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Original Article

Progress in Medical Physics 2024; 35(2): 45-51

Published online June 30, 2024 https://doi.org/10.14316/pmp.2024.35.2.45

Copyright © Korean Society of Medical Physics.

Analysis of the Multi-Leaf Collimator Quality Assurance for the HalcyonTM Linear Accelerator

Jin Jegal1,2 , Hyojun Park1,2 , Seonghee Kang1,2,3 , Jung-in Kim1,2,3,4 , Chang Heon Choi1,2,3,4

1Department of Radiation Oncology, Seoul National University Hospital, Seoul, 2Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, 3Biomedical Research Institute, Seoul National University Hospital, Seoul, 4Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea

Correspondence to:Chang Heon Choi
(dm140@naver.com)
Tel: 82-2–2072–4157
Fax: 82-2–765–3317

Received: February 29, 2024; Revised: April 5, 2024; Accepted: April 17, 2024

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Purpose: Accurate operation of the multi-leaf collimator (MLC), a key technology in intensity modulated radiation therapy (IMRT), is essential for safe and optimal radiation treatment. The HalcyonTM linear accelerator has a collimator with low leakage and radiation transmission, making it suitable for IMRT. The limitations of the existing HalcyonTM MLC quality assurance (QA) method were supplemented with a mathematical method, and the results were analyzed.
Methods: Electric portal imaging device (EPID) images obtained by performing the MLC QA plan on the HalcyonTM was analyzed using Python. The picket fence tests were performed and compared using the maximum pixel value and mathematical methods. Dose rate, gantry speed, and leaf speed variation plan were performed for dose transmission comparison.
Results: For the maximum pixel value, the minimum distance between leaf junctions was 13.86 mm, and the maximum was 16.06 mm. However, for the mathematical method, the minimum and maximum were 14.54 mm and 15.68 mm, respectively. This suggests that setting the peak value to the highest value may cause an error in interpretation due to the limitations of the pixels of the EPID image. Performing QA on the remaining items confirmed that the measured values were within 3% of tolerance.
Conclusions: The presented analysis method applied to the MLC QA can derive more reasonable and valid values than existing methods, which will help with MLC monitoring by reducing errors in excessive interpretation.

KeywordsHalcyonTM linear accelerator, Multi-leaf collimator quality assurance, Picket fence test, Electric portal imaging device image

Intensity modulated radiation therapy (IMRT) is a standard and optimal technique for tumor treatment [1,2]. The latest IMRT technique is volumetric modulated arc therapy (VMAT). VMAT performs dynamic multi-leaf collimator (MLC) IMRT by rotating 360° once or twice, allowing for continuous control of the linear accelerator’s rotation speed and dose rate [3]. As the impact of IMRT, which controls MLC modulation, increases in radiation therapy, the correlation between leaf location accuracy and interleaf or adjacent leaf transmission as well as the accuracy of delivered IMRT fields have been a subject of research [4]. The American Association of Physicists in Medicine Task group reports also referred the importance of quality assurance (QA) protocol for IMRT [5,6]. TG-142 recommends testing the MLC system, including the leaf position accuracy, for gantry rotation as it may affect leaf motion due to gravitational effects imposed on the leaf carriage system.

The picket fence test can be used to qualitatively assess positional accuracy [7]. Additionally, the dose rate and gantry speed, which can affect the leaf carriage system, can be monitored by comparing the picket fence dose distribution and the open beam. Analysis methods include evaluating the MLC travel speed by using the MLC log files or vendor software, but electric portal imaging device (EPID) can also be used for immediate quantitative analysis [5,8-10]. Using EPID, the distance of leaves from junction to junction and the picket fence length can be measured by detecting changes in each position of the image acquired with subpixel precision. Leaf transmission can also be evaluated by calculating the area of the region of interest (ROI) representing the calibrated unit (CU) value in each case.

