Quality Assurance for Image- Guided Radiation Therapy IGRT-QA
Abstract
Image-Guided Radiation Therapy (IGRT) is a
technique that utilizes imaging during each treatment session to enhance the
precision and accuracy of radiation therapy. By guiding the delivery of
radiation to the planned target volume, IGRT ensures that the intended dose is
accurately administered while sparing surrounding healthy tissues. Quality
Assurance (QA) for IGRT6 on a linear accelerator (linac) is crucial to
verify that the system is functioning correctly and delivering the prescribed
dose effectively. The American Association of Physicists in Medicine (AAPM)
Task Group 179 (TG-179) recommends periodic QA9,10 to maintain the integrity
of IGRT systems. Key components of IGRT QA for linacs include ensuring the
alignment of the radiation beam with the target by verifying the accuracy of
the gantry, collimator and couch movements. Assessing the quality of images
used for guidance, such as those from kilovolt (kV) or megavolt (MV) cone-beam
CT (CBCT), is essential for precise patient positioning. Customizing the QA
process for individual patients, taking into account anatomical changes and
setup errors, is also important. Regularly evaluating the performance of the
IGRT system, including the calibration of imaging devices and the accuracy of
dose delivery, ensures consistent performance. Additionally, ensuring that the
IGRT system adheres to regulatory standards and safety guidelines is crucial
for maintaining safety and compliance. By implementing a comprehensive QA
program4, healthcare providers can ensure the consistent and
reliable performance of IGRT systems, ultimately improving patient outcomes.
Keywords: Image-Guided
Radiation Therapy (IGRT), Quality Assurance (QA), Linear Accelerator (Linac),
Varian TrueBeam, Elekta Infinity, Isocenter Verification, Winston-Lutz Test,
MC2 Varian Mobius Phantom, Catphan Phantom, Cone-Beam CT (CBCT), On-Board
Imaging (OBI), Doselab Software, Geometric Accuracy, Image Quality,
Patient-Specific QA, System Performance, Safety Interlocks, AAPM TG-179
1. Introduction
This paper
outlines the IGRT QA protocol implemented at our institution for Linear
Accelerators. Quality Assurance (QA) ensures the safe and efficient application
of technologies. Common QA tests include Imaging System QAs such as kV 2D, MV
EPID and kV CBCT, with the machine isocenter serving as the reference point.
Safety Checks: Daily safety checks are
recommended to prevent collisions between imaging equipment and the couch or
patient. Monthly IGRT QA encompasses image quality checks, scale checks orientation
accuracy, uniformity, noise, high contrast spatial resolution and low contrast
detectability. Annual QA checks include CT number accuracy and stability,
imaging dose and imaging system performance (specifically kV system checks like
kVp and mAs accuracy and stability).
2.
Materials and Methods7-9
At our clinic,
patient treatments are administered using two advanced linear accelerators: the
Varian TrueBeam Linear Accelerator and the Elekta Infinity Linear Accelerator.
To maintain precise and reliable image-guided radiation therapy (IGRT), we have
established a comprehensive quality assurance (QA)1 protocol that encompasses daily and monthly and annual checks,
as well as safety interlock assessments.
2.1. Daily QA procedures
For daily machine
performance evaluations, we utilize specialized phantoms for each linear
accelerator. The Varian TrueBeam system is assessed using the Isocal Phantom,
which ensures the accuracy and stability of imaging and treatment parameters.
For the Elekta Infinity system, we employ the QUASAR™ Penta-Guide Phantom,
which is specifically designed to evaluate the performance of Cone Beam
Computed Tomography (CBCT) imaging systems. These daily checks help identify
any deviations early and maintain consistent imaging quality and treatment
accuracy.
