Three-dimensional semiconductor detector array ArcCHECK for radiation detection and quality assurance of medical linear acceleration

doi:10.21203/rs.3.rs-2772366/v1

Monitoring the radiation is an integral aspect of the physical practice of linear accelerators used in medical facilities to identify and treat radiation exposure. It is challenging for conventional two-dimensional dose test methodologies to meet the testing requirements of the volumetric modulated arc therapy (VMAT) irradiation technology. This study aimed to analyze multiple dose variations and mechanical correctness in linac operation using the ArcCHECK phantom and verify the accuracy of the method to improve the efficiency and precision of accelerator radiation monitoring.

Health sciences/Oncology/Cancer

Physical sciences/Physics/Nuclear physics

Physical sciences/Engineering/Biomedical engineering

Volumetric modulated arc therapy technology 1

ArcCHECK 2

Quality assurance 3

Radiation detection 4

semiconductor detector 5

Radiation monitoring is an important part of the physical practice of large linac equipment in medical institutions. A medical linac produces many types of radiation with high energy, which are used in radiotherapy to improve the tumor control rate. However, high-energy rays with strong penetration abilities are harmful to the human environment. Therefore, it is necessary to conduct in-depth radiation testing and quality assurance for linacs.

Volumetric modulated arc therapy (VMAT) is a way to create arc therapy treatment plans based on conventional intensity-modulated radiation therapy (IMRT) ^1–3. Continuous single (or multiple) arc therapy in VMAT can be achieved with gantry rotation speed, multi-leaf collimator (MLC) leaf movement, and dose rate variation during radiation delivery, which is more complex than IMRT. For VMAT treatment plans, the accuracy of the MLC position, position deviation caused by gravity during gantry rotation, and beam flatness and symmetry have a significant impact on the implementation of radiotherapy ^4,5. Hence, there is a need to conduct daily radiation detection and quality assurance research on the VMAT technique to ensure the safe and accurate implementation of radiotherapy. The Netherlands Commission on Radiation Dosimetry (NCS) Report 24 issued VMAT-related quality assurance (QA) for the linac regarding individual testing and combined validation of the gantry, MLC motion, and dose rate variation ⁶.

The ArcCHECK SunNuclear (SNC) Machine QA is a cylindrical semiconductor ionization chamber detector. Its special three-dimensional (3D) detector distribution allows for precise QA in both geometric and delivery aspects ⁷. Many studies have investigated linac quality management in ArcCHECK. The majority of studies have focused on measured data in terms of gantry, MLC, flatness, and symmetry. The duration of this study was short. Except for the isocenter deviation, which was continuously measured for 12 months, the other parameters had only one set of data. The study neither assessed the long-term stability nor measured its sensitivity to machine errors. In addition, the independent verification method is not a common daily or monthly inspection method used in medical centers.

In this study, ArcCHECK was used as the detector to verify its feasibility for linac QA. The measurements were compared with those of conventional QA methods over the long term. The QA test items performed in this study included gantry (angle, isocenter, and velocity) QA, MLC QA, dynamic beam flatness, and symmetry QA, which were measured continuously for 12 months. The stability of the method and its sensitivity to machine offset were evaluated through monitoring research using twice-monthly test data and approximately 200 daily inspection data.

2.1 Equipment

2.1.1 Accelerator

The linac used in this study was TrueBeam linac from Varian with a test energy of 6 MV.

2.1.2 ArcCHECK Phantom

ArcCHECK phantom was used for verification. The ArcCHECK is a cylindrical phantom made of polymethyl methacrylate (PMMA) with a 3D array of 1386 semiconductor detectors with a spacing of 10 mm and diameter cavity of 15 cm in the center for placing the ionization chambers and homogeneous inserts. ArcCHECK also has two inclinometers to measure the rotation angle around the cylinder axis and tilt of the axis. The temperature sensor measured the ambient temperature of the detector area. ArcCHECK enables the measurement of absolute and relative values in real-time, with measurements saved every 50 ms ^8,9. In addition to being used for Patient-Specific QA, we used its Machine QA feature, which enables gantry QA, MLC QA, and beam flatness and symmetry QA.