However, when using EPID, the distance of the leaves from junction to junction is measured by the eye with the highest CU value at the leaf junction. Thus, the results of this method may be subjective, not quantitative. In contrast, radiological imaging technology® (RIT), which is a representative software that can perform MLC QA, can analyze not only the length of leaf junctions but also the full width at half maximum in the leaf pair junction [11]. To utilize this software, a reference EPID template is required for each linear accelerator, and the discrepancy is analyzed through comparison with the reference template. In this study, we developed an analytical method for EPID picket fence images using Python and their packages. Rather than simply obtaining the max value or performing interpolation, the value was obtained by performing linear fitting up to the 5th order and estimating the extreme value. Additionally, the areas of the open beam and picket fence, which can determine the contribution of gantry speed and dose rate to the motion of the MLC, were analyzed in Python.

MLC QA was performed for the HalcyonTM linear accelerator, which contains a 6-MV flattening filter free and a dual-layer MLC [12]. HalcyonTM was equipped with an EPID imager that records 1,280×1,280-pixel transmission images during treatment. The MLC of the HalcyonTM linear accelerator was a 1.0-cm wide double-layered collimator with low leakage and transmission of radiation, making it optimal for IMRT and VMAT [13]. The MLC QA was comprised of four sections depending on the variable of interest: picket fence test, dose rate, gantry speed, and leaf speed; this was offered by Varian Medical Systems for the machine QA program. The MLC operating direction of all plans was from patient right to patient left (PL), and the operating conditions of each plan are summarized in Table 1. The picket fence patterns were designed to assess the accuracy of MLC leaf positions. As the gantry rotates, the leaves create a junction at 1.5-mm intervals, irradiate for approximately 4 seconds, and then move 1.5 cm in the PL direction to create 10 junctions and create EPID images. EPID images were extracted by digital imaging and communications in medicine format and written by Python 3.6. Pandas 1.1.5, Numpy 1.19.5, Sympy, and Scikit-learn packages were used for analysis.

Table 1 Plan parameters for the MLC QA

MLC QA planDelivered MUGantry rotationDose rate
Picket fence test480.0179.0°–187.0°800 MU/s
Dose rate61.1179.0°–242.8°~800 MU/s (variation)
Gantry speed383.0179.0°–242.8°800 MU/s
Leaf speed245.0179.0°–187.0°800 MU/s

MLC, multi-leaf collimator; QA, quality assurance; MU, monitor unit.

1. Picket fence test

Fig. 1 shows the picket fence test performed under the conditions in Table 1. ROIs were determined for 49 leaf pairs by referring to the Halcyon templet for RIT software. There were 10 junctions, and each pixel value was fitted with a 5th-order polynomial equation centered on the peak point as follows:

Figure 1.Electric portal imaging device image for picket fence test written by Python. X and Y axes represent the pixel number. A white vertical line in the figure is a leaf junction.

px=a0+a1x++a5x5

where p(xi)=yj for each i, j∈{0, 1, 2,..., n} and n is the number of data. The solution was obtained to the value that minimized the sum squared error, i=1n yiyi' 2. The inflection point of each fitted function was set as the peak, and the minimum and maximum values were obtained by measuring the distance between peak points at each junction. The data with fitted function were described at Fig. 2. The results are summarized in Table 2. We compared our results with a method measuring the distance between the maximum pixels of each leaf junction and that obtained by the RIT software. Ours and the RIT’s results were measured within tolerance, and the method of measuring distance was using the maximum pixels over or under-estimated.

Table 2 Comparison of the mathematics method by fitting with a general method using pixel value

MethodMin (mm)Max (mm)
A 5th-order polynomial equation14.5415.68
Maximum pixel value distance13.8616.06
RIT software14.1015.53

Figure 2.Fitting curve samples are represented by the blue line, the pixel number (data) is the blue dot, and the red dot is the peak value at junction, where JN (N=1, 2, 3,…, 10) is the junction and LM (M=1, 2, 3,…, 49) is the leaf pair of the picket fence test.

