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Progress in Medical Physics 2024; 35(2): 36-44

Published online June 30, 2024 https://doi.org/10.14316/pmp.2024.35.2.36

Copyright © Korean Society of Medical Physics.

Dosimetric Evaluations of HyperArc and RapidArc in Stereotactic Radiosurgery for a Single Brain Metastasis

So-Yeon Park , Noorie Choi , Na Young Jang

Department of Radiation Oncology, Veterans Health Service Medical Center, Seoul, Korea

Correspondence to:Na Young Jang
(mumuki79@gmail.com)
Tel: 82-2-2225-4647
Fax: 82-2-2225-4640

Received: February 21, 2024; Revised: March 12, 2024; Accepted: March 25, 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: This study assessed and compared the dosimetric performance of HyperArc and RapidArc in stereotactic radiosurgery (SRS) for a single brain metastasis.
Methods: Twenty patients with intracranial brain metastases, each presenting a distinct target volume, were retrospectively selected. Subsequently, volumetric modulated arc therapy (VMAT) plans were designed using RapidArc (VMATRA) and HyperArc (VMATHA) for each patient. For planning comparisons, dose-volumetric histogram (DVH) parameters for planning target volumes (PTVs) and normal brain regions were computed across all VMAT plans. Subsequently, their total monitor units (MUs), total beam-on times, and modulation complexity scores for the VMAT (MCSv) were compared. A statistical test was used to evaluate the dosimetric disparities in the DVH parameters, total MUs, total beam-on times, and MCSv between the VMATHA and VMATRA plans.
Results: For the PT Vs, VMATHA presented a higher homogeneity index (HI) than VMATRA. Moreover, VMATHA p resented s ignificantly s maller g radient i ndex ( GI) v alues (P<0.001) than VMATRA. Thus, VMATHA demonstrated better performance in the DVH parameters for the PTV than VMATRA. For normal brain tissues, VMATHA p resented l ower v olume r eceiving 5 0% o f t he prescription dose and V2Gy to the normal brain tissues than VMATRA (P<0.0001). While the total MUs required for VMATHA was significantly higher than those for VMATRA, the total beam-on time for VMATHA was superior to that for VMATRA.
Conclusions: Thus, VMATHA exhibited superior performance in achieving rapid dose fall-offs (as indicated by the GI) and a higher HI at the PTV compared to VMATRA in brain SRS. This advancement positions HyperArc as a significant development in the field of radiation therapy, offering optimized treatment outcomes for brain SRS.

KeywordsHyperArc, Stereotactic radiosurgery, Brain metastasis, Volumetric modulated arc therapy, Dosimetric comparison

Stereotactic radiosurgery (SRS), a crucial treatment technique for intracranial brain metastases, involves delivering high radiation doses to an intracranial target in a single session. Since its introduction by Leksell, SRS has demonstrated substantial efficacy, achieving local control rates of 80%–95% over 5–10 years with minimal long-term toxicity [1-8].

Generally, precisely delivering high radiation doses to the intracranial target while minimizing exposure to surrounding critical organs represents a critical aspect of SRS. Hence, ensuring proper patient immobilization and positioning during SRS is crucial. Initially, frame-based SRS systems using fixed head frames were employed to ensure accurate target localization during planning and delivery [9-11]. However, these systems presented notable disadvantages, including patient discomfort, frame slippage, and a slight risk of complications such as bleeding and infections [11]. Consequently, linear accelerators (Linacs) with multileaf collimator (MLC) delivery systems emerged as the preferred alternatives.

Notably, advancements in radiotherapy, particularly volumetric modulated arc therapy (VMAT), have enabled the treatment of large, complex, or multiple brain lesions using multiple arcs. For instance, RapidArc (Varian Medical Systems) is a VMAT technique offering coplanar or noncoplanar, isocentric delivery. Through 1–3 rotations of a Linac gantry, this technique can deliver intensity-modulated radiation doses with high conformality, producing superior treatment plans compared to multi-field intensity-modulated radiation therapy, while reducing the treatment duration [12,13]. HyperArc (Varian Medical Systems) is another novel isocentric VMAT technique particularly designed for MLC-based noncoplanar stereotactic radiotherapy. This technique features automated optimization and delivery, focusing on minimizing planning workloads by automatically determining ideal isocenters, collimator angles, and arc arrangements. This approach ensures maximal conformality and minimal radiation spillage into surrounding tissues [14-16]. Notably, HyperArc combines up to four arcs: a full arc with a couch angle of 0° and three half arcs with couch angles of ±45° and ±90°. It utilizes standard immobilization devices and incorporates automated couch transitions to enhance delivery efficiency [17,18].

