Dosimetric characteristics of 6 MV photons from TrueBeam STx medical linear accelerator: Simulation and experimental data

s of small fields and of ranges 
with high dose gradients. The RAZOR has 
sensitive volume and the radius cavity are 0.01 
cm
3
 and 1.0 mm, respectively. 
Fig.1. Photograph of Blue Phantom (IBA, 
Germany) scanning water phantom below 
TrueBeam STx head in the experimental setup. 
Measurements include PDD along the 
central axis and crossline profile at the depth of 
maximum dose (1.5 cm) scans were made for 
FF 6 MV with 10×10 cm
2
 field size. To avoid 
any ripple effect in the measurement, a PDD 
scan was started from the bottom of the tank 
moving toward the surface of the water. Data 
processing and analysis were performed using 
IBA’s OmniPro Accept with application 
setting for a geometric mean smoothing 
function with a value of 3 mm. Appropriate 
stopping power ratio factors were used for 
electron ionization values to PDD values 
conversion. Measured PDD curve was 
compared to Golden Beam Data (GBD) and 
the reproducibility of the measurement data. 
GBD was provided by Varian, which often use 
for commissioning measurements. A 
comparison of the PDD with the Gamma Index 
criteria of 2%/2 mm yielded a gamma pass rate 
of 97%. Therefore, the accuracy of this 
measurement can be considered to be within 
2%/2 mm. 
D. Geant4/GATE 
For many years, the Geant4-based 
GATE MC code has been developed as an 
open-source MC program for nuclear medicine 
simulation, with a focus on PET and SPECT 
imaging [23]. This toolkit allows creating a 
simulation on the basis of simple macro-
command instead of handling tedious C++ 
syntaxes of Geant4 code. It helps a quicker 
learning phase for new users and makes a small 
size of the GATE work folder easy to share 
within the community. Details about the GATE 
capabilities and validation are presented 
elsewhere [23-25]. 
A new GATE v8.2 was used for this 
simulation [26]. 6 Varian PhS files of smaller 
size (2 GBs) were imported into GATE and 
used for the downstream of the jaws as a 
source in the Linac. These individual files were 
then concatenated to one large PhS file. After 
exiting the PhS plane, the particle passes 
DOSIMETRIC CHARACTERISTICS OF 6 MV PHOTONS FROM TRUEBEAM STX  
40 
through the second collimator as the Y and X 
jaws and MLC. Data for the material and 
geometry of the Linac components were 
obtained from the TrueBeam STx Monte Carlo 
package [19]. 
Geant4 Electromagnetic physics package 
3 (G4EmStandardPhysics_option3) was used 
for precise dose calculations and particle-
matter interactions or radiation transport in the 
simulation. G4EmStandardPhysics_option3 
designed for any applications required higher 
accuracy of electrons, hadrons, and ion 
tracking without a magnetic field. The package 
has been presented in Poon and Arce at al. for 
radiotherapy application [27, 28] and 
recommended in Varian documents [19]. The 
range cuts for gamma, electron, and positron 
are fixed to 0.1 mm in a water phantom, 1 mm 
in the world volume, and 10 mm in TrueBeam 
material volume, respectively. 
A virtual water phantom with 30×30×30 
cm
3
 volume was installed at an SSD equal to 
100 cm as the measurement. It is used for the 
MC estimation of the absorbed dose 
distribution. Voxel size was set to 3×3×3 mm
2
for a field size of 10×10 cm
2
. 
E. Gamma Index method 
Data analysis was based on comparisons 
between GATE simulations and measurements 
using the Gamma Index method [29], which 
became a ―gold standard‖ method for the 
comparison in dose distribution [13]. This 
method was conducted with a percentage dose 
difference (ΔD) criteria and distance to 
agreement (DTA) of of 1%/1mm and 
2%/2mm. If the Gamma Index value is greater 
than unity, it indicates a position where the 
agreement between the measured and 
simulated dose maps do not meet the 
predefined criteria. Passing criteria were met if 
the gamma index was no larger than 1. An 
important feature of this method is that the 
final assessment of the dose distribution 
quality. For the regions of significant 
disagreement, the Gamma Index value is 
greater than unity that will be apparent relative. 
The gamma pass rate was defined as a 
quotient of the passing points and all points. 
For the global Gamma Index passing criteria of 
2%/2 mm, a good agreement, a high 
agreement, and a reasonable agreement 
between the measured and simulated dose 
distribution were observed with over 99%, 
95% and 90% of the points of PDD and cross-
plane profile, respectively. 
III. RESULTS AND DISCUSSION 
6 PhS files stored 3×10
8
 photon, 
electron, and positron particles, which have 
been simulated. Approximately 6×10
9
 particle 
histories from 6 PhS files were performed such 
that the statistical uncertainty in the dose for 
the voxels inside the radiation field was less 
than 0.2% at the depth of maximum water 
phantom. The simulation results take into 
account the electron and positron 
contamination in the photon beam. 