2. Dose rate, gantry speed, and leaf speed variation

Dose rate, gantry speed, and leaf speed plan were performed and compared with the open beam plan offered by Varian Medical Systems for the machine QA program as shown in Fig. 3. The open beam plans stand in for static field and did not include the MLC movement. We compared the areas of the open beam and MLC plan for each ROI, and results are summarized in Tables 35. Rcorr represents the dynamic multi-leaf collimator value/open value×100 as the corrected reading. The number of ROIs was set to be Figs. 3d, e, f at each bend shaped pair.

Table 3 Results of gantry speed plan for MLC QA

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.376232.7341013.760.500.24
Pair 20.421643.0932713.63−0.45
Pair 30.464413.3958513.68−0.12
Pair 40.484953.5402013.700.04
Pair 50.464723.3934113.69−0.02
Pair 60.421983.0900513.66−0.27
Pair 70.376142.7391713.730.29

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit.


Table 4 Results of dose rate plan for MLC QA

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.069740.5071213.750.740.41
Pair 20.072960.5387713.54−0.79
Pair 30.075830.5579113.59−0.43
Pair 40.077000.5643413.65−0.04
Pair 50.076180.5579113.650.03
Pair 60.073440.5388813.63−0.16
Pair 70.069730.5075413.740.65

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit.


Table 5 Results of leaf speed plan for MLC QA

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.051531.810942.85−1.430.96
Pair 20.060592.103092.88−0.20
Pair 30.066412.261612.941.72
Pair 40.060942.096972.910.67
Pair 50.051701.805122.87−0.76

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit.


Figure 3.Electric portal imaging device images of open beam and each intensity modulated radiation therapy plan for the multi-leaf collimator quality assurance. (a) and (d) are for the dose rate plan, (b) and (e) are for the gantry speed plan, and (c) and (f) are for the leaf speed plan.

The accuracy of the MLC movement can be assessed by the picket fence test with an error standard of <1 mm. It is determined by comparing the measurement with the planned leaf junctions at a spacing of 1.5 cm. However, if the brightest part of the leaf junction in the EPID image or the maximum pixel value was chosen, the movement of the MLC cannot be evaluated accurately. Also, when we monitored monthly, it was difficult to decide the origin of the deflective movement.

As shown in Table 2, the method of comparing the general pixel values mentioned above lead to over- or under-estimation depending on the EPID pixel number. Junctions for those cases are shown in Fig. 4. There were discontinuous pixel values in the EPID image, which might be caused by the calibration of EPID’s a-silicon detector or due to limitations in resolution. These values can also cause confusion in monitoring trends in MLC movement.

Figure 4.Over- or under-estimation of cases of the general method using pixel value. The blue solid line is the 5th-order polynomial fitting function, and the red dot is the inflection point of the fitting function. The red arrow represents over- or under-estimated cases if the maximum pixel value distance method was used.

Next, EPID images were obtained by analyzing the dose rate, gantry speed, and leaf speed plan offered by Varian Medical Systems for the machine QA program. The ability of HalcyonTM to modulate the dose rate and gantry speed to achieve the specified values was evaluated. Next, the MLC leaf speed control was evaluated after having validated the MLC accuracy and Halcyon’s TM ability to vary the dose rate and gantry speed. The procedures were intended to determine whether there was a problem with the performance of the MLC by analyzing quantitative changes in EPID measurement values, including movements of the open beam and MLC, through ratio values. Varian Medical System presented a tolerance of ±3.0% for each difference value and 1.5% for the average deviation of all difference values. It was possible to determine that these three factors of IMRT treatment had little effect on MLC movement, and that accurate treatment would be performed.

The leaf pair trend of peak values of each junction, as measured by the presented method, is shown in Fig. 5. The peak value of each junction shows a tendency to move in the PL direction as it goes down for each leaf pair. It can be inferred that this is because the movement of the fence in the plan is irradiated while moving in the PL direction, but it is judged to be a meaningless and infinitesimal movement of <0.5 mm. Analysis of these MLC movements will be helpful for future monitoring and will be beneficial data for making decisions when problems arise.