This study evaluates and compares the dosimetric performance of HyperArc and RapidArc in SRS for a single brain metastasis, intending to share the acquired clinical experiences and insights regarding the clinical implementation of HyperArc.

1. Patient and plan preparation

From January 2017 to December 2022, twenty patients diagnosed with intracranial brain metastases, each presenting a distinct target volume, were retrospectively selected at our institution. These patients had previously undergone SRS using the VMAT technique.

For diagnostic purposes, computed tomography scans of the treatment sites of all patients were recorded using an immobilization technique. These scans were performed using Discover RT (GE Healthcare), yielding images with a resolution of 512×512 pixels and a slice thickness of 1 mm.

An oncologist delineated the gross target volumes (GTVs) and normal organs based on T1-weighted and T2 magnetic resonance imaging. For this analysis, the selection of normal organs was exclusively confined to normal brain tissues, thus excluding other organs. The planning target volume (PTV) was defined by extending an isotropic margin of 1 or 2 mm from the GTV. To facilitate dosimetric evaluations and plan optimizations, two concentric rings were created around the PTV at distances of 3 and 13 mm, termed as Ring3mm and Ring13mm, respectively. Notably, any overlap of the PTV with the normal brain tissue was excluded from the normal brain structure to improve PTV coverage.

2. Treatment planning

Each VMAT plan in this study used 6 MV photon beams without flattening filter from a TrueBeam STx, equipped with a high-definition 120TM MLC system (Varian Medical Systems). These plans were specifically designed as HyperArc plans (VMATHA) including four automatically arranged arcs: a single full arc (couch rotation: 0°) and three half arcs (couch rotations: ±45° and ±90°). To minimize the dispersion of low doses to normal brain tissues, a half noncoplanar arc was excluded based on the tumor location. The HyperArc software automatically determined the isocenter by centering it on the PTV. Additionally, ideal collimator angles were automatically configured.

All VMATHA plans were optimized using a photon optimizer (version 16.1; Varian Medical Systems) with a uniform voxel resolution of 2.5 mm. Furthermore, the jaw-tracking function was employed to minimize leakage doses to normal organs. Prescription doses for the PTV varied from 14–25 Gy in one fraction, according to tumor characteristics and locations. Throughout the optimization process, we adhered to the planning constraints outlined in the report of the American Association of Physicists in Medicine Task Group 101 [19], focusing on sparing the normal brain tissue and thus preventing further complications. Table 1 lists the planning constraints for both the target volume and normal brain tissue in brain SRS. To optimize the target coverage and minimize dose leakage to surrounding normal tissues, the automatic lower dose objective and SRS normal tissue objective techniques were applied. To further enhance the dosimetric plan quality, each VMATHA plan was reoptimized using the existing dose distribution as its base distribution. Subsequently, dose calculations were performed using the Acuros XB advanced dose calculation algorithm (version 16.1; Varian Medical Systems) on a 2 mm calculation grid. Plan normalization ensured that 100% of the prescribed dose covered 95% of the PTV.

Table 1 Planning constraints for the planning target volume, normal brain tissue, and ring structures surrounding the PTV at distances of 3 and 13 mm in brain stereotactic radiosurgery plans

StructurePlanning constraintsPriority
PTVV100% >95%150
D100% >100% of the prescription dose150
Normal brainV2Gy <20%100
Ring3mmDmax <70% of the prescription dose100
Ring13mmDmax <30% of the prescription dose100

PTV, planning target volume; Vn%, volume receiving n% of the prescription dose; Dn%, dose received by n% of the target volume; VnGy, volume receiving n Gy; Dmax, maximum dose; Ringnmm, ring structure surrounding the PTV at a distance of n mm.


For comparative analysis, each VMAT plan was also created as a RapidArc plan (VMATRA) using 6 MV photon beams without flattening filter from a Clinac iX equipped with a Millennium 120 MLC system (Varian Medical Systems). Each VMATRA plan comprised one full arc with a couch rotation of 0° and one half noncoplanar arc with a couch rotation of 90° (or 270°). In these plans, collimator angles were manually set to 30° and 330°. All VMATRA plans were optimized using the progressive resolution optimizer (version 8.9; Varian Medical Systems), following identical planning protocols and constraints as those applied for the VMATHA plans.

