METHODS
BrainLab ExacTrac Dynamic System, BrainLab Phantom Pointer, and BrainLab Head Phantom were
utilized. Imaging and positioning accuracy were assessed through manual and couch-induced displacement
tests. This article does not contain any studies with human participants or animals performed
by any of the authors. Ethical approval was not required for this study as it was conducted
using phantoms only.
RESULTS
The study evaluated the accuracy of BrainLab ETD and Varian cone-beam computed tomography
(CBCT) systems using a shared reference. Both systems demonstrated sub-millimeter precision, with
translational errors ranging between 0.06 mm (ETD Z-axis) and 0.04 mm (ETD X-axis), and rotational
errors below 0.12°. Maximum observed errors did not exceed 0.3 mm for translational or
0.2° for rotational movements. For combined errors 3D translational accuracy showed no significant
difference p=0.482) between ETD (0.11 mm) and CBCT (0.09 mm). The 6D error magnitude was
slightly lower for ETD (0.14 mm vs. 0.20 mm, p<0.001). Correlation analysis revealed that single-axis
correlations between ETD and CBCT were not significant (p>0.05) except for pitch errors (p=0.023),
which showed moderate correlation. The mean DIFF 6D vector (0.23 mm), representing the positional
offset between ETD and CBCT detections, was significantly larger than both ETD 6D and CBCT
6D errors (p<0.001 and p=0.002, respectively).
CONCLUSION
Both systems offer high precision in detecting positional shifts. The ETD System"s ability to provide continuous,
real-time monitoring enhances patient safety by allowing real-time correction of positioning
errors, making it a valuable tool for stereotactic radiotherapy applications. Further studies with patient
data are needed to validate the clinical application of these findings.
Keywords: Cone-beam computed tomography; ExacTrac dynamic; stereotactic radiation therapy
In recent years, the integration of imaging systems
like the BrainLab ExacTrac Dynamic (ETD) Imaging
System with linear accelerators such as the Varian
TrueBeam STX has further refined SRT capabilities.
These systems provide real-time imaging and positioning
adjustments, allowing for highly accurate target
localization and dose delivery.[
Numerous studies have underscored the importance
of regular calibration and QA checks in maintaining the
accuracy of SRT systems. The use of phantoms and specialized
QA tools, such as the Varian Machine Performance
Check (MPC) software and the BrainLab Phantom
Pointer, has been instrumental in verifying the alignment
and performance of imaging and treatment delivery components.[
This study aims to provide a comprehensive evaluation
of the imaging and positioning accuracy of SRT
systems using a series of calibration and QA tests. By
employing tools such as the BrainLab Phantom Pointer,
BrainLab Head Phantom, and various displacement
tests, we seek to validate the precision of these systems in
clinical settings. Our methodology includes detailed calibration
routines for CBCT, MV, and kV imaging systems,
as well as performance checks using the Varian MPC
software. Additionally, we assess the systems" response to
manual and couch-induced displacements to thoroughly
examine their accuracy under different conditions.
The findings of this study will contribute to the
existing body of knowledge on the importance of
stringent QA protocols in SRT. By ensuring that all
mechanical and dosimetric settings are accurately calibrated, this research aims to enhance the reliability of
stereotactic treatments and improve patient outcomes.
This work underscores the critical role of continuous
quality assurance in the evolving landscape of radiation
oncology, highlighting the necessity of meticulous
calibration and evaluation practices to maintain the
high standards required for effective cancer treatment.
Calibration and Quality Control
Imaging Systems Calibration
Initial calibration routines were performed to enhance
the imaging quality of both linear accelerators. These
included dark field and flood field calibrations, identification
and exclusion of dead detector pixels, and adjustment
of kV collimator jaw settings. Subsequently,
similar calibration processes were carried out for the
CBCT system. The isocenters of the kV and MV imaging
systems were aligned with the isocenter of the
linear accelerator using the Varian IsoCal phantom. By
rotating the gantry through 360 degrees and capturing
images, any deviations of the geometric center from
the device"s central axis were detected and corrected.
Machine Performance Check (MPC)
The MPC involved comprehensive mechanical and dosimetric
tests of the treatment devices. The Varian MPC
software and MPC check phantom were used to conduct
numerous quality control tests, including checks
on gantry, collimator, and couch alignment, as well as
the accuracy of the jaws and MLC positions. These tests
ensured that all calibrations were verified and met established
limits before starting the measurements.
