% for depth dose and dose profiles. Mesbahi et al. simulated and compared the source model of Varian 21EX and Elekta SL-25 for 10 × 10 and 20 × 20 cm2 fields using MCNP4C MC code while Tayalati et al. modelled an Elekta Synergy linac for5 × 5 -30 × 30 cm2 at 6MV using GEANT4/GATE MC code [7, 8]. Bencheikh et al. simulated an accurate source model of Varian 2100 linac for 6 × 6, 10 × 10 and 15 × 15 cm side of a square field at 6 MV photon beam using BEAMnrc/DOSXYZnrc codes. While Gonzalez et al. used the PENELOPE MC code to develop an accurate source model of an Elekta Precise linac equipped with a dynamic micro-multileaf collimator for 7 × 7 cm2

field size [9, 10], these studies concluded by evaluating the accuracy of their source model with either Gamma criterion or local percentage difference.

The purpose of this study was to accurately simulate a 6MV photon beam and test the variations of parallel and elliptical beam sources based on the dosimetric characteristics of the linac model using the BEAMnrc/DOSXYZnrc package.

Materials and Methods

Measurements

The accuracy of an MC source model of a given linac depends on the percentage depth dose and lateral profiles of the physical dose measurements. The measurement was carried out on the Elekta Precise Linac at the Department of Radiotherapy, Lagos University Teaching Hospital (LUTH), Nigeria using the MP3-P water scanning system produced by PTW Freiburg GmbH. The water scanning system comprises ofanMP3-P water tank of 35 × 25 × 38 cm3and Tandem Dual-channel Electrometer for data acquisition from the 31010 Semiflex ionisation

chamber inserted into the tank during measurement. Physical dose measurement was characterized for 10 × 10 cm2field at 6MV photon beam. Conventionally, 31010 ion chamber (vented sensitive volume of 0.125cm3) is designed mainly for dose distribution measurement due to its high uniform spatial resolution. The percentage depth doses, and lateral beam profiles were measured at 100 cm source to surface distance (SSD). These measurement data were extracted, smoothened, and interpolated at 2 mm step width using MEPHYSTO Software suite.

Monte Carlo simulation

Development of an MC source model for Elekta Precise Linac using the BEAMnrc package code requires the manufacturers’ specification, which serves as a guide. The accuracy of a source model depends on the deviation of simulated data compared with the physically measured data. Previous MC studies have determined the accuracy of their models by accepting a local difference percentage which is less than 2% deviation from the physical measurement [11-12]. Any missing detail from the manufacturers’ specifications can affect the accuracy of the simulation. This study had the manufacturer’s specification for Elekta Precise linac head. Linac head (geometrical) modeling: This study modelled the x-ray beam for 10 cm side of a square field at 6MV. The component modules (CMs) used in the simulations are the SLABS, CONS3R, FLATFILT, CHAMBER, MIRROR, MLCQ and JAWS which were able to model the target, primary collimator, flattening filter, ionization chamber, mirror, MLC and diaphragm (back-up jaws), respectively, as shown in Fig. 1.For nifty simulation, the model was divided into the upstream and downstream segment. The upstream model is machine and particle-specific. It has a general influence on the simulated beam while the downstream model contains only the MLC and back-up jaws, which defines the dosimetric shape of the simulated beam. The phase-space file that was generated for upstream model was used as incident source for downstream model. The geometries of the CMs were defined alongside with the materials. However, few materials/ alloys are not on the PEGS4 (material list). This study created and added a tungsten alloy to the available material list to suit manufacturers’ specification for MLC and backup-jaws material. In addition, a phase-space file is generated after a successful simulation. It contains all the characteristic of the simulated beam, which could be analyzed using the BEAMDP analysing software or used as the source file for dose calculation in DOSXYZnrc MC Software. For efficient simulations, the numbers of histories were carefully selected to have uncertainty that is less than 1%.



Figure 1. BEAMnrc Monte Carlo simulation of a Precise Linac model

Incident source modeling: This study evaluated the sensitivity of two initial electron source parameters. Beam radius of the parallel initial electron-beam source (ISOURC = 0) was varied between 0.001 and 0.4 cm for 6.00 MeV monoenergetic incident beam, which is a uniform distribution of radial intensity. In mean angular spread electron-beam (2D Gaussian distribution) source (ISOURC=19), the standard deviation values of the 2D Gaussian distribution (sigma or symbol σ) beam was varied between 0.05 and 2 mm at 6.00 MeV. The Gaussian distribution could be described by the full width half maximum (FWHM) which is approximately 2.35σ (2√2ln2σ).In BEAMnrc transport parameters, factors such as energy transport beam, number of histories, background medium for CMs, maximum simulation hours, Global electron and photon cutoff energies (ECUT and PCUT) are defined. For this simulation, ECUT and PCUT were set to 0.7 and 0.01 MeV, respectively. In addition, the EGSnrc transport parameter, boundary crossing and electron-step algorithms were set to EXACT and PRESTA-II, respectively.