3. Evaluations of treatment plans

The evaluation of dosimetric quality in this study involved a comprehensive analysis of the dose-volumetric histogram (DVH) parameters for each plan, focusing on the target coverage and doses delivered to normal organs. The DVH parameters assessed for the PTV included minimum, maximum, and mean doses. Additionally, the conformity index (as proposed by Paddick et al. [CIpaddick]), gradient index (GI), and homogeneity index (HI) were computed as follows [20-22]:

CIpaddick=TVprescription doseTV×TVprescription doseVprescription dose
GI=Vprescription dose 50%Vprescription dose
HI=Maximum dosePrescription dose

where TVprescription dose represents the target volume encompassed by the prescription dose, TV denotes the total target volume, Vprescription dose represents the volume receiving the prescription dose, and Vprescription dose 50% denotes the volume receiving 50% of the prescription dose.

Similarly, the DVH parameters analyzed for the normal organs included the volumes receiving 100% and 50% of the prescription dose (V100% and V50%, respectively), along with the volumes receiving 18 Gy (V18Gy), 12 Gy (V12Gy), and 2 Gy (V2Gy).

Treatment efficiency and deliverability were evaluated using the total beam-on time, total monitor units (MUs), and modulation complexity score for VMAT (MCSv). The MCSv metric, developed by Masi et al. [23], evaluates the complexities in MLC motion and beam apertures in VMAT plans. A lower MCSv value indicates increased modulation complexity. The foregoing metrics were computed using an inhouse software developed in MATLAB R2021a (MathWorks).

Statistical analysis was performed using the Shapiro–Wilk test to determine the normality of the datasets. Depending on the obtained results, either the Wilcoxon signed-rank test or a paired t-test was employed to evaluate the dosimetric differences in the DVH parameters, total MUs, beam-on times, and MCSv between the VMATHA and VMATRA plans. Statistical significance was established at P<0.05. Overall analyses were conducted using the statistical program PRISM (version 8.4.3; GraphPad Software Inc.).

1. DVH parameters

Table 2 summarizes the average DVH parameters for the PTV in both the VMATHA and VMATRA plans. Our analysis revealed statistically significant differences in all the considered DVH parameters between VMATHA and VMATRA (P<0.05), except for CIpaddick (P=0.152). Notably, compared to VMATRA, VMATHA presented lower minimum dose values and considerably higher maximum and mean dose values. Additionally, compared to VMATRA, VMATHA presented significantly elevated HI values. Moreover, VMATHA presented significantly smaller GI values (P<0.001), suggesting a more effective dose fall-off outside the PTV, compared to VMATRA. Overall, the quality of the DVH parameters for the PTV was superior in VMATHA compared to VMATRA.

Table 2 Average DVH parameters for planning target volumes in brain stereotactic radiosurgery plans

DVH parameterVMATRAVMATHAP-value
Volume (cm3)9.77±7.42-
Minimum dose (%)95.28±1.9790.81±4.08<0.0001
Maximum dose (%)112.77±11.12142.60±6.60<0.0001
Mean dose (%)105.94±5.39119.33±4.57<0.001
CIpaddick1.10±0.130.99±0.09-
HI1.13±1.111.43±0.66<0.0001
GI3.79±0.762.56±0.17<0.001

Data are presented as mean±standard deviation.

DVH, dose-volumetric histogram; VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; CIpaddick, conformity index suggested by Paddick et al.; HI, homogeneity index; GI, gradient index.


Table 3 summarizes the average DVH parameters for the normal brain tissue in both the VMATHA and VMATRA plans. Across all parameters, VMATHA exhibited significantly lower values than VMATRA. In particular, the differences in V50% and V2Gy for the normal brain tissue between VMATHA and VMATRA were substantial (23.37 cm³ and 37.29 cm³, respectively, with P<0.0001 for V50% and 11.28% and 17.82%, respectively, with P<0.0001 for V2Gy).

Table 3 Average DVH parameters for normal brain regions in brain stereotactic radiosurgery plans

DVH parameterVMATRAVMATHAP-value
Volume (cm3)1,460.82±296.47-
V100% (cm3)11.03±9.329.40±7.05<0.001
V50% (cm3)37.29±25.4323.37±16.70<0.0001
V18Gy (%)0.28±0.320.08±0.080.021
V12Gy (%)1.04±0.920.46±0.460.002
V2Gy (%)17.82±11.1811.28±9.49<0.0001

Data are presented as mean±standard deviation.

DVH, dose-volumetric histogram; VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; Vn%, volume receiving n% of the prescription dose; VnGy, volume receiving at least n Gy.