2.1.3 Daily QA devices

2.1.3.1 MPC

The machine performance check (MPC) application is an EPID image-based tool for verifying the TrueBeam linac, including mechanical isocenter deviation, MLC leaf deviation, and beam uniformity deviation. The aSi1000 EPID utilizes a 40 × 43 cm² panel with a backscatter absorber plate between the detection panel and gantry. The detector matrix is 1024 x 768 with a resolution of 0.39 mm ^10,11.

2.1.3.2 Winston-Lutz (EPID)

The Winston-Lutz test is used to verify the consistency of the beam and mechanical isocenter ^12,13. A BB phantom and EPID were used to collect the images. The results were analyzed and counted using a 3D function in SunCHECK.

2.1.3.3 Iso-Align front pointer verification

Iso-Align was used as an isocenter verification tool. Insert the Front Pointer tray with the Front Pointer rod and rotate the gantry 0–360° to visually detect and record the front pointer deviation.

2.1.3.4 Picket fence (EPID)

The Picket fence method was used to verify the MLC in place deviation, and the test was performed during gantry rotation ¹⁴. The images were collected using EPID during beam-out and then imported into the SunCHECK software for analysis.

2.1.3.5 Flatness & Symmetry (EPID)

A flatness and symmetry phantom with a test field size of 10 cm × 10 cm was used. The images were received by the EPID image plate and then analyzed and counted using SunCHECK software.

2.1.3.6 Ionization chamber

The measurement of the beam flatness and symmetry was performed using a PTW OCTAVIUS Detector 729 two dimensional (2D)-array ionization chamber and solid water phantom at SSD 100 cm, underwater 10 cm, with a test field size of 20 × 20 cm ¹⁵.

2.2 Methods

The TrueBeam linac was used to evaluate the performance of the ArcCHECK SNC Machine QA tool while using different detection tools for comparison. Table 1 lists the QA-related items, independent verification methods, and test frequencies (Table 1).

Table 1

QA-related items, independent verification methods, and test frequencies.
Item		Equipment	Frequency
Gantry QA	Gantry isocenter, The correlation between gantry isocenter and angle, The correlation between the rotation speed of gantry and angle	ArcCHECK	Monthly
		MPC isocenter size	Daily
		Winston-lutz-EPID	Monthly
		Iso-Align front pointer verification	Monthly
MLC QA	Absolute mean deviation	ArcCHECK	Monthly
		MPC MLC test	Daily
		picket fence-EPID	Monthly
Flatness & Symmetry QA	Flatness / Symmetry	ArcCHECK	Monthly
		EPID	Daily
		Ionization chamber	Monthly

2.2.1 Gantry

Gantry QA can be used to test the gantry angle, speed, and isocenter deviation. The Gantry QA tests can be implemented in static or dynamic delivery. A) For static beam delivery, measurements were performed using an ArcCHECK phantom. The Jaw was set to a specific field of X = 1 cm and Y = 25 cm, and a collimator angle of 9° and at least 50 MU at any desired gantry angle were delivered without stopping measurement. Test data for all angles were stored in the same file and as a .acm file after the measurements were completed. B) For dynamic beam delivery, we employed Jaw and MLC to create a 1 cm x 25 cm "plus" ( + ) shaped field in both X and Y directions, as shown in Fig. 1, and obtained a gantry angle of 181–179°, collimator angle of 9°, and at least 1200 MU. Measurements were performed using the ArcCHECK phantom, and all data were stored as. acm file. The principle of the method is that the measurements are collected in ArcCHECK. Because the position and shape of the beam are known, both the shortest distance from each collimator angle to the isocenter can be calculated, and the actual deviation can be calculated from the angle between the incident beams. In this study, we used both static and dynamic QA test methods, and continuous monitoring for 12 months. Independent verification, with continuous monitoring for 12 months, using daily MPC, monthly Winston-lutz tests, and monthly Iso-Align front pointer validation as comparison.