Figure 5.Peak values trend as measured by setting the inflection point corresponding to the leaf pair. S, superior; PL, patient left; I, inferior; PR, patient right.

The MLC currently plays the most important role in IMRT and VMAT treatment, and its accuracy is related to the accuracy of treatment. The major items of the MLC QA are the picket fence test, dose rate, gantry speed, and leaf speed variation. The aim of the picket fence test is to determine whether the MLC can be accurately driven to the correct position according to the IMRT plan. Generally, the picket fence test is performed by measuring the distance between the brightest parts, with the maximum CU value of the leaf junction created by the MLC QA plan. However, this may be obtained as a discontinuous value due to limitations in EPID pixel resolution, resulting in under- or over-estimation of the value of the distance between leaf junctions. The mathematical method of fitting the value of the leaf junction presented in this study to the 5th order and finding the peak value based on the extrema complemented and yielded reasonable results. The QA of the remaining items (dose rate, gantry speed, and leaf speed variation) were performed by calculating each ROI’s area and comparing the values when the MLC is driven and not driven during open beam. The method presented in this study will enable the MLC QA to be appropriately analyzed and will help monitor its operation.

All relevant data are within the paper and its Supporting Information files.

Conceptualization: Chang Heon Choi, Jung-in Kim. Data curation: Jin Jegal, Hyojun Park. Formal analysis: Jin Jegal. Investigation: Jin Jegal, Hyojun Park. Methodology: Seonghee Kang. Project administration: Chang Heon Choi. Resources: Chang Heon Choi, Jung-in Kim. Software: Jin Jegal, Hyojun Park. Supervision: Jung-in Kim, Chang Heon Choi. Validation: Seonghee Kang, Chang Heon Choi. Visualization: Jin Jegal, Hyojun Park. Writing – original draft: Jin Jegal. Writing – review & editing: Seonghee Kang, Chang Heon Choi, Jung-in Kim.

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Article

Original Article

Progress in Medical Physics 2024; 35(2): 45-51

Published online June 30, 2024 https://doi.org/10.14316/pmp.2024.35.2.45

Copyright © Korean Society of Medical Physics.

Analysis of the Multi-Leaf Collimator Quality Assurance for the HalcyonTM Linear Accelerator

Jin Jegal1,2 , Hyojun Park1,2 , Seonghee Kang1,2,3 , Jung-in Kim1,2,3,4 , Chang Heon Choi1,2,3,4

1Department of Radiation Oncology, Seoul National University Hospital, Seoul, 2Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, 3Biomedical Research Institute, Seoul National University Hospital, Seoul, 4Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea

Correspondence to:Chang Heon Choi
(dm140@naver.com)
Tel: 82-2–2072–4157
Fax: 82-2–765–3317

Received: February 29, 2024; Revised: April 5, 2024; Accepted: April 17, 2024

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Purpose: Accurate operation of the multi-leaf collimator (MLC), a key technology in intensity modulated radiation therapy (IMRT), is essential for safe and optimal radiation treatment. The HalcyonTM linear accelerator has a collimator with low leakage and radiation transmission, making it suitable for IMRT. The limitations of the existing HalcyonTM MLC quality assurance (QA) method were supplemented with a mathematical method, and the results were analyzed.
Methods: Electric portal imaging device (EPID) images obtained by performing the MLC QA plan on the HalcyonTM was analyzed using Python. The picket fence tests were performed and compared using the maximum pixel value and mathematical methods. Dose rate, gantry speed, and leaf speed variation plan were performed for dose transmission comparison.
Results: For the maximum pixel value, the minimum distance between leaf junctions was 13.86 mm, and the maximum was 16.06 mm. However, for the mathematical method, the minimum and maximum were 14.54 mm and 15.68 mm, respectively. This suggests that setting the peak value to the highest value may cause an error in interpretation due to the limitations of the pixels of the EPID image. Performing QA on the remaining items confirmed that the measured values were within 3% of tolerance.
Conclusions: The presented analysis method applied to the MLC QA can derive more reasonable and valid values than existing methods, which will help with MLC monitoring by reducing errors in excessive interpretation.