Thus, employing HyperArc for generating VMAT plans appears to enhance the dose fall-off outside the PTV and deliver a higher maximum dose within the PTV, while more effectively minimizing the exposure to surrounding normal tissues in brain SRS. For illustrative purposes, the dosimetric evaluations included dose distributions from VMATHA and VMATRA for two representative patients, as depicted in Fig. 1. The DVHs for these patients are presented in Fig. 2.

Figure 1.Representative dose distributions of brain stereotactic radiosurgery cases (patient #3 and #14): Dose distributions of volumetric modulated arc therapy plans created using RapidArc volumetric modulated arc therapy (VMATRA) (a) and HyperArc (VMATHA) (b) for patient #3. Dose distributions of VMATRA (c) and VMATHA (d) for patient #14. Doses are depicted by color wash with 2 Gy (the lowest dose) in blue and 30 Gy (the highest dose) in red.

Figure 2.Representative dose-volumetric histograms of brain stereotactic radiosurgery cases (patient #3 and #14). Volumetric modulated arc therapy plans generated using RapidArc (RA) (a) and HyperArc (HA) (b) plotted using solid and dashed lines, respectively, for the planning target volume (PTV) and normal brain.

2. Total MU, total beam-on time, and MCSv

Table 4 presents the average total MUs, beam-on times, and MCSv values. The data indicate that the VMATHA plans are more complex than the VMATRA plans, with statistically significant differences (MCSv values of 0.539 for VMATHA and 0.619 for VMATRA, P<0.001). This complexity is further emphasized by the mean total MUs and beam-on times. Specifically, the average total MUs for VMATHA significantly exceeded those for VMATRA (6,704.85 and 4,245.60, respectively, with P<0.0001). However, the total beam-on time for VMATRA exceeded that for VMATHA (424.56 seconds compared with 287.35 seconds, respectively, with P<0.0001). Notably, these beam-on times were calculated using different dose rates for each system: 600 MU/min for VMATRA and 1,400 MU/min for VMATHA. This variation in dose rates is a critical factor in understanding the differences in beam-on times between the two systems, reflecting the inherent complexities and operational characteristics of each technology.

Table 4 Average total monitor units, beam-on time, and modulation complexity scores of volumetric modulated arc therapy plans for brain stereotactic radiosurgery

VMATRAVMATHAP-value
MU4,245.60±750.586,704.85±1,497.61<0.0001
Beam-on time (s)424.56±75.06287.35±64.18<0.0001
MCSv0.619±0.0320.539±0.045<0.001

Data are presented as mean±standard deviation.

VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; MU, monitor unit; MCSv, modulation complexity score for volumetric modulated arc therapy generated by Masi et al. (2013) [23].

In this investigation, we compared the performance of HyperArc and RaipdArc in generating brain SRS VMAT plans. In particular, we focused on analyzing the DVH parameters for both the target volume and surrounding normal organs, total MUs, beam-on times, and MCSv values. Compared with VMATRA, VMATHA demonstrated a superior dose gradient from the PTV and a higher maximum dose in the PTV, while effectively minimizing dose exposure to the normal brain tissue. However, these improvements in plan dosimetric quality corresponded to an increase in the overall modulation complexity of the plan and total MUs, potentially affecting treatment deliverability and efficiency. Although VMATHA required approximately 1.6 times more total MUs than VMATRA, using TrueBeam STx with HyperArc, operating with 6 MV flattening filter free beams at a 1,400 MU/min dose-rate, could reduce the total beam-on time by a factor of 1.5 times compared with Clinac iX.

Similar studies, such as the study conducted by Vergalasova et al. [16], have explored the dosimetric quality of HyperArc at SRS treatment sites, further comparing it with the dosimetric qualities of other popular treatment plans such as GammaKnife and RapidArc. Their findings indicate that both RapidArc and HyperArc achieve conformality and low-dose brain spillage levels comparable to those of GammaKnife, while significantly reducing the beam-on time. Notably, the quality of the VMATRA plans varies substantially based on optimization constraint settings and the planner, while the VMATHA plans consistently demonstrate better dosimetric performance.

Our study highlights that using a next-generation treatment planning system for VMATHA produces a significantly higher HI than VMATRA plans, particularly if a dose constraint for the maximum dose is not established during optimization. Plans with higher HI values—traditionally considered less desirable for conventional fractionated radiotherapy—may be acceptable for hypo-fractionated therapies such as stereotactic body radiation therapy (SBRT) and SRS, particularly if the maximum dose is constrained within the GTV. This concept aligns with GammaKnife units, often prescribed for PTV with a 50% isodose line, implying a maximum dose within the target volume that is twice the prescription dose. The VMATHA planning approach facilitated further improvements in plan quality, including sharp dose gradients and reduced exposure to surrounding brain regions. This technique also opens up possibilities for dose escalations in treating larger tumors while maintaining normal tissue tolerance. Studies, such as those by Dong et al. [24] and others, have demonstrated the feasibility of dose escalations in treating various cancers using noncoplanar dose delivery techniques such as 4π radiotherapy [25,26]. Future studies could explore the viability of dose escalations using VMATHA planning for brain metastases and other tumor sites.