2.2. Monthly QA Procedures
Our monthly QA
procedures involve more detailed imaging quality assessments. For both
megavoltage (MV) and kilovoltage (kV) imaging systems of the Varian TrueBeam
and Elekta Infinity we use the MC2 Varian Mobius Phantom. This phantom
facilitates thorough testing of imaging performance, including spatial
resolution, contrast and geometric accuracy. To verify the CBCT image quality
on both systems, we use the Catphan Phantom, which provides objective metrics
for image quality such as uniformity, noise and low-contrast detectability.
2.3. Isocenter Verification
We perform the
Winston Lutz test to ensure accurate isocenter alignment. This test uses a Cube
Phantom containing a precisely positioned 2mm sphere at its centre, allowing
for high-precision analysis of the congruence between the radiation isocenter,
the mechanical isocenter and the imaging isocenter. Additionally, we use a
marker block with a central fiducial and four internal discs to assess
geometric accuracy and scaling, contributing to accurate patient positioning
and dose delivery.
2.4. Collision checks and safety interlocks
Safety is a critical
aspect of our IGRT protocols. We conduct collision checks to verify the
functionality of safety interlocks in both kV and MV imaging systems. These
interlocks are designed to automatically halt system operations if there is a
risk of a collision between moving components, such as the gantry and the
patient table. In particular, if a collision is detected between the kV/MV
systems and the KVS arm pedal or device body, the motion of the arm, gantry,
couch and On-Board Imager (OBI) console will be immediately stopped (Figure
1). This safety mechanism is vital to protecting both patients and clinical
staff by preventing accidental hardware collisions during imaging and
treatment.
Figure 1:
KVD, KVS and MVD.
2.5. OBI check
On-Board Imaging
(OBI) systems play a crucial role in modern radiation therapy by facilitating
precise patient positioning and ensuring accurate treatment delivery. These
systems integrate both kilovolt (kV) and megavolt (MV) imaging capabilities,
allowing for high-resolution imaging and verification of treatment parameters
before and during the therapy sessions. Maintaining the performance and
accuracy of OBI systems is fundamental to achieving optimal clinical outcomes
and enhancing patient safety.
2.6. Isocenter verification procedure
The isocenter
verification process is designed to confirm the precise alignment of the
radiation beam with the target treatment area. This verification is critical
for maintaining the congruence of the mechanical, radiation and imaging
isocenters, thereby minimizing discrepancies that could affect treatment
accuracy. The verification begins with the setup of a specialized cube phantom
on the treatment couch. This phantom, featuring a 2mm sphere positioned at its
center, is meticulously aligned with the laser or light field markers to
establish a baseline for imaging accuracy. Ensuring this alignment is critical,
as even minor misplacements can lead to significant deviations in treatment
delivery. Following the setup, images are captured using both the kV and MV
imaging systems at multiple gantry angles, specifically at 0, 90, 180 and 270
degrees. These varied perspectives provide a comprehensive assessment of the
imaging system’s alignment with the treatment beam and help identify potential
discrepancies that could arise from mechanical or imaging system inaccuracies (Figure
2). The acquired images are then subjected to detailed analysis. The goal
is to verify that the 2mm sphere at the centre of the phantom appears in a
consistent position across all images, regardless of the imaging modality or
gantry angle. To achieve this, we utilize the Doselab QA analysing software
(Varian Medical Systems), which offers robust image processing and comparison
tools. The analysis is conducted with a stringent tolerance of ±1 mm, ensuring
that only minimal and clinically acceptable variances are permitted. Any
discrepancies beyond this threshold prompt a detailed investigation and
necessary calibration of the imaging system.
2.7. Quality Assurance (QA) procedure for the marker
block with fiducial and discs
In addition to
isocenter verification, our clinic employs a QA procedure involving a marker
block with a central fiducial and four internal discs. This test serves as an
additional layer of validation for the geometric accuracy and scale of the
imaging systems, which is critical for precise patient positioning and
treatment targeting. The process begins by positioning the marker block on the
treatment couch. Special attention is given to ensure that the fiducial is
centrally located and the four discs are correctly oriented within the block.