Keywords: HalcyonTM linear accelerator, Multi-leaf collimator quality assurance, Picket fence test, Electric portal imaging device image

Introduction

Intensity modulated radiation therapy (IMRT) is a standard and optimal technique for tumor treatment [1,2]. The latest IMRT technique is volumetric modulated arc therapy (VMAT). VMAT performs dynamic multi-leaf collimator (MLC) IMRT by rotating 360° once or twice, allowing for continuous control of the linear accelerator’s rotation speed and dose rate [3]. As the impact of IMRT, which controls MLC modulation, increases in radiation therapy, the correlation between leaf location accuracy and interleaf or adjacent leaf transmission as well as the accuracy of delivered IMRT fields have been a subject of research [4]. The American Association of Physicists in Medicine Task group reports also referred the importance of quality assurance (QA) protocol for IMRT [5,6]. TG-142 recommends testing the MLC system, including the leaf position accuracy, for gantry rotation as it may affect leaf motion due to gravitational effects imposed on the leaf carriage system.

The picket fence test can be used to qualitatively assess positional accuracy [7]. Additionally, the dose rate and gantry speed, which can affect the leaf carriage system, can be monitored by comparing the picket fence dose distribution and the open beam. Analysis methods include evaluating the MLC travel speed by using the MLC log files or vendor software, but electric portal imaging device (EPID) can also be used for immediate quantitative analysis [5,8-10]. Using EPID, the distance of leaves from junction to junction and the picket fence length can be measured by detecting changes in each position of the image acquired with subpixel precision. Leaf transmission can also be evaluated by calculating the area of the region of interest (ROI) representing the calibrated unit (CU) value in each case.

However, when using EPID, the distance of the leaves from junction to junction is measured by the eye with the highest CU value at the leaf junction. Thus, the results of this method may be subjective, not quantitative. In contrast, radiological imaging technology® (RIT), which is a representative software that can perform MLC QA, can analyze not only the length of leaf junctions but also the full width at half maximum in the leaf pair junction [11]. To utilize this software, a reference EPID template is required for each linear accelerator, and the discrepancy is analyzed through comparison with the reference template. In this study, we developed an analytical method for EPID picket fence images using Python and their packages. Rather than simply obtaining the max value or performing interpolation, the value was obtained by performing linear fitting up to the 5th order and estimating the extreme value. Additionally, the areas of the open beam and picket fence, which can determine the contribution of gantry speed and dose rate to the motion of the MLC, were analyzed in Python.

Materials and Methods

MLC QA was performed for the HalcyonTM linear accelerator, which contains a 6-MV flattening filter free and a dual-layer MLC [12]. HalcyonTM was equipped with an EPID imager that records 1,280×1,280-pixel transmission images during treatment. The MLC of the HalcyonTM linear accelerator was a 1.0-cm wide double-layered collimator with low leakage and transmission of radiation, making it optimal for IMRT and VMAT [13]. The MLC QA was comprised of four sections depending on the variable of interest: picket fence test, dose rate, gantry speed, and leaf speed; this was offered by Varian Medical Systems for the machine QA program. The MLC operating direction of all plans was from patient right to patient left (PL), and the operating conditions of each plan are summarized in Table 1. The picket fence patterns were designed to assess the accuracy of MLC leaf positions. As the gantry rotates, the leaves create a junction at 1.5-mm intervals, irradiate for approximately 4 seconds, and then move 1.5 cm in the PL direction to create 10 junctions and create EPID images. EPID images were extracted by digital imaging and communications in medicine format and written by Python 3.6. Pandas 1.1.5, Numpy 1.19.5, Sympy, and Scikit-learn packages were used for analysis.

Table 1 . Plan parameters for the MLC QA.