Both the 4π and VMATHA planning approaches represent noncoplanar planning techniques on conventional C-arm-type Linacs. The 4π optimization method begins with a vast array of more than 1,162 beams distributed across the 4π angle space with a 6° gap between adjacent beams [26], requiring the machine to traverse several noncoplanar beams. In contrast, the VMATHA optimization integrates noncoplanar beams with VMAT using four couch angles and one isocentric irradiation, potentially achieving shorter dose delivery times for multiple targets compared to the 4π technique.

An essential aspect of VMATHA planning is the SRS normal tissue objective, aimed at achieving the most closely packed dose possible while minimizing the dose spread across targets. In this regard, our findings indicated that compared to the VMATRA plans, the VMATHA plans involved more intricate MLC patterns with smaller segments (lower MCSv values). However, as reported by Ohira et al. [27], lower MCSv values in VMAT plans could lead to reduced dosimetric accuracy, as evidenced by gamma pass rates, highlighting the need for meticulous dosimetric validation before clinically implementing VMATHA plans.

In this study, we utilized two different MLC systems: high-definition 120TM MLCs and Millennium MLCs from TrueBeam STx and Clinac iX, respectively. Notably, these MLCs exhibit evident differences in their geometric and dosimetric characteristics. Numerous studies have explored the effects of different MLC systems on planning and deliverability. In planning comparisons, the dosimetric impacts of different MLC systems on plan quality have been observed to be minimal [28-30]. Subramanian et al. [30] demonstrated that MLCs with a 5 mm width completely satisfied the RTOG-0813 treatment planning criteria for lung SBRT. The dosimetric superiority of VMATHA resulted from the delivery technique and optimization algorithm for planning rather than the different types of MLC systems.

Increased exposure of nontarget tissues to low radiation doses represents a crucial concern when using single-isocentric VMATHA for multiple targets. This is attributed to shared MLC leaf pairs between two or more targets, preventing the effective blocking of radiation exposure to normal tissues surrounding multiple metastases [31]. Wu et al. [32] demonstrated that employing a collimator optimization algorithm could significantly reduce the dose exposure to normal brain tissues while maintaining similar target coverages. This algorithm, developed for VMATHA planning, solves the “island blocking” issue, and our results demonstrate that compared to multi-isocentric irradiation, VMATHA provides a lower volume of brain tissues receiving a dose >4 Gy. Despite the slightly higher V2Gy of the VMATHA plan, this low-dose spread is not deemed clinically significant, specifically considering that it remains below the levels typically used for whole-brain irradiation in a single fraction.

Compared with VMATRA, VMATHA demonstrated superior performance in achieving rapid dose fall-offs (as indicated by the GI) and a higher HI at the PTV, while more effectively sparing normal tissues in brain SRS. The enhanced dosimetric plan quality achieved by HyperArc highlights its potential as a new and preferred option for delivering SRS doses, particularly in high-precision treatments for brain SRS. This advancement presents HyperArc as a significant development in the field of radiation therapy, offering optimized treatment outcomes for patients undergoing brain SRS.

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023-00253604).

Conceptualization: So-Yeon Park, Na Young Jang. Data curation: So-Yeon Park, Noorie Choi. Formal analysis: So-Yeon Park. Funding acquisition: So-Yeon Park. Investigation: So-Yeon Park. Methodology: So-Yeon Park. Na Young Jang. Project administration: So-Yeon Park. Resources: Noorie Choi, Na Young Jang. Software: So-Yeon Park. Supervision: Na Young Jang. Validation: Noorie Choi. Visualization: So-Yeon Park, Na Young Jang. Writing – original draft: So-Yeon Park. Writing – review & editing: So-Yeon Park, Na Young Jang.

This study was approved by the Institutional Review Board of Veterans Health Service Medical Center (IRB No. 2023-01-001). The requirement for written informed consent was waived by the IRB due to the retrospective nature of the study.

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Article

Original Article

Progress in Medical Physics 2024; 35(2): 36-44

Published online June 30, 2024 https://doi.org/10.14316/pmp.2024.35.2.36

Copyright © Korean Society of Medical Physics.