Dose calculation in a homogenous water phantom: DOSXYZnrc MC code calculates dose distribution in the simulated homogenous water phantom. The phase space file that contains the linac beam characteristic was incident on the water phantom. The source-to-

surface distance (SSD) was defined in this study at 100 cm. The EGSnrc parameters in the DOSXYZnrc code were set in a similar condition to the BEAMnrc MC code. Dose distribution was calculated in a homogenous water phantom of 30 × 30 × 30 cm3. Each rectilinear voxel in the water phantom was defined as 0.2 × 0.2× 0.2 cm3 for accurate dose calculation. Depth doses and dose profiles were evaluated for each simulated beam. Accuracy and sensitivity of variant parameters of the simulated data were benchmarked with measured data using the Gamma (γ) criteria of 2%/2mm and 3%/3mm (percentage-dose- difference/distance-to-agreement). This Monte Carlo study used a Linux OS Laptop equipped with 8 Intel® Core i7-7700HQ processors. The processors operate at 2.80GHz clock speed for 16 GB RAM.

Analysis

To validate our simulation, we used the gamma (γ) evaluation method which has generally been accepted as the gold standard method for dose distribution comparison. Unlike local percentage difference that considers only the dose difference, γ analysis considered the spatial criterion which is highly sensitive in detecting errors [13, 14]. This method incorporates pass-fail criteria for both dose-difference (DD) and distance-to-agreement (DTA) of dose distributions to calculate a dimensionless metric called the gamma index [15, 16]. It shows the magnitude of agreement between DD and DTA [17, 18]. For this study, we used γ criteria of 2%/2mm and 3%/3mm which are acceptable in clinical dosimetry [15].

Results

Percentage depth dose

Figures 2 and 5 show the percentage depth doses for variations in the parallel and angular spread electron beam sources for 10 × 10 cm2 field at 6MV alongside with physical measurements. In Figure 3, the γ index for 3%/3mm tolerance is displayed for calculated dose distribution for the parallel electron beam. It shows that the indexes are greater than one (>1) around surface dose and build-up regions. These regions are the most sensitive part of a depth dose profile. In Figure 4, surface dose increases with an increase in incident beam radius. Figure 6 depicts the γ index test of 3%/3mm for variations in sigma values of the angular spread electron source beam. In Figure 7, the surface dose increases with an increase in sigma values. Increase in surface dose with an increase in beam size could be caused by further scattering in the bremsstrahlung beam when passing through the collimators, flattening filters and air chamber. This will increase the secondary electrons that are deposited on the surface of the homogeneous water phantom.



(a)

(b)

Figure 2. Profiles of the percentage depth doses for parallel beam source

with (a) measured data and simulated data for 0.001, 0.01, 0.05, 0.1, 0.25,

and 0.4 cm beam radius, and (b) inlays to magnify the entrance dose

region (blue panel) and exit dose region (red panel).

Figure 3. Profile of 3%/3mm Gamma index test with depth dose for simulated

data

Figure 4. Correlation between surface dose (%) and source beam radius for

the simulated electron transport.

(a)



(b) Figure 5. Profiles of the percentage depth doses for elliptic beam source

with (a) measured data, and simulated data for 0.05, 0.5, 1.0, 1.5, and 2.5

mm mean angular spread, and (b) inlays to magnify the entrance dose

region (blue panel) and exit dose region (red panel).

Figure 6. Profile of 3%/3mm Gamma index test with depth dose for simulated

data

Figure 7. Correlation between surface dose (%) and source beam sigma value for the

simulated electron transport.