MLC QA planDelivered MUGantry rotationDose rate
Picket fence test480.0179.0°–187.0°800 MU/s
Dose rate61.1179.0°–242.8°~800 MU/s (variation)
Gantry speed383.0179.0°–242.8°800 MU/s
Leaf speed245.0179.0°–187.0°800 MU/s

MLC, multi-leaf collimator; QA, quality assurance; MU, monitor unit..


Results

1. Picket fence test

Fig. 1 shows the picket fence test performed under the conditions in Table 1. ROIs were determined for 49 leaf pairs by referring to the Halcyon templet for RIT software. There were 10 junctions, and each pixel value was fitted with a 5th-order polynomial equation centered on the peak point as follows:

Figure 1. Electric portal imaging device image for picket fence test written by Python. X and Y axes represent the pixel number. A white vertical line in the figure is a leaf junction.

px=a0+a1x++a5x5

where p(xi)=yj for each i, j∈{0, 1, 2,..., n} and n is the number of data. The solution was obtained to the value that minimized the sum squared error, i=1n yiyi' 2. The inflection point of each fitted function was set as the peak, and the minimum and maximum values were obtained by measuring the distance between peak points at each junction. The data with fitted function were described at Fig. 2. The results are summarized in Table 2. We compared our results with a method measuring the distance between the maximum pixels of each leaf junction and that obtained by the RIT software. Ours and the RIT’s results were measured within tolerance, and the method of measuring distance was using the maximum pixels over or under-estimated.

Table 2 . Comparison of the mathematics method by fitting with a general method using pixel value.

MethodMin (mm)Max (mm)
A 5th-order polynomial equation14.5415.68
Maximum pixel value distance13.8616.06
RIT software14.1015.53


Figure 2. Fitting curve samples are represented by the blue line, the pixel number (data) is the blue dot, and the red dot is the peak value at junction, where JN (N=1, 2, 3,…, 10) is the junction and LM (M=1, 2, 3,…, 49) is the leaf pair of the picket fence test.

2. Dose rate, gantry speed, and leaf speed variation

Dose rate, gantry speed, and leaf speed plan were performed and compared with the open beam plan offered by Varian Medical Systems for the machine QA program as shown in Fig. 3. The open beam plans stand in for static field and did not include the MLC movement. We compared the areas of the open beam and MLC plan for each ROI, and results are summarized in Tables 35. Rcorr represents the dynamic multi-leaf collimator value/open value×100 as the corrected reading. The number of ROIs was set to be Figs. 3d, e, f at each bend shaped pair.

Table 3 . Results of gantry speed plan for MLC QA.

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.376232.7341013.760.500.24
Pair 20.421643.0932713.63−0.45
Pair 30.464413.3958513.68−0.12
Pair 40.484953.5402013.700.04
Pair 50.464723.3934113.69−0.02
Pair 60.421983.0900513.66−0.27
Pair 70.376142.7391713.730.29

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit..



Table 4 . Results of dose rate plan for MLC QA.

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.069740.5071213.750.740.41
Pair 20.072960.5387713.54−0.79
Pair 30.075830.5579113.59−0.43
Pair 40.077000.5643413.65−0.04
Pair 50.076180.5579113.650.03
Pair 60.073440.5388813.63−0.16
Pair 70.069730.5075413.740.65

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit..



Table 5 . Results of leaf speed plan for MLC QA.

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.051531.810942.85−1.430.96
Pair 20.060592.103092.88−0.20
Pair 30.066412.261612.941.72
Pair 40.060942.096972.910.67
Pair 50.051701.805122.87−0.76

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit..



Figure 3. Electric portal imaging device images of open beam and each intensity modulated radiation therapy plan for the multi-leaf collimator quality assurance. (a) and (d) are for the dose rate plan, (b) and (e) are for the gantry speed plan, and (c) and (f) are for the leaf speed plan.

Discussion

The accuracy of the MLC movement can be assessed by the picket fence test with an error standard of <1 mm. It is determined by comparing the measurement with the planned leaf junctions at a spacing of 1.5 cm. However, if the brightest part of the leaf junction in the EPID image or the maximum pixel value was chosen, the movement of the MLC cannot be evaluated accurately. Also, when we monitored monthly, it was difficult to decide the origin of the deflective movement.