Dosimetric Evaluations of HyperArc and RapidArc in Stereotactic Radiosurgery for a Single Brain Metastasis

So-Yeon Park , Noorie Choi , Na Young Jang

Department of Radiation Oncology, Veterans Health Service Medical Center, Seoul, Korea

Correspondence to:Na Young Jang
(mumuki79@gmail.com)
Tel: 82-2-2225-4647
Fax: 82-2-2225-4640

Received: February 21, 2024; Revised: March 12, 2024; Accepted: March 25, 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: This study assessed and compared the dosimetric performance of HyperArc and RapidArc in stereotactic radiosurgery (SRS) for a single brain metastasis.
Methods: Twenty patients with intracranial brain metastases, each presenting a distinct target volume, were retrospectively selected. Subsequently, volumetric modulated arc therapy (VMAT) plans were designed using RapidArc (VMATRA) and HyperArc (VMATHA) for each patient. For planning comparisons, dose-volumetric histogram (DVH) parameters for planning target volumes (PTVs) and normal brain regions were computed across all VMAT plans. Subsequently, their total monitor units (MUs), total beam-on times, and modulation complexity scores for the VMAT (MCSv) were compared. A statistical test was used to evaluate the dosimetric disparities in the DVH parameters, total MUs, total beam-on times, and MCSv between the VMATHA and VMATRA plans.
Results: For the PT Vs, VMATHA presented a higher homogeneity index (HI) than VMATRA. Moreover, VMATHA p resented s ignificantly s maller g radient i ndex ( GI) v alues (P<0.001) than VMATRA. Thus, VMATHA demonstrated better performance in the DVH parameters for the PTV than VMATRA. For normal brain tissues, VMATHA p resented l ower v olume r eceiving 5 0% o f t he prescription dose and V2Gy to the normal brain tissues than VMATRA (P<0.0001). While the total MUs required for VMATHA was significantly higher than those for VMATRA, the total beam-on time for VMATHA was superior to that for VMATRA.
Conclusions: Thus, VMATHA exhibited superior performance in achieving rapid dose fall-offs (as indicated by the GI) and a higher HI at the PTV compared to VMATRA in brain SRS. This advancement positions HyperArc as a significant development in the field of radiation therapy, offering optimized treatment outcomes for brain SRS.

Keywords: HyperArc, Stereotactic radiosurgery, Brain metastasis, Volumetric modulated arc therapy, Dosimetric comparison

Introduction

Stereotactic radiosurgery (SRS), a crucial treatment technique for intracranial brain metastases, involves delivering high radiation doses to an intracranial target in a single session. Since its introduction by Leksell, SRS has demonstrated substantial efficacy, achieving local control rates of 80%–95% over 5–10 years with minimal long-term toxicity [1-8].

Generally, precisely delivering high radiation doses to the intracranial target while minimizing exposure to surrounding critical organs represents a critical aspect of SRS. Hence, ensuring proper patient immobilization and positioning during SRS is crucial. Initially, frame-based SRS systems using fixed head frames were employed to ensure accurate target localization during planning and delivery [9-11]. However, these systems presented notable disadvantages, including patient discomfort, frame slippage, and a slight risk of complications such as bleeding and infections [11]. Consequently, linear accelerators (Linacs) with multileaf collimator (MLC) delivery systems emerged as the preferred alternatives.

Notably, advancements in radiotherapy, particularly volumetric modulated arc therapy (VMAT), have enabled the treatment of large, complex, or multiple brain lesions using multiple arcs. For instance, RapidArc (Varian Medical Systems) is a VMAT technique offering coplanar or noncoplanar, isocentric delivery. Through 1–3 rotations of a Linac gantry, this technique can deliver intensity-modulated radiation doses with high conformality, producing superior treatment plans compared to multi-field intensity-modulated radiation therapy, while reducing the treatment duration [12,13]. HyperArc (Varian Medical Systems) is another novel isocentric VMAT technique particularly designed for MLC-based noncoplanar stereotactic radiotherapy. This technique features automated optimization and delivery, focusing on minimizing planning workloads by automatically determining ideal isocenters, collimator angles, and arc arrangements. This approach ensures maximal conformality and minimal radiation spillage into surrounding tissues [14-16]. Notably, HyperArc combines up to four arcs: a full arc with a couch angle of 0° and three half arcs with couch angles of ±45° and ±90°. It utilizes standard immobilization devices and incorporates automated couch transitions to enhance delivery efficiency [17,18].

This study evaluates and compares the dosimetric performance of HyperArc and RapidArc in SRS for a single brain metastasis, intending to share the acquired clinical experiences and insights regarding the clinical implementation of HyperArc.