Proper positioning of the marker block is essential to establish a reliable
reference point for imaging assessments. After setup, kV and MV images are
captured to assess the alignment of the imaging system with the marker block.
These images are carefully analyzed to identify any discrepancies between the
expected and observed positions of the fiducial and discs. Consistency between
the kV and MV images indicates accurate system calibration, while discrepancies
could signal the need for further adjustments.
To test the imaging
system’s performance under dynamic conditions, a controlled table shift is
performed. The treatment couch is moved to a known offset position and a new
set of images is acquired. This step evaluates whether the fiducial and discs
maintain their expected positions relative to the isocenter, even when the
patient positioning changes. Such testing simulates real-world treatment
scenarios where patient movement or positioning adjustments may occur. The next
step involves quantifying any discrepancies between the expected and actual
positions of the fiducial and discs. Precise measurements are taken to
determine whether these differences fall within the acceptable tolerance,
typically within 1mm (Refer to Fig. 3). If the discrepancies exceed this
threshold, adjustments are made to the alignment of the imaging system or the
treatment couch to restore accuracy.
Once adjustments are
implemented, the verification process is repeated to confirm that the system is
performing within the established QA criteria. This iterative approach ensures
that any issues are fully resolved before the imaging system is cleared for
clinical use.
2.8. Significance of QA for marker block with fiducial
and discs
Maintaining a
rigorous QA process for the marker block with fiducial and discs offers several
critical benefits:
2.8.1. Accuracy: By ensuring the radiation beam is precisely targeted at
the fiducial and discs, the QA process directly contributes to the accuracy of
radiation delivery to the intended treatment area.
2.8.2. Consistency: The procedures help maintain consistent
treatment delivery across multiple sessions, which is especially important for
fractionated treatment plans where reproducibility is key.
2.8.3. Safety: The QA process minimizes the risk of unintended
irradiation of healthy tissues surrounding the target area, enhancing overall
patient safety. Through adherence to these detailed QA protocols, our clinic
upholds the highest standards of accuracy, consistency and safety in radiation
therapy treatments. This meticulous approach not only enhances treatment
efficacy but also reinforces our commitment to patient care and safety.
Figure 2:
Cube phantom with 2mm sphere at the centre.
Figure 3: Marker block with 1 fiducial at the
center and four discs inside the block.
2.9. Machine
Performance Check (MPC)2,3,9
The Machine Performance Check (MPC) is a critical
application developed by Varian to verify the mechanical and dosimetric
performance of the TrueBeam Linear Accelerator. This integrated, image-based
tool is a cornerstone of daily quality assurance (QA) practices, ensuring that
the linear accelerator (linac) operates with optimal accuracy and reliability.
Regular use of MPC not only supports high treatment precision but also enhances
patient safety by identifying potential issues before clinical use.
The MPC process begins with the placement of the
IsoCal phantom within the treatment room. This phantom is strategically
positioned at a predefined location, serving as a reference object for various
geometric and dosimetric evaluations. The IsoCal phantom is specifically
designed to facilitate comprehensive system checks, including the verification
of imaging alignment, radiation beam properties and mechanical accuracy of the
linac components (Figure 4a and Figure 4b).
Once the phantom is in place, the system initiates
data acquisition by capturing a series of images with and without the IsoCal
phantom at specific positions. These images include both kilovoltage (kV) and
megavoltage (MV) modalities, providing a dual-perspective assessment that is
crucial for performing accurate geometric and dosimetric checks. The use of
both imaging modalities allows for cross-verification of measurements and
enhances the robustness of the QA process.
2.10. Automated geometric
checks
The MPC system performs a series of automated
geometric checks that assess the mechanical accuracy and alignment of critical
components of the linac. The treatment isocenter, which represents the focal
point where the radiation beams converge, is thoroughly evaluated to confirm
its size and location relative to the imaging devices. Maintaining a precise
treatment isocenter is essential for delivering radiation to the intended
target area while avoiding healthy tissues.