Lateral beam profile dose

Lateral profiles in water for 10 × 10 cm2 field at 5 and 10 cm depths are displayed in Figures 8 and 9. Figures 8a and 8c contain the lateral profiles as a function of beam radius in parallel electron beam source at 5 cm and 10 cm while Figure 8b and 8d showed the 3%/3 mm γ index test for dose distributions in Figure 4a and 4c. From Figure 8 it shows that the sensitivity of a lateral profile lies in the penumbra region. Variations of electron beam radii are significant at this sensitive region with γ index > 1 especially for beam radius above 0.05cm. At 10 cm depth doses, there are γ indexes with a numerical value of 2.9 for a beam radius of 0.4 cm. In Figure 9a and 9c, the lateral beam profiles of elliptical beam variation at 5 and 10 cm depth dose in a water phantom is displayed; while Figure 9b and 9d contain the γ index test for Figure 9a and 9c. From the curves, it follows that the penumbra region of dose profile is sensitive to variation in the elliptical beam where there are larger disparities as a standard deviation of σ increases.

(a)



(b)

(c)

(d)

Figure 8. Lateral beam profile for parallel beam source for (a) comparison

between measured and simulated beam radii of 0.001, 0.01, 0.05, 0.1, 0.25

and 0.4 cm at 5 cm depth dose; (b) 3%/3mm γ index test for the lateral

beam profile (c) relative dose profile of measured and simulated data at

10 cm depth in the water phantom; (d) 3%/3mm γ evaluation of relative

dose.

(a)

(b)

(c)



(d)

Figure 9. Lateral beam profile for elliptical beam source for (a) comparison

between measured and simulated beam radii of 0.001, 0.01, 0.05, 0.1, 0.25

and 0.4 cm at 5 cm depth dose (b) 3%/3mm γ index test for the lateral

beam profile (c) measured and simulated data at 10 cm depth in the

water phantom (d) 3%/3mm γ evaluation of the relative dose.

According to TG 105, dose discrepancies between measured and simulated data should be within an appropriate DD/DTA criterion [19]. Table 1 and 2 summarize the sensitivity of the parallel and elliptical electron incident parameters to dose distribution in a homogenous water phantom using the 2%/2mm and 3%/3 mm γ criteria (these γ index criteria are taken as a gold standard [20-22]). As shown in the Tables, parallel source beam radii of 0.001 to 0.4 cm generated the pass rates of 97.46 to 96.61% and 98.31 to 96.61% for 2%/2mm and 3%/3mm while σ variations (0.05 to 2 mm) in elliptical beam source resulted into pass rates of 97.46 to 96.61% and 98.31 to 97.46%. Variations of the beam two incident electron parameters within reasonable numerical values are inconsequential to the maximum depth dose and a γ pass rate of the depth dose profile to a great extent. Unlike the depth dose profile, lateral beam profiles are sensitive to the beam radius of the parallel source and Gaussian distribution of the elliptical source. As beam radius increases from 0.001 cm to 0.4cm, γ pass rate decreases from 85.19 to 61.73% and 92.59 to 67.90% at 5 cm depth for 2%/2mm and 3%/3mm. While at 10 cm depth dose, the γ pass rate decreases from 90.12 to 56.79% and 93.83 to 69.14%. For Gaussian distributions, increase in σ value from 0.05 to 2.0 mm causes a decrease in γ pass rate from 83.95 to 50.61% and 91.36 to 56.79% at 5 cm depth for 2%/2mm and 3%/3mm. Also, the γ pass rate decreases at 10 cm depth from 87.65 to 50.62% and 93.83 to 58.03%. At beam radius 0.001cm and sigma 0.05 mm for parallel and elliptical incident sources, a simulated model of the Elekta Precise linac is accurate. However, 1 mm change in parallel beam radius causes 4.4 and 6.3% change in lateral beam profiles at 5 and 10 cm depth doses. Likewise, 1 mm change in Gaussian

distribution of the elliptical beam causes 14.8 and 16.5% in lateral beam profiles at the same depth doses.

Table1. Gamma Index pass rate for different radius parameters in parallel beam source.

R

(cm)

DMax(cm) PDD Profile (5 cm depth) Profile (10 cm

depth)

2%/2mm 3%/3mm 2%/2mm 3%/3mm 2%/2mm 3%/3mm

0.001 1.5 97.46% 98.31% 85.19% 92.59% 90.12% 93.83%

0.01 1.5 97.76% 98.31% 75.31% 87.65% 82.72% 88.89%

0.05 1.5 97.76% 98.31% 70.37% 85.19% 82.72% 88.89%

0.1 1.5 97.76% 98.31% 69.14% 90.12% 67.90% 93.3%

0.25 1.5 97.46% 97.46% 64.20% 71.61% 71.61% 74.07%

0.4 1.5 96.61% 96.61% 61.73% 67.90% 56.79% 69.14%

Table 2. Gamma Index pass rate for different sigma values in elliptical beam source