Materials and Methods

1. Patient and plan preparation

From January 2017 to December 2022, twenty patients diagnosed with intracranial brain metastases, each presenting a distinct target volume, were retrospectively selected at our institution. These patients had previously undergone SRS using the VMAT technique.

For diagnostic purposes, computed tomography scans of the treatment sites of all patients were recorded using an immobilization technique. These scans were performed using Discover RT (GE Healthcare), yielding images with a resolution of 512×512 pixels and a slice thickness of 1 mm.

An oncologist delineated the gross target volumes (GTVs) and normal organs based on T1-weighted and T2 magnetic resonance imaging. For this analysis, the selection of normal organs was exclusively confined to normal brain tissues, thus excluding other organs. The planning target volume (PTV) was defined by extending an isotropic margin of 1 or 2 mm from the GTV. To facilitate dosimetric evaluations and plan optimizations, two concentric rings were created around the PTV at distances of 3 and 13 mm, termed as Ring3mm and Ring13mm, respectively. Notably, any overlap of the PTV with the normal brain tissue was excluded from the normal brain structure to improve PTV coverage.

2. Treatment planning

Each VMAT plan in this study used 6 MV photon beams without flattening filter from a TrueBeam STx, equipped with a high-definition 120TM MLC system (Varian Medical Systems). These plans were specifically designed as HyperArc plans (VMATHA) including four automatically arranged arcs: a single full arc (couch rotation: 0°) and three half arcs (couch rotations: ±45° and ±90°). To minimize the dispersion of low doses to normal brain tissues, a half noncoplanar arc was excluded based on the tumor location. The HyperArc software automatically determined the isocenter by centering it on the PTV. Additionally, ideal collimator angles were automatically configured.

All VMATHA plans were optimized using a photon optimizer (version 16.1; Varian Medical Systems) with a uniform voxel resolution of 2.5 mm. Furthermore, the jaw-tracking function was employed to minimize leakage doses to normal organs. Prescription doses for the PTV varied from 14–25 Gy in one fraction, according to tumor characteristics and locations. Throughout the optimization process, we adhered to the planning constraints outlined in the report of the American Association of Physicists in Medicine Task Group 101 [19], focusing on sparing the normal brain tissue and thus preventing further complications. Table 1 lists the planning constraints for both the target volume and normal brain tissue in brain SRS. To optimize the target coverage and minimize dose leakage to surrounding normal tissues, the automatic lower dose objective and SRS normal tissue objective techniques were applied. To further enhance the dosimetric plan quality, each VMATHA plan was reoptimized using the existing dose distribution as its base distribution. Subsequently, dose calculations were performed using the Acuros XB advanced dose calculation algorithm (version 16.1; Varian Medical Systems) on a 2 mm calculation grid. Plan normalization ensured that 100% of the prescribed dose covered 95% of the PTV.

Table 1 . Planning constraints for the planning target volume, normal brain tissue, and ring structures surrounding the PTV at distances of 3 and 13 mm in brain stereotactic radiosurgery plans.

StructurePlanning constraintsPriority
PTVV100% >95%150
D100% >100% of the prescription dose150
Normal brainV2Gy <20%100
Ring3mmDmax <70% of the prescription dose100
Ring13mmDmax <30% of the prescription dose100

PTV, planning target volume; Vn%, volume receiving n% of the prescription dose; Dn%, dose received by n% of the target volume; VnGy, volume receiving n Gy; Dmax, maximum dose; Ringnmm, ring structure surrounding the PTV at a distance of n mm..



For comparative analysis, each VMAT plan was also created as a RapidArc plan (VMATRA) using 6 MV photon beams without flattening filter from a Clinac iX equipped with a Millennium 120 MLC system (Varian Medical Systems). Each VMATRA plan comprised one full arc with a couch rotation of 0° and one half noncoplanar arc with a couch rotation of 90° (or 270°). In these plans, collimator angles were manually set to 30° and 330°. All VMATRA plans were optimized using the progressive resolution optimizer (version 8.9; Varian Medical Systems), following identical planning protocols and constraints as those applied for the VMATHA plans.