The imaging isocenter coincidence check is another
vital aspect of the geometric evaluation. This check ensures that the kV and MV
imaging systems are accurately aligned with each other. Misalignment between
these imaging systems could lead to discrepancies in patient positioning and
impact treatment accuracy.
In addition to isocenter evaluations, the MPC system
verifies the collimator rotation offset to ensure the accuracy of the
collimator's rotational movements. The collimator shapes the radiation beam and
any deviation in its rotation could affect the treatment field's geometry.
Similarly, gantry positioning is assessed to verify that the gantry moves to
the correct angles during treatment delivery.
The treatment couch, which can move in up to six
degrees of freedom, undergoes checks to confirm its positioning accuracy. These
checks are crucial for patient positioning, especially when complex treatment
plans require precise couch adjustments. The Multi-Leaf Collimator (MLC) leaf
positions are also evaluated for accuracy and reproducibility, as the MLC is
responsible for modulating the radiation beam shape according to the treatment
plan. The jaw positioning, which further defines the treatment field, is
checked to ensure it meets the expected parameters.
2.11. Automated dosimetric
checks
In addition to geometric evaluations, MPC conducts
automated dosimetric checks that focus on the consistency and stability of the
radiation beam. One of the primary checks involves measuring the beam output
constancy. This test assesses whether the radiation dose delivered by the linac
remains consistent over time, which is crucial for achieving the prescribed
treatment dose.
The beam profile constancy is also evaluated to ensure
that the radiation intensity is uniform across the treatment field. Any changes
in the beam profile could affect dose distribution and potentially lead to
under- or over-treatment of certain areas. The beam centre shift check verifies
whether there are any shifts in the central axis of the radiation beam, which
could affect the alignment with the target area.
2.12. Analysis
and reporting
After completing the automated checks, the MPC system
performs an analysis of the acquired images and data. This automated analysis
compares the measured geometric and dosimetric parameters against predefined
tolerances set by clinical protocols and manufacturer specifications. By
automating this process, MPC minimizes the risk of human error and enhances the
efficiency of the QA process.
The system generates a detailed report that summarizes
the results of all checks performed during the MPC procedure. This report
includes quantitative measurements, graphical representations of image analyses
and indications of whether the parameters meet the acceptable limits. The
report serves as a critical document for clinical record-keeping and for
identifying any trends in system performance that may require attention.
2.13. Review and
corrective actions
Once the report is generated, it is reviewed by a
qualified medical physicist or a member of the QA team. This review process
involves assessing whether all measured parameters fall within the established
tolerance limits. If the results are within acceptable ranges, the linac is
cleared for clinical use.
However, if any parameters are found to be out of
tolerance, immediate corrective actions are implemented. These actions may
include recalibrating the imaging system, adjusting mechanical components or
performing additional tests to isolate the cause of the discrepancy. The goal
of these corrective measures is to restore the linac’s performance to optimal
levels before it is used for patient treatments.
The MPC procedure is designed to be highly automated,
which not only reduces the need for manual intervention but also enhances the
consistency and repeatability of the QA process. By automating data
acquisition, analysis and reporting, MPC ensures that the linac’s performance
is thoroughly evaluated on a daily basis with minimal variability in the QA
outcomes.
Figure 4: a) Isocal Phantom b) MPC SET UP
2.14. Digital measurement
accuracy for On-Board Imaging (OBI)
To verify the digital measurement accuracy of the
On-Board Imaging (OBI) system, an additional procedure involving the blade
calibration tool is conducted. The blade calibration tool is placed precisely
at the isocenter of the treatment field, providing a reference for evaluating
the digital measurement accuracy of the OBI system.
A kV image is acquired with the calibration tool in
place. The image is then analysed using the measuring tools integrated into the
OBI system. Specifically, the superior/inferior (S/I) and right/left (R/L)
dimensions of a 10 cm x 10 cm square are measured. The expected measurement
value is 10 ± 0.1 cm, providing a stringent criterion for digital measurement
accuracy (Figure 5).