Picket Fence and Winston-Lutz Tests
To verify the positional accuracy of the MLCs and optimize
the success rate of the Hancock Winston-Lutz
test, the picket fence test was conducted. Using the
Eclipse planning system, plans were created at gantry
angles of 0, 90, 180, and 270 degrees, covering all
MLCs. Each plan included a defined irradiation time
of 20 MU for each subfield, ensuring proper alignment
and gaps between MLC pairs.
The Hancock Winston-Lutz test involved creating twelve different areas with 6 MV photon energy, incorporating specific gantry, couch, and collimator angles. Each area was designed to prevent leakage doses by positioning the MLC intersection areas outside the radiation field. The BrainLab phantom pointer (ballbearing) was positioned according to the light field centers at gantry angles of 0 and 90 degrees. Port images obtained from the irradiation were analyzed using the SNC Machine software, and necessary corrections were applied. The device"s isocenter was then radiologically positioned with high precision, and the BrainLab ETD system was calibrated to this point.
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Evaluation of Imaging and Positioning Accuracy
Before evaluating the imaging and positioning accuracies
of the stereotactic radiotherapy systems, all
mechanical and dosimetric settings of the treatment
devices were verified for accuracy, and necessary adjustments
were made. The isocenters of both systems
were precisely calibrated.
BrainLab Phantom Pointer (Ball-Bearing) Tests
Manual Displacement Test
We moved the ball-bearing (BB) in one direction in 3D
space to a known distance "d" and measured the displacement
(Fig.
Couch Displacement Test
Following the manual displacement tests, the BrainLab
Phantom Pointer was repositioned to the isocenter, and displacements were applied using the treatment
couch. These displacements were detected both by the
BrainLab ETD system and the integrated kV and MV
port imaging systems. The responses of the BrainLab
ETD system and the kV and MV port imaging systems
to these displacements were separately analyzed.
Comparison of Positioning Accuracy with BrainLab
ETD and Varian CBCT Using Head Phantom
This part of the study was designed to evaluate the relative
accuracy of the BrainLab ETD and Varian CBCT
systems in detecting displacements applied via a clinically
validated treatment couch. Rather than assuming
CBCT as the gold standard, both systems were independently
assessed using a shared reference. Translational
displacements were validated with a micrometer,
while rotational measurements used the treatment
couch as a consistent reference. This strategy allows a
methodologically unbiased comparison of six degrees
of freedom (6DOF) detection capabilities.
The head phantom was fitted with a BrainLab head
and neck positioning mask to ensure stability and reproducibility
during measurements (Fig.
Couch Displacement Protocol
Couch displacements were systematically applied
in four predefined distances of 0.1 mm, 0.5 mm,
1.0 mm, and 3.0 mm along each axis (X, Y, Z). For
each distance, displacements were performed in both
positive and negative directions, resulting in eight
measurements per axis. Rotational displacements
(yaw) were also included in the protocol, following
the same sequence of movements and distances. Each
displacement was performed sequentially, first along
the translational axes (X, Y, Z) and then for yaw, ensuring
a comprehensive evaluation of the system"s
6DOF detection capabilities.
To avoid bias present in previous studies, which often compared ETD directly against CBCT while implicitly treating CBCT as the baseline, this study was designed to measure both systems independently against the same source of displacements: the treatment couch. By using the couch as the reference for both ETD and CBCT, the study evaluated their relative performance on an equal basis.
Measurement and Data Recording
For each applied displacement, six values were recorded,
reflecting positional deviations across all degrees
of freedom: translations (X, Y, Z) and rotations (yaw,
pitch, roll). Translational displacements were validated
against a high-precision micrometer, ensuring accuracy
within 0.02 mm. This validation established the
treatment couch as a reliable reference for translational
movements along the X, Y, and Z axes.
To evaluate rotational detection, displacements were applied along the yaw axis, incorporating the inherent reliability of the treatment couch as the reference. Although the rotational accuracy of the couch was not independently validated, its use as a consistent reference allowed for a relative comparison of the BrainLab ETD and Varian CBCT systems. Yaw rotations were included to evaluate the relative performance of the two systems, rather than to establish absolute rotational accuracy.
For each displacement, the 6D error magnitude (E6D) was calculated using the following formula:
where Δi represents the deviation in each degree of freedom. This calculation was performed for each applied displacement to provide a comprehensive measure of positional error for both systems.