3. Evaluations of treatment plans

The evaluation of dosimetric quality in this study involved a comprehensive analysis of the dose-volumetric histogram (DVH) parameters for each plan, focusing on the target coverage and doses delivered to normal organs. The DVH parameters assessed for the PTV included minimum, maximum, and mean doses. Additionally, the conformity index (as proposed by Paddick et al. [CIpaddick]), gradient index (GI), and homogeneity index (HI) were computed as follows [20-22]:

CIpaddick=TVprescription doseTV×TVprescription doseVprescription dose
GI=Vprescription dose 50%Vprescription dose
HI=Maximum dosePrescription dose

where TVprescription dose represents the target volume encompassed by the prescription dose, TV denotes the total target volume, Vprescription dose represents the volume receiving the prescription dose, and Vprescription dose 50% denotes the volume receiving 50% of the prescription dose.

Similarly, the DVH parameters analyzed for the normal organs included the volumes receiving 100% and 50% of the prescription dose (V100% and V50%, respectively), along with the volumes receiving 18 Gy (V18Gy), 12 Gy (V12Gy), and 2 Gy (V2Gy).

Treatment efficiency and deliverability were evaluated using the total beam-on time, total monitor units (MUs), and modulation complexity score for VMAT (MCSv). The MCSv metric, developed by Masi et al. [23], evaluates the complexities in MLC motion and beam apertures in VMAT plans. A lower MCSv value indicates increased modulation complexity. The foregoing metrics were computed using an inhouse software developed in MATLAB R2021a (MathWorks).

Statistical analysis was performed using the Shapiro–Wilk test to determine the normality of the datasets. Depending on the obtained results, either the Wilcoxon signed-rank test or a paired t-test was employed to evaluate the dosimetric differences in the DVH parameters, total MUs, beam-on times, and MCSv between the VMATHA and VMATRA plans. Statistical significance was established at P<0.05. Overall analyses were conducted using the statistical program PRISM (version 8.4.3; GraphPad Software Inc.).

Results

1. DVH parameters

Table 2 summarizes the average DVH parameters for the PTV in both the VMATHA and VMATRA plans. Our analysis revealed statistically significant differences in all the considered DVH parameters between VMATHA and VMATRA (P<0.05), except for CIpaddick (P=0.152). Notably, compared to VMATRA, VMATHA presented lower minimum dose values and considerably higher maximum and mean dose values. Additionally, compared to VMATRA, VMATHA presented significantly elevated HI values. Moreover, VMATHA presented significantly smaller GI values (P<0.001), suggesting a more effective dose fall-off outside the PTV, compared to VMATRA. Overall, the quality of the DVH parameters for the PTV was superior in VMATHA compared to VMATRA.

Table 2 . Average DVH parameters for planning target volumes in brain stereotactic radiosurgery plans.

DVH parameterVMATRAVMATHAP-value
Volume (cm3)9.77±7.42-
Minimum dose (%)95.28±1.9790.81±4.08<0.0001
Maximum dose (%)112.77±11.12142.60±6.60<0.0001
Mean dose (%)105.94±5.39119.33±4.57<0.001
CIpaddick1.10±0.130.99±0.09-
HI1.13±1.111.43±0.66<0.0001
GI3.79±0.762.56±0.17<0.001

Data are presented as mean±standard deviation..

DVH, dose-volumetric histogram; VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; CIpaddick, conformity index suggested by Paddick et al.; HI, homogeneity index; GI, gradient index..



Table 3 summarizes the average DVH parameters for the normal brain tissue in both the VMATHA and VMATRA plans. Across all parameters, VMATHA exhibited significantly lower values than VMATRA. In particular, the differences in V50% and V2Gy for the normal brain tissue between VMATHA and VMATRA were substantial (23.37 cm³ and 37.29 cm³, respectively, with P<0.0001 for V50% and 11.28% and 17.82%, respectively, with P<0.0001 for V2Gy).

Table 3 . Average DVH parameters for normal brain regions in brain stereotactic radiosurgery plans.

DVH parameterVMATRAVMATHAP-value
Volume (cm3)1,460.82±296.47-
V100% (cm3)11.03±9.329.40±7.05<0.001
V50% (cm3)37.29±25.4323.37±16.70<0.0001
V18Gy (%)0.28±0.320.08±0.080.021
V12Gy (%)1.04±0.920.46±0.460.002
V2Gy (%)17.82±11.1811.28±9.49<0.0001

Data are presented as mean±standard deviation..

DVH, dose-volumetric histogram; VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; Vn%, volume receiving n% of the prescription dose; VnGy, volume receiving at least n Gy..



Thus, employing HyperArc for generating VMAT plans appears to enhance the dose fall-off outside the PTV and deliver a higher maximum dose within the PTV, while more effectively minimizing the exposure to surrounding normal tissues in brain SRS. For illustrative purposes, the dosimetric evaluations included dose distributions from VMATHA and VMATRA for two representative patients, as depicted in Fig