By adhering to this procedure, the accuracy of the
digital measurements taken by the OBI system is validated, supporting
high-precision imaging and treatment delivery. This step is particularly
important for maintaining spatial accuracy in patient positioning and ensuring
that the treatment plan is executed as intended.
Through the comprehensive execution of the MPC and
digital measurement accuracy procedures, our clinic maintains a robust QA
framework that underpins the safety, accuracy and reliability of our radiation
therapy services. These practices reinforce our commitment to delivering
high-quality care and achieving optimal treatment outcomes for our patients.
Figure 5: Digital Measurement Accuracy for OBI.
2.15. Mechanical
position QA: KVD and KVS
The Mechanical Position Quality Assurance (QA) process
for kilovolt detectors (KVD) and kilovolt sources (KVS) is an essential step in
maintaining the precision of imaging systems used in radiation therapy. One of
the critical components of this process is the mechanical centre check, which
ensures that the imaging components are accurately aligned with the treatment isocentre.
To conduct the mechanical centre check, the gantry is
first rotated to a 90-degree position at the coordinates (0,0,100). This
specific positioning allows for precise measurement of distances relative to
the isocenter. The QA procedure involves measuring the distance between the
isocenter and the surface of the KVS, as well as the distance between the
isocenter and the grid surface. These measurements are vital for verifying the
alignment of the imaging components with the treatment beam. Maintaining a tolerance
of ±2 mm for these measurements is crucial, as even small deviations can lead
to inaccuracies in patient positioning and treatment delivery.
By routinely performing this mechanical center check,
our clinic ensures that the KVD and KVS systems remain within acceptable
operational parameters, thereby contributing to the overall accuracy and safety
of radiation therapy treatments.
2.16. Image quality
QA
Image quality assurance (QA) is a systematic process
designed to ensure that imaging systems consistently produce high-quality
images. This process involves regular data recording, analysis and evaluation
to monitor image quality over time. By implementing a structured QA program,
our clinic can identify potential issues early and make data-driven decisions
to enhance image quality, which is critical for accurate diagnosis and
treatment planning.
One of the primary tools used in our image quality QA
process is the MC2 Varian Mobius Phantom. This phantom is specifically designed
for both megavoltage (MV) and kilovoltage (kV) imaging QA, allowing for a
streamlined approach to testing. During the QA process, the MC2 phantom is
positioned within the imaging system according to established protocols (Figure
6a). Since the phantom is compatible with both MV and kV imaging, only a
single setup is required, which improves efficiency and reduces setup variability.
The imaging procedures involve capturing images with
both MV and kV modalities. The MC2 phantom contains various test objects that
assess critical image quality parameters, including resolution, contrast,
noise, uniformity and image scaling. These parameters are indicative of the
imaging system's ability to produce clear and accurate images under clinical
conditions.
Once the imaging is completed, the acquired images are
analysed using Dose Lab TG-142 software. This software is designed to automate
the analysis process by comparing the imaging metrics against the TG-142
guidelines. It provides quantitative assessments of resolution, contrast, noise
levels, image uniformity and scaling accuracy (Figure 6b and 6c). The
automated nature of the software minimizes human error and ensures a consistent
evaluation of the imaging system's performance.
Figure 6: a) MC2 Varian
Mobius Phantom b) KV Image c) MV
Image
3. CBCT Image Quality: Catphan CTP
604 Phantom
In addition to the MC2 Varian Mobius Phantom, Catphan
phantoms are employed for cone-beam computed tomography (CBCT) imaging quality
assurance. These phantoms are critical for evaluating CBCT systems' performance
across a range of parameters, including uniformity, contrast, contrast-to-noise
ratio (CNR), spatial resolution, slice thickness and geometric distortion.
The process begins by accurately positioning the
Catphan phantom on the treatment couch following the manufacturer's guidelines.