Statistical Analysis
All data were analyzed using IBM SPSS Statistics (version
29.0.1, IBM Corp., Armonk, NY, USA). Due to
the small sample sizes and non-normal distribution
of error values (verified via visual inspection and distribution
metrics), nonparametric statistical methods
were used throughout the study. The Wilcoxon
signed-rank test was applied to compare paired error measurements between systems, and correlation
analyses were conducted to evaluate inter-system
agreement across individual and combined axes. Statistical
significance was defined at p<0.05.
All graphical representations were created using the
BoxPlotR web-based tool.[
Micrometer vs ETD Accuracy
ETD system detected all manual displacements of
the Ball-Bearing (BB) phantom (0.10-3.00 mm) with
0.02 mm precision, as measured using the integrated
micrometer at two-digit sensitivity (0.01 mm resolution).
No discrepancies were observed between applied
displacements and ETD-reported values, even at the
smallest tested increment (0.10 mm).
This confirms the system's reliability for subsequent
tests requiring 0.1 mm clinical sensitivity. The results validate
ETD"s capability to resolve submillimeter positional
deviations, ensuring its suitability for high-precision stereotactic
radiotherapy quality assurance protocols.
Couch Displacement Detection
The precision of BrainLab ETD, MV and kV imaging
systems in detecting couch-induced displacements was
evaluated using the Ball-Bearing (BB) phantom. Couch
shifts (0.10-3.00 mm) were applied along all translational
axes (X, Y, Z) using service mode commands (two-digit
precision, 0.01 mm resolution), with displacements
recorded by the ETD and orthogonal MV/kV imaging
systems at clinical one-digit precision (0.1 mm).
The ETD system detected all 30 couch displacements
with 0.1 mm maximum error per axis, mirroring its performance
in micrometer-based tests. For MV/kV systems,
individual axis errors did not exceed 0.1 mm, and
3D vector errors reached a maximum of 0.14 mm (mean:
0.08±0.04 mm). Wilcoxon signed rank tests confirmed
that there were no significant differences between MV
and kV systems (p>0.05), with errors exhibiting random
distribution (no inter-system correlation, r<0.2) (Fig.
These results validate the treatment couch as a reliable reference for submillimeter positional adjustments in clinical workflows. The consistency between the ETD, MV, and kV systems underscores their collective precision for stereotactic radiotherapy applications that require a tolerance of 0.1 mm.
Head Phantom Movements
Translations and Rotations Detected by ETD and
CBCT
The descriptive statistics for positional errors demonstrated
submillimeter precision across both the ETD and CBCT systems. For translational axes, mean errors
ranged between 0.06 mm (ETD Z axis) and 0.04
mm (ETD X axis), with standard deviations <0.07 mm.
Rotational errors were similarly minimal, with mean
deviations <0.12° and standard deviations <0.07°. The
maximum observed errors did not exceed 0.3 mm or
0.2°, underscoring the high precision of both systems.
Although significant differences (p<0.05) were observed for specific axes (e.g., ETD demonstrated smaller errors in Y/Z translations and pitch rotations, CBCT on the X axis and yaw), the absolute error magnitudes were clinically negligible. For example, the mean difference in the Y-axis errors between ETD and CBCT was 0.04 mm, and the pitch rotation discrepancies averaged 0.09°.
For combined errors, 3D translational accuracy
showed no significant difference between systems
(p=0.482), with mean errors of 0.11 mm (ETD) and
0.09 mm (CBCT). The boxplots for translational errors
(X, Y, Z) and rotational vector magnitude (RVM)
errors (Fig.
The 6D error magnitude, incorporating translational
and rotational deviations, was slightly lower for
ETD (0.14 mm vs. 0.20 mm, p<0.001). The boxplot
depicting 6D error magnitude (Fig.
For the combined 6D error, which integrates translational and rotational deviations, the correlation coefficient was not significant (p=0.653). This highlights the independent detection of 6D errors by the two systems, with no systematic relationship observed across the overall error magnitudes. These findings emphasize the robustness of both systems for stereotactic radiotherapy, with clinically negligible differences in precision. The correlation analysis further supports the conclusion that ETD and CBCT systems independently achieve high accuracy in positional detection, with minor variations likely attributable to system-specific operational characteristics rather than systematic biases.
Comparison of ETD and CBCT Error Magnitudes
Using a Difference Vector
Our study evaluated both the ETD and CBCT systems
against a shared reference, a phantom displaced
by the treatment couch and we showed that both systems
achieve sub-millimeter precision in error detection.
While some prior literature assumes CBCT as the
ground truth, this methodology may lead to inaccurate
estimations of error magnitudes when comparing ETD
detections directly to CBCT. This can exaggerate the
magnitude, as the difference reflects not only the magnitude
but also the directional offset between the two
systems. Conversely, if the vectorial directions of ETD
and CBCT errors align closely, the magnitude may
appear artificially reduced, masking the true extent of
discrepancies. To evaluate the discrepancy, we computed
a difference vector (DIFF-6D), representing the
positional offset between ETD and CBCT detections.
The mean DIFF-6D was 0.23 mm, larger than both
ETD-6D and CBCT-6D. Statistical analysis confirmed
that the DIFF- 6D vector was significantly greater than
the 6D errors of ETD and CBCT systems (p<0.001 and
p=0.002 respectively) (Fig.
These results highlight the potential overestimation of ETD errors when compared directly to CBCT without an independent reference. While CBCT remains a clinically indispensable tool due to its anatomical visualization capabilities, our findings emphasize the importance of unbiased methodologies for evaluating detection capabilities of imaging systems.
Since thermoplastic masks do not completely eliminate
potential movements during stereotactic treatments,
patient motion of varying magnitudes may occur
between each arc of dose delivery, which can lead
to overdosing of normal tissues near the target.[
Setup and intrafraction movements can be corrected
using the ExacTrac X-ray 6D imaging system, allowing
for reduced GTV-PTV margins.[
CBCT is essential in all radiotherapy treatments
due to its superior visualization of anatomical structures
and soft tissues, offering 3D and transaxial images
before treatment. Geometric uncertainties are detected
more clearly in SRS/SBRT through integrating
multiple imaging techniques.[
In SRS for multiple brain metastases, correct isocenter
positioning during treatment is crucial to ensure
precise dose delivery to all targets.[
To determine the accuracy of the ExacTrac system,
the first step is to verify patient positioning accuracy
relative to the linear accelerator isocenter. Secondly,
imaging and couch corrections must be confirmed.
[
A Winston-Lutz (WL) phantom, with a small metal
sphere inside, is used to determine the ExacTrac X-ray
isocenter relative to the radiation isocenter. The difference
between the sphere"s center projection and the
treatment area center should be <0.7 mm.[
In our study, we determined the radiation isocenter
using similar methods. After examining the 2D images
taken at different gantry, collimator, and couch
angles, the radiation isocenter was determined by
shifting the metal sphere on the ETD phantom with
micrometer precision. The BB phantom provided with
the ETD system was used to ensure sub-millimeter
accuracy in detecting the isocenter, showing minimal
deviation. The ETD system performed accurately in
all measurements taken at 90 different points, detecting
the BB without error, with maximum deviations of
0.1 mm from the radiation isocenter.
Verbakel et al.[
When comparing ExacTrac and CBCT in terms of
setup errors, ExacTrac showed more accurate results.
Chow et al.[
Kojima et al.[
Spadea et al.[
Li et al.[
Since fixed phantoms were used in our study, future
studies will compare the performance of these systems
with real patient data to further evaluate their clinical
applicability.
While CBCT remains indispensable for anatomical visualization in clinical workflows, this study demonstrates that both ETD and CBCT independently achieve the precision required for stereotactic radiotherapy applications. Future research should further investigate system performance in dynamic clinical scenarios especially in pediatric patients to validate these findings.
Conflict of Interest Statement: The authors declare no direct conflict of interests.
Funding: This study was supported by the Ege University Scientific Research Projects Coordination Unit under Project No. 21799. The authors would like to express their gratitude to Ege University for their financial and institutional support.
Use of AI for Writing Assistance: No AI technologies utilized.
Author Contributions: Concept - D.Y., E.T., Y.Z.H., N.O., S.K.; Design - D.Y., E.T., Y.Z.H., N.O., S.K.; Supervision - D.Y., E.T., Y.Z.H., N.O., S.K.; Data collection and/ or processing - E.T., Y.Z.H.; Data analysis and/or interpretation - D.Y., E.T., Y.Z.H.; Literature search - D.Y., E.T.; Writing - D.Y., E.T.; Critical review - D.Y., E.T., Y.Z.H., N.O., S.K.
Peer-review: Externally peer-reviewed.