σ

(mm)

DMax(cm) PDD Profile (5 cm depth) Profile (10 cm

depth)

2%/2mm 3%/3mm 2%/2mm 3%/3mm 2%/2mm 3%/3mm

0.05 1.5 97.46% 98.31% 83.95% 91.36% 87.65% 93.83%

0.5 1.5 97.46% 97.46% 69.14% 76.54% 69.14% 76.54%

1.0 1.5 97.46% 97.46% 67.90% 70.37% 64.19% 70.37%

1.5 1.5 97.46% 97.46% 62.96% 64.19% 61.72% 65.43%

2.0 1.5 96.61% 97.46% 50.61% 56.79% 50.62% 58.03%

Discussion

This study modelled an accurate Elekta Precise linac using the parallel and elliptical incident beam sources and also analyzed the sensitivity of varied incident parameters of the two different electron beam sources on dose distribution in a homogenous water phantom. It is essential for a reliable linac head model to simulate the physical linac head accurately. This study found an accurate MC model at abeam radius of 0.01 mm and sigma of 0.05 mm for parallel and elliptical incident sources, respectively. This study simulated 6 MV beams with an exactly nominal energy value of 6.0 MeV. Previous studies have used nominal values between 5.7 and 6.3 to simulate a linac of 6MV beams[23], [24]. Variations in mean energy would have influenced the tweaking of the simulated parameters, especially the incident beam radius and Gaussian distribution over several iterations. However, a study simulated a fixed beam radius (1.1 mm) and fine-tuned the mean energy to find an accurate MC model around 5.9 – 6.2 MV [25].

It could be challenging to choose the best incident electron beam source for MC simulation. This study has



investigated the dosimetric sensitivity of an initial electron source, which is more pronounced in angular spread electron source beam than parallel source beam. In parallel incident beam, there will be less interaction of fast-moving electrons as they approach the target, unlike the Gaussian distribution beams where it could be deduced that electron particles interact/scatter more before the exit window. This interaction increases the intensity of the fast-moving electrons as they approach the target. Increase in σ will cause more interaction of particles and therefore increases the intensity of the particles colliding with the target. For clinical relevance, the angular spread beam source would be of better advantage regarding angular divergence spread (fitting parameter) of the beam in critical treatment where physical measurement is complex. However, the angular divergence parameter (Ө< 0.80) of the angular spread beam is insensitive to dose distribution of field sizes less than 10 × 10 cm2 [26]. This is valuable for large treatment fields such as conformal, head and neck IMRT, and VMAT. Dosimetry of small fields at 4 and 6 MV beams are more sensitive to variations of mean energy and beam radius compared to their large fields. Although, the sensitivity to variations in beam radius and angular spread is more pronounced in large fields ( ≥ 35 × 35 cm2) lateral profiles at ≥ 10 MV incident beams [24].

Discrepancies between the measured data and accurate simulated data may not be caused by MC simulation of radiation transport since the accepted uncertainty in our model is < 1% [14, 19]. These disparities at the shoulder/ penumbra region could be attributed to additional electron interaction and scattering in the linac head which were not fully accounted for experimentally or errors in the ionization chamber set up or irregular response of the ion chamber at a shallow depth where contaminant electrons are dominant in the water phantom. Even though some geometrical components of the linac heads were provided, maybe a higher degree of accuracy could have been achieved if the manufacturers have provided the full detail of the linac head.

Conclusions

Accurate Monte Carlo model of a linac depends on skillful iteration to match with physical dose measurements and the availability of full linac details. We found the best fitting parameter for the two initial source beams utilized in this study. These accurate source models show the potential to be used for efficient and effective dose calculation in patient-specific radiotherapy treatment and further dosimetry

studies. This study shows that the two initial electron beam sources are significant to dose distribution of 10 × 10 cm2 field at 6 MV beams, but sensitivity is more pronounced in angular spread electron beam source.

Acknowledgement

The authors acknowledge the opportunity to use the Elekta Precise Linac at LUTH, Nigeria.

Abbreviations

LINAC: Linear accelerator; SSD: Source-to-surface distance; ECUT: Electron cut off energy; PCUT: Photon cutoff energies; FWHM: Full width half maximum; CMs: Component modules.

Author Contributions

OMO carried out the simulation, analyzed the measurement and drafted the manuscript. MOA and RDJ supplied the measurement data and edited the drafted manuscript. RIO edited the drafted manuscript. All authors read and approved the final manuscript.

Competing Interests

The authors have declared that no competing interest exists.

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