Two CBCT scans are performed as part of the QA protocol: a head scan at 100 kVp
and 20 mA and a pelvis scan at 125 kVp and 80 mA. Both scans use a
source-to-image distance (SID) of 150 cm, a reconstruction matrix of 512 x 512
and a slice thickness of 2.0 mm. These specific settings replicate clinical
conditions and provide a thorough assessment of the imaging system's
capabilities.
The images generated from these scans are then
analyzed using Doselab Software (Figure 7). The software evaluates
uniformity across the entire field of view, which is critical for consistent
image quality. It also measures the contrast resolution to determine the
system's ability to differentiate between varying tissue densities. The contrast-to-noise
ratio (CNR) is assessed to gauge image clarity, balancing contrast with the
inherent noise of the imaging system.
Spatial resolution tests are conducted to verify the
system's ability to detect small anatomical details, which is crucial for
identifying subtle changes in tissue. Slice thickness accuracy is measured to
ensure that the z-axis representation of scanned objects is true to life.
Finally, geometric distortion checks are performed to identify and correct any
discrepancies in the image that might arise from system inaccuracies.
Figure 7: Catphan CTP604.
3.1. Image quality
(EPID) for treatment verification2,5,9
Electronic Portal Imaging
Devices (EPIDs) play a vital role in radiation therapy by providing real-time
imaging to verify patient positioning and monitor treatment delivery. These
devices use MV imaging, which offers the advantage of capturing images directly
with the treatment beam, thereby aligning the treatment verification process
with the therapeutic dose delivery.
To ensure high image quality
with EPIDs, our clinic utilizes both the MC2 Varian Mobius Phantom and the Las
Vegas Phantom (Figure
8). The Las Vegas
Phantom, in particular, is instrumental in evaluating the geometric accuracy
and contrast resolution of MV images.
The QA process involves
setting up the Las Vegas Phantom at the top of the MV detector (MVD). The
phantom is carefully positioned to ensure it is entirely within the field of
view of the imaging system. Planar MV images of the phantom are then acquired using
the EPID. These images are analysed to assess geometric accuracy by examining
the spatial arrangement of the holes in the phantom. Accurate positioning of
these holes in the images indicates that the EPID is correctly aligned with the
treatment isocentre.
In addition to geometric
checks, contrast resolution is evaluated by analysing the visibility of holes
with varying diameters and depths. This assessment helps determine the EPID's
ability to differentiate between subtle contrasts in the imaging field. The
results of the contrast resolution analysis are compared to the manufacturer's
specified values for each energy level to ensure compliance with clinical
standards.
By maintaining rigorous
image quality QA protocols for both CBCT and EPID systems, our clinic ensures
that all imaging modalities used in radiation therapy meet the highest
standards of accuracy and reliability. This approach is integral to achieving
precise patient positioning, effective treatment planning and safe delivery of
therapeutic doses.
Through these meticulous QA
processes, our clinic upholds a commitment to delivering high-quality care
while minimizing risks associated with imaging and treatment delivery in
radiation therapy.
2.5X
6X
Figure 8: Las Vegas
Phantom and MV Images for different Energies.
4. Results and Discussion
The comprehensive Quality Assurance (QA) analysis was
conducted using DoseLab QA analyzing software, focusing on a range of imaging
and mechanical performance parameters of our radiation therapy systems. By
systematically evaluating these metrics, we aimed to ensure that all imaging
systems, including On-Board Imaging (OBI), Cone-Beam Computed Tomography (CBCT)
and Electronic Portal Imaging Devices (EPIDs), meet stringent clinical
standards.