2.2.2 MLC

MLC errors can be evaluated by measuring and analyzing the differences between MLC and Jaw positions. The analysis required two measurements: the first for the jaw to form a fixed-size field (MLC retraction) and the second for the MLC to form a fixed-size field (jaw retraction). The two measurement fields were identical in size, and the corresponding deviation of the beam edge detector corresponded to the MLC or jaw-in-place error. The measurement field size can be set according to the user interface guidelines. Two plans were generated on the Eclipse treatment planning system with the same field size and at least 50 MU, one of which only used jaw and the other only used MLC. The ArcCHECK phantom was used for the measurements, and all data for the Jaw and MLC were stored separately as. acm files.

To obtain the synthesis results of the MLC and Jaw measurements, the penumbra transfer fit function needs to be measured. The penumbra transfer function (PTF) is a fit of the penumbra shape used to convert the percentage difference between the MLC and Jaw measurements to a distance in millimeters. Each linac has a specific PTF. Once the PTF is generated, the parameters are saved in the ASCII file format on the local computer. After the measurements are saved, the software reads two measurements (Jaws and MLC) and normalizes them to their maximum value (the maximum value of each measurement is 1). It calculates the percentage difference between the Jaw and MLC as one result, and then calculates the distance difference between the MLC and Jaw using the PTF.

Independent verification, with continuous monitoring for 12 months, using daily MPC and monthly Picket Fence tests as a comparison.

2.2.3 Flatness & Symmetry

The flatness and symmetry test, using the ArcCHECK phantom, measured the flatness and symmetry of the beam in the Y-axis direction and the symmetry in the X-axis direction. Similarly, both dynamic and static deliveries exist. A) For static beam delivery, a field size of X = 25 cm and Y = 25 cm at the isocenter was created in Eclipse, at least 50 MU at any selected gantry angle was delivered without stopping measurement, and all data were stored in one file. B) For dynamic beam delivery, a field size of X = 25 cm and Y = 25 cm was created in Eclipse with a full arc and at least 1200 MU was delivered. In this study, we used a dynamic QA test method and continuous monitoring for 12 months. Independent verification used the daily test EPID flatness and symmetry analysis results and monthly test results in the 2D-array ionization chamber for comparison.

3.1 Gantry angle, gantry speed, and gantry rotation

Fig. 2 shows the results of the initial gantry speed measurement, where the average gantry speed is 3 °/s and the maximum speed deviation is approximately 0.1°/s. The left side of Fig. 2 shows the gantry start and end angles, total angle of movement, beam-out time, average gantry speed in degrees/second, and maximum deviation of the gantry speed relative to the average, whereas the right side shows a plot of the gantry rotation speed versus the gantry angle. Each point in the graph represents an update (once every 50 ms). Over a period of 12 months, the average speed of the gantry was 3 °/s and the average maximum speed deviance was 0.1 °/s. Fig. 3 shows the results of the initial gantry isocenter measurements. The left side of this figure shows the isocenter schematic, whereas the right side shows the magnified results of the isocenter schematic, illustrating the predicted center deviation from the beam center. Fig. 4 shows the first measurement distance between the center of each angular beam and ArcCHECK isocenter. The distribution of points in Fig. 4 shows the trend during the gantry rotation.

Fig. 5 compares the gantry isocentric deviation validation results for ArcCHECK, MPC, Winston-lutz, and Iso-Align. As shown in the figure, the four datasets exhibited the same tendency. The mean and standard deviation of the isocentric deviation at 12 months for ArcCHECK was 0.292 ± 0.016 mm. Over the 12 months, the mean and standard deviation of the isocenter deviations for the MPC, Winston-lutz, and iso-alignment tests were 276 ± 0.021, 0.256 ± 0.017, and 0.292 ± 0.028 mm, respectively. ArcCHECK yielded superior results compared to MPC and Winston-lutz. This is probably because the ArcCHECK measurements were calculated using dynamic measurements of the gantry rotation.