As shown in Table 2, the method of comparing the general pixel values mentioned above lead to over- or under-estimation depending on the EPID pixel number. Junctions for those cases are shown in Fig. 4. There were discontinuous pixel values in the EPID image, which might be caused by the calibration of EPID’s a-silicon detector or due to limitations in resolution. These values can also cause confusion in monitoring trends in MLC movement.

Figure 4. Over- or under-estimation of cases of the general method using pixel value. The blue solid line is the 5th-order polynomial fitting function, and the red dot is the inflection point of the fitting function. The red arrow represents over- or under-estimated cases if the maximum pixel value distance method was used.

Next, EPID images were obtained by analyzing the dose rate, gantry speed, and leaf speed plan offered by Varian Medical Systems for the machine QA program. The ability of HalcyonTM to modulate the dose rate and gantry speed to achieve the specified values was evaluated. Next, the MLC leaf speed control was evaluated after having validated the MLC accuracy and Halcyon’s TM ability to vary the dose rate and gantry speed. The procedures were intended to determine whether there was a problem with the performance of the MLC by analyzing quantitative changes in EPID measurement values, including movements of the open beam and MLC, through ratio values. Varian Medical System presented a tolerance of ±3.0% for each difference value and 1.5% for the average deviation of all difference values. It was possible to determine that these three factors of IMRT treatment had little effect on MLC movement, and that accurate treatment would be performed.

The leaf pair trend of peak values of each junction, as measured by the presented method, is shown in Fig. 5. The peak value of each junction shows a tendency to move in the PL direction as it goes down for each leaf pair. It can be inferred that this is because the movement of the fence in the plan is irradiated while moving in the PL direction, but it is judged to be a meaningless and infinitesimal movement of <0.5 mm. Analysis of these MLC movements will be helpful for future monitoring and will be beneficial data for making decisions when problems arise.

Figure 5. Peak values trend as measured by setting the inflection point corresponding to the leaf pair. S, superior; PL, patient left; I, inferior; PR, patient right.

Conclusions

The MLC currently plays the most important role in IMRT and VMAT treatment, and its accuracy is related to the accuracy of treatment. The major items of the MLC QA are the picket fence test, dose rate, gantry speed, and leaf speed variation. The aim of the picket fence test is to determine whether the MLC can be accurately driven to the correct position according to the IMRT plan. Generally, the picket fence test is performed by measuring the distance between the brightest parts, with the maximum CU value of the leaf junction created by the MLC QA plan. However, this may be obtained as a discontinuous value due to limitations in EPID pixel resolution, resulting in under- or over-estimation of the value of the distance between leaf junctions. The mathematical method of fitting the value of the leaf junction presented in this study to the 5th order and finding the peak value based on the extrema complemented and yielded reasonable results. The QA of the remaining items (dose rate, gantry speed, and leaf speed variation) were performed by calculating each ROI’s area and comparing the values when the MLC is driven and not driven during open beam. The method presented in this study will enable the MLC QA to be appropriately analyzed and will help monitor its operation.

Conflicts of Interest

The authors have nothing to disclose.

Availability of Data and Materials

All relevant data are within the paper and its Supporting Information files.

Author Contributions

Conceptualization: Chang Heon Choi, Jung-in Kim. Data curation: Jin Jegal, Hyojun Park. Formal analysis: Jin Jegal. Investigation: Jin Jegal, Hyojun Park. Methodology: Seonghee Kang. Project administration: Chang Heon Choi. Resources: Chang Heon Choi, Jung-in Kim. Software: Jin Jegal, Hyojun Park. Supervision: Jung-in Kim, Chang Heon Choi. Validation: Seonghee Kang, Chang Heon Choi. Visualization: Jin Jegal, Hyojun Park. Writing – original draft: Jin Jegal. Writing – review & editing: Seonghee Kang, Chang Heon Choi, Jung-in Kim.