4.1. Winston-lutz
test results
The Winston-Lutz test, a critical procedure for
verifying the isocentric accuracy of the treatment machines, was conducted
using the Varian cube phantom. This test involved acquiring images at various
gantry, collimator and couch angles to assess the precision of the radiation
beam's alignment with the treatment isocenter. The cube phantom, which features
a 2mm sphere at its centre, served as the focal point for these evaluations. The
images acquired during the Winston-Lutz test were analysed using Dose Lab
software, focusing on the displacement of the sphere relative to the treatment
isocenter. The analysis revealed that the isocentric deviation remained within
the acceptable tolerance of ±1 mm across all tested angles (Figure 9a and
9b). This result indicates that both the Varian TrueBeam and Elekta
Infinity Linear Accelerators are maintaining precise mechanical and imaging
alignment, which is crucial for ensuring accurate treatment delivery and
patient safety. The consistent results across different gantry, collimator and
couch positions further demonstrate the stability of the imaging and treatment
systems, reducing the risk of geometric uncertainties during clinical use.
Figure 9: a)
b)
4.2. Image quality
QA: MV and kV imaging analysis
For the quality assurance of both megavoltage (MV) and
kilovoltage (kV) imaging systems, the MC2 Varian Mobius Phantom was employed.
This phantom offers a robust platform for evaluating multiple image quality
parameters, including resolution, contrast, noise, uniformity and image
scaling.
The QA procedure involved capturing both MV and kV
images of the phantom under clinical conditions, followed by an in-depth
analysis using DoseLab software. The software provided detailed metrics that
allowed us to quantify the performance of the imaging systems.
The resolution assessment showed that the imaging
systems could clearly distinguish fine structures within the phantom,
indicating a high degree of spatial accuracy. The contrast evaluation confirmed
that the systems could effectively differentiate between varying tissue
densities, enhancing the visibility of anatomical structures. Noise levels were
consistently low, contributing to clearer and more precise images.
Uniformity checks across the imaging field
demonstrated a balanced and consistent image quality, which is vital for
reducing variability in treatment planning. The image scaling results confirmed
that the imaging systems maintained accurate geometric representation, ensuring
that measurements taken from images are reliable for clinical decision-making (Figure
10a and 10b).
These findings validate that both the MV and kV
imaging systems are performing within the recommended guidelines, ensuring
their readiness for clinical application and supporting high-precision patient
treatments.
Figure 10: a) MV Image Analysis b) KV
image Analysis
4.3. CBCT image
quality evaluation10
The image quality of Cone-Beam Computed Tomography
(CBCT) systems, integrated into both the Elekta Infinity and Varian TrueBeam
Linear Accelerators, was also thoroughly evaluated. The CBCT QA process
involved using Catphan phantoms, which are specifically designed to test
various aspects of image quality. The CBCT imaging systems were subjected to
rigorous testing across key parameters, including uniformity, contrast,
contrast-to-noise ratio (CNR), spatial resolution, slice thickness and
geometric distortion. These tests are integral to ensuring that CBCT images
provide accurate and reliable data for patient positioning and treatment
verification.
Uniformity analysis demonstrated that the CBCT systems
maintained consistent image quality across the entire field of view, reducing
the likelihood of image artifacts that could compromise treatment accuracy. The
contrast and CNR evaluations highlighted the systems' ability to differentiate
between different tissue types while maintaining clear and sharp images.
Spatial resolution tests confirmed that the CBCT systems could accurately
depict fine anatomical details, which is critical for precise treatment planning.
The slice thickness analysis validated that the z-axis representation of
scanned objects matched clinical specifications, contributing to accurate 3D
reconstructions of patient anatomy.
Geometric distortion checks indicated minimal deviation, ensuring that CBCT images accurately represent the spatial relationships within the scanned volume. These findings align with clinical standards and reinforce the systems' suitability for use in daily clinical practice (Figure 11).
Figure 11: CBCT Image Quality Analysis.
5. Discussion: Clinical
Implications of QA Results
The robust performance of our imaging systems, as
demonstrated by the QA results, underscores the effectiveness of our QA
protocols. Maintaining image quality within specified tolerances is critical
for achieving precise patient positioning and treatment accuracy, which
directly impacts clinical outcomes.