3.2 Multi-leaf collimator

Fig. 6 shows a schematic of the initial MLC test results. The upper left panel displays the findings of the jaw test, the upper right panel displays the results of the MLC test, and the lower panel displays the MLC/jaw overlap difference graph. As shown in the illustration, the beamsides overlap. If the MLC and Jaw are precisely aligned, the difference will be near zero (green), and the color changes to yellow and red if the variation is greater than zero. If the deviation is greater than zero, the MLC is offset from the Jaw.

Fig. 7 shows the MLC test results for ArcCHECK, MPC, and the Picket Fence. It displays the maximum leaf deviation values of the MLC during the 12 months. The figure illustrates that the trends of the three measurement techniques were consistent. In the ArcCHECK test, the average maximum leaf deviation of MLC was 0.392 ± 0.049 mm. The average maximum leaf deviations of MPC and PicketFence were 0.482 ± 0.049 and 0.355 ± 0.031 mm, respectively.

3.3 Flatness and symmetry

Fig. 8 shows a schematic of the results of the first measurement of the dynamic flatness. The results of the flatness in the Y-axis direction tested in ArcCHECK, EPID, and the ionization chamber matrix are shown in Fig. 9. Fig. 10 and 11 show the results of the symmetries of the x- and y-axis directions of the radiation field tested in the ArcCHECK, EPID, and ionization chamber matrices, respectively. The results of ArcCHECK, EPID, and the ionization chamber matrix were 101.02% ± 0.21%, 100.86% ± 0.17%, and 100.90% ± 0.18% in the X-axis direction and 101.11% ± 0.21%, 100.95% ± 0.17%, and 100.91% ± 0.18% in the Y-axis direction, respectively.

As shown in Fig. 9 - 11, the trends in flatness and symmetry over the past 12 months were consistent.

Currently, VMAT and IMRT are the most widely used radiation treatments. It is feasible to achieve a highly conformal dose distribution during IMRT therapy by adjusting the flux intensity in each subbeam. However, to achieve a suitable conformal dose distribution for IMRT, the MU in IMRT is substantially greater than that in traditional radiation treatment. This can lead to prolonged treatment time and higher intra-fraction motion in the patient. Furthermore, high MU levels may result in greater OAR dosages and an increased risk of secondary cancers ^16,17.

Since 2008, VMAT technology ^{3, 18–20} has been used as a revolutionary radiation therapy modality that solves the numerous drawbacks of IMRT techniques. Additionally, it can be administered at a highly conformal dosage distribution. VMAT differs from IMRT in that the linac gantry rotates around the patient when the beam is activated. The acceleration gantry rotation speed, dosage rate, and beam shape can be constantly modified simultaneously. It is possible to provide radiation treatment while the gantry is rotating. It significantly increases radiation therapy efficiency, decreases treatment time, and may deliver radiation coverage to the exact target area with fewer doses. It offers greater protection to organs at risk without lowering the tumor dose (Gerald J. Kutcher et al., 1994; Nath et al., 1994.).

The TG142 report mentions that under routine gantry QA, the gantry isocenter deviation is an annual verification measurement item ²³. However, the use of only normal QA items in VMAT treatment mode is insufficient. In this study, the Winston-Lutz approach is utilized for the independent verification of the gantry isocenter test, which allows the isocenter deviation of the radiation field to be determined. Nonetheless, the isocenter deviation of the gantry during rotation could not be evaluated because it was measured under a static field. The mechanical isocenter was validated using the iso-align phantom, which is both convenient and efficient. However, the radiation field isocenter cannot be validated, and the human component reduces measurement accuracy. A variety of VMAT model-based validation programs and techniques have been proposed; for example, approaches based on EPID pictures to validate the gantry can provide a combined validation result of gantry angle and dose rate, but individual information about gantry angle cannot be retrieved ^11,24,25. As previously stated, the dose rate, gantry speed, and MLC leaf position of VMAT constantly change. In addition, ArcCHECK offers the benefit of a comprehensive measurement strategy for VMAT quality assurance. Using the advantage of the arc detection point, ArcCHECK analyzes the isocenter location acquired as the isocenter of the rotating radiation field of the gantry by utilizing multiple incident beam angles and outgoing beam intensities. Consequently, the average outcomes over the 12 months of our investigation were greater than the Winton-Lutz values. The experiments in this study revealed that the mean and standard deviation of the rotating gantry isocenter for 12 months were 0.292 ± 0.016 mm, and the dynamic gantry isocenter findings were all less than 1 mm during the measurement procedure, which conforms to the tolerance specified in the TG142 report ²³.