A. PDD curve 
In this paper, the quantity PDD defined 
as the quotient, expressed as a percentage, of 
the absorbed dose at a predefined depth (dx) to 
the absorbed maximum dose at a fixed 
reference depth of d0 = 1.5 cm, along the 
central axis of the beam. Fig. 2 shows the 
comparison between measured and GATE 
estimated PDD with SSD = 100 cm for a 
10×10 cm
2
 field for FF 6 MV photon beam and 
the Gamma Index distribution. The maximum 
dose was detected at 1.5 cm of depth in both 
measurement and simulation. The statistical 
uncertainty of bins scoring PDD was between 
0.02% to 0.04% and all bins scoring more than 
50% of the maximum absorbed dose was 
0.02% to 0.2%. The distribution of Gamma 
N. D. TON, B. D. LINH, Q.T. PHAM 
41 
Index is shown in Fig. 2, there is only one 
point was larger than 1. The evaluation using 
the Gamma Index with 2%/2 mm criteria for 
PDD obtained was greater than 98%. There is a 
good agreement between the computed and 
measured PDD. 
Fig. 3 shows the percentage dose 
difference of the PDD relative to the maximum 
dose of the measurement as equation 1. This 
discrepancy is never greater than 2%.mm, 
2%. 
 |
| (1) 
Where, D1 and D2 are the value of 
simulated and measured, respectively, D0 is the 
maximum dose of the measurement. 
Fig.2: Comparison of measured (black line) and GATE simulation estimated (red circle) PDD of TrueBeam 
STx FF 6 MV photon beam with SSD =100 cm for 10×10 cm
2
 field. The distribution of gamma index points 
of PDD (blue plus). 
Fig. 3: Percentage dose difference between the simulated and measured PDD relative to the maximum dose 
of the measurement. 
DOSIMETRIC CHARACTERISTICS OF 6 MV PHOTONS FROM TRUEBEAM STX  
42 
Fig. 4: Comparison of measured (black line) and GATE simulation estimated (red circle) cross-plane profile at 1.5 cm for 
TrueBeam STx FF 6 MV photon beam for 10×10 cm2 field. The distribution of gamma index points of the cross-plane 
profile (blue plus). 
Fig. 5: Percentage dose difference between the simulated and measured cross-plane profile at 1.5 cm of depth 
relative to the maximum dose of the measurement. 
B. Cross-plane Profile 
The comparison of profile at 1.5 cm in 
the water tank between the simulation and 
measure is shown in Fig. 4. The statistical 
uncertainty of the simulation is in the range of 
0.02% to 0.2%. For the Gamma Index criteria 
of 2%/2 mm, the distribution is presented in 
Fig. 4 and the average pass rates for the cross-
plane profile was ≥ 94%. This agreement 
notably worsens with the more stringent 
criteria of 1%/1 mm. Although the gamma 
indices in the penumbra region (at ±5 cm) are 
bigger than those in the inside field, there is 
still a reasonable agreement between 
computed and measured the cross-plane 
profile at 1.5 cm depth. 
For cross-plane profiles inside the field 
region, the percentage dose difference is less 
than 1.5%. The result of the percentage dose 
difference is shown in Fig. 5. The differences 
N. D. TON, B. D. LINH, Q.T. PHAM 
43 
in the penumbra region (at ±5 cm) are bigger 
than those in the inside field. These 
differences probably represent the number of 
simulated particles in which less number of 
simulated particles will be found in the 
penumbra and result in a big statistical 
fluctuations in MC simulation and a big 
difference relative [30, 31]. 
IV. CONCLUSIONS 
The aim of this work is to validate the 
potential application Geant4/GATE software 
for the Varian TrueBeam STx. In this study, 
the characteristics of 6 MV photons of 
TrueBeam STx include PDD and crossline 
profile, which was simulated based on 
Geant4/GATE using Varian PhS file, and 
Varian manufacturer’s information. A PDD 
curve and beam profile for 10×10 cm
2
 field 
size in a water phantom using Geant4/GATE 
simulation show a good agreement with 
measured dose data for FF 6 MV photon beam 
produced by the Linac. The percentage dose 
difference and Gamma Index method were 
used for comparison. The agreement between 
simulations and experimental data proved that 
Geant4/GATE can be used for accurate Monte 
Carlo dose estimation. 
ACKNOWLEDGMENT 
The authors would like to thank 
VINATOM for the support under the grant 
number CS/19/04-02, MSc. Nguyen Ngoc 
Quynh at INST for his kind help in the run of 
Geant4/GATE and all medical physicist at 
Department of Radiation Oncology and 
Radiosurgery, 108 Military Central Hospital. 
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