Fig 1.

Figure 1.Electric portal imaging device image for picket fence test written by Python. X and Y axes represent the pixel number. A white vertical line in the figure is a leaf junction.
Progress in Medical Physics 2024; 35: 45-51https://doi.org/10.14316/pmp.2024.35.2.45

Fig 2.

Figure 2.Fitting curve samples are represented by the blue line, the pixel number (data) is the blue dot, and the red dot is the peak value at junction, where JN (N=1, 2, 3,…, 10) is the junction and LM (M=1, 2, 3,…, 49) is the leaf pair of the picket fence test.
Progress in Medical Physics 2024; 35: 45-51https://doi.org/10.14316/pmp.2024.35.2.45

Fig 3.

Figure 3.Electric portal imaging device images of open beam and each intensity modulated radiation therapy plan for the multi-leaf collimator quality assurance. (a) and (d) are for the dose rate plan, (b) and (e) are for the gantry speed plan, and (c) and (f) are for the leaf speed plan.
Progress in Medical Physics 2024; 35: 45-51https://doi.org/10.14316/pmp.2024.35.2.45

Fig 4.

Figure 4.Over- or under-estimation of cases of the general method using pixel value. The blue solid line is the 5th-order polynomial fitting function, and the red dot is the inflection point of the fitting function. The red arrow represents over- or under-estimated cases if the maximum pixel value distance method was used.
Progress in Medical Physics 2024; 35: 45-51https://doi.org/10.14316/pmp.2024.35.2.45

Fig 5.

Figure 5.Peak values trend as measured by setting the inflection point corresponding to the leaf pair. S, superior; PL, patient left; I, inferior; PR, patient right.
Progress in Medical Physics 2024; 35: 45-51https://doi.org/10.14316/pmp.2024.35.2.45

Table 1 Plan parameters for the MLC QA

MLC QA planDelivered MUGantry rotationDose rate
Picket fence test480.0179.0°–187.0°800 MU/s
Dose rate61.1179.0°–242.8°~800 MU/s (variation)
Gantry speed383.0179.0°–242.8°800 MU/s
Leaf speed245.0179.0°–187.0°800 MU/s

MLC, multi-leaf collimator; QA, quality assurance; MU, monitor unit.


Table 2 Comparison of the mathematics method by fitting with a general method using pixel value

MethodMin (mm)Max (mm)
A 5th-order polynomial equation14.5415.68
Maximum pixel value distance13.8616.06
RIT software14.1015.53

Table 3 Results of gantry speed plan for MLC QA

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.376232.7341013.760.500.24
Pair 20.421643.0932713.63−0.45
Pair 30.464413.3958513.68−0.12
Pair 40.484953.5402013.700.04
Pair 50.464723.3934113.69−0.02
Pair 60.421983.0900513.66−0.27
Pair 70.376142.7391713.730.29

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit.


Table 4 Results of dose rate plan for MLC QA

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.069740.5071213.750.740.41
Pair 20.072960.5387713.54−0.79
Pair 30.075830.5579113.59−0.43
Pair 40.077000.5643413.65−0.04
Pair 50.076180.5579113.650.03
Pair 60.073440.5388813.63−0.16
Pair 70.069730.5075413.740.65

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit.


Table 5 Results of leaf speed plan for MLC QA

ROI measurement pointDMLC value (CU)Open value (CU)RcorrDifference to average (%)Average deviation (%)
Pair 10.051531.810942.85−1.430.96
Pair 20.060592.103092.88−0.20
Pair 30.066412.261612.941.72
Pair 40.060942.096972.910.67
Pair 50.051701.805122.87−0.76

MLC, multi-leaf collimator; QA, quality assurance; ROI, region of interest; DMLC, dynamic multi-leaf collimator; CU, calibrated unit.


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Korean Society of Medical Physics

Vol.35 No.2
June 2024

pISSN 2508-4445
eISSN 2508-4453
Formerly ISSN 1226-5829

Frequency: Quarterly

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