During beam on, the MLC continues to translate at a variable rate. According to the TG-142 report, the maximum deviation in leaf motion should be measured monthly ²³. Many approaches to MLC quality assurance, such as the Picket Fence test and linac log file analysis ²⁶, have been proposed. In this study, the MPC and Picket Fence tests, which are both based on EPID image analysis to assess MLC in-position accuracy, were employed as independent verifications. MPC analyses are based on the collimator's center of rotation (Barnes and Greer 2017b), whereas Picket Fence analyses are based on the image's center ¹⁴. In contrast, ArcCHECK evaluates the leaf-in-place accuracy by examining the jaw’s alignment with the MLC. LoSasso et al. studied MLC leakage and found that MLC leakage increased with the field size created by the jaw and increased with the beam energy ²⁷. Varian's TrueBeam linac has jaw tracking technology (JTT), which allows the jaw to move with the MLC, lowering leakage from the MLC leaves and hence the OAR dose at the target area ²⁸. ArcCHECK is ideally suited for examining linac with the JTT and for examining jaw alignment with the MLC. In this study, it was determined that the results of ArcCHECK and independent verification were sufficiently consistent.

Flatness and symmetry QA are the fundamental parameters of monthly QA ²³. In regular quality assurance, the gantry angle for evaluating flatness and symmetry is 0° and is assessed in static gantry mode. In this study, flatness and symmetry measurements were performed in the dynamic gantry mode. Because there is no flatness symmetry test item in the MPC, the daily independent verification inspection uses the EPID image results. In addition, we employed monthly ionization-chamber matrix measurements for independent verification. The independent verification results and ArcCHECK test results were consistent. However, owing to the uniqueness of the ArcCHECK phantom form, the flatness in the x-direction cannot be measured. Consequently, it has not been suggested as a standard tool for routine QA. However, ArcCHECK evaluates the symmetry of flatness during gantry rotation, which is more appropriate for quality assurance in VMAT mode.

Radiation monitoring is an integral aspect of the physical operation of large linac equipment in medical institutes. It is challenging for traditional 2D dose-testing methodologies to satisfy the testing requirements of the VMAT arc radiation technology. In this study, ArcCHECK 3D arrays of cylindrical semiconductor radiation detectors were utilized to determine the gantry isocentric deviation, MLC in-place deviation, and beam flatness symmetry in the linac mode of operation utilizing measurement results from a particular radiation dose delivery method. To establish a relationship between dose deviation and mechanical performance, twelve months of long-term monitoring and comparison of the experimental outcomes with normal daily and monthly testing were performed.

ArcCHECK can be utilized as a high-confidence quality assurance tool for VMAT therapy technology and can execute multiple linac dynamic performance quality assurance, which is beneficial for clinical applications.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Funding

JD was supported by grants from the Health research Program of Shaanxi Provincial Health Commission (No. 2022D037). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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No competing interests reported.

Three-dimensional semiconductor detector array ArcCHECK for radiation detection and quality assurance of medical linear acceleration

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials And Methods

2.1.1 Accelerator

2.1.2 ArcCHECK Phantom

2.1.3 Daily QA devices

2.2.1 Gantry

2.2.2 MLC

2.2.3 Flatness & Symmetry

3 Results

3.1 Gantry angle, gantry speed, and gantry rotation

3.2 Multi-leaf collimator

3.3 Flatness and symmetry

4 Discussion

5 Conclusion

Declarations

References

Additional Declarations

Status:

Version 1