The success of the Winston-Lutz test with deviations
well within the ±1 mm tolerance highlights the mechanical precision of our
linear accelerators. This precision is essential for ensuring that the
radiation beam accurately targets the treatment area, minimizing the exposure
of healthy tissues.
The excellent performance of the MV and kV imaging
systems, particularly in terms of resolution, contrast and noise management,
enhances the accuracy of patient setup and treatment verification. These
imaging modalities play a crucial role in Image-Guided Radiation Therapy
(IGRT), where accurate image quality translates to improved treatment delivery.
CBCT systems, with their demonstrated high-quality
imaging metrics, provide an additional layer of accuracy by allowing for 3D
visualization of the treatment area. This capability is particularly beneficial
for adaptive radiation therapy, where treatment plans may be modified based on
daily imaging results.
Overall, the consistent and high-quality performance
of our imaging systems contributes to a safer and more effective radiation
therapy environment. These results not only validate our current QA practices
but also reinforce our commitment to maintaining rigorous standards in clinical
imaging and treatment processes.
By continuously monitoring and analysing QA data, we
can proactively address any emerging issues, ensuring that our systems remain
reliable and that patient treatments are delivered with the highest possible
accuracy. These efforts ultimately enhance patient safety and improve treatment
outcomes, aligning with our clinic's mission to provide exceptional care in
radiation oncology.
6. Conclusion
Establishing a comprehensive Quality Assurance (QA)
program is essential for monitoring the mechanical stability and image quality
of Image-Guided Radiotherapy (IGRT) systems. Regular and systematic testing is
crucial for detecting any performance deficits that may arise. Although the
specific implementation, frequency and tolerances of QA tests can vary
depending on the institution and equipment, having a well-defined set of
guidelines is imperative. These guidelines ensure that the system performance
remains consistent and reliable, ultimately contributing to the safety and
efficacy of patient treatments.
A robust QA program should include daily, monthly and
annual checks, each targeting different aspects of the IGRT system. Daily
checks focus on immediate safety and operational readiness, while monthly
checks delve into more detailed image quality and mechanical accuracy
assessments. Annual checks provide a comprehensive evaluation of the system's
overall performance, including dosimetric accuracy and long-term stability. By
adhering to these guidelines, institutions can maintain high standards of
treatment delivery, minimize the risk of errors and ensure that patients
receive the best possible care. Continuous monitoring and timely corrective
actions based on QA findings are key to sustaining the optimal performance of
IGRT systems.
7.
Reference
- Klein EE, et al. "Task Group
142 report: Quality assurance of medical accelerators," Med. Phys, 2009;36: 4197-4212.
- Olch AJ. "Quality assurance of electronic portal imaging
devices in radiation therapy," Med
Phys, 2006;33: 1938-1952.
- Boyer
AL, et al. "Basic applications of multileaf collimators," Med Phys, 1994;21: 1261-1271.
- Pawlicki AJ, Dunscombe PB, Boyer AL and Mundt CR. "Quality
assurance in radiation oncology: A primer for physicists and
oncologists," Med. Phys.,
2005;32: 1202-1203.
- Siebers JV, LoSasso J and Chui CS. "A method for improving the
accuracy of portal dose images for dosimetric verification," Med. Phys., 2000;27: 1051-1059.
- Wong JW, et al.
"The use of active breathing control (ABC) to reduce margin for breathing
motion," Int J Radiat Oncol
Biol Phys, 1999;44: 911-919.
- Olch AJ. "Dosimetric performance of an amorphous silicon
electronic portal imaging device," Med Phys, 2005;32: 2536-2544.
- Klein EE, et al. "Task Group
142 report: Quality assurance of medical accelerators," Med. Phys, 2009;36: 4197-4212.
- Olch AJ. "Quality assurance of electronic portal imaging devices in radiation therapy," Med Phys, 2006;33: 1938-1952.
- Boyer
AL, et al. "Basic applications of multileaf collimators," Med Phys, 1994;21: 1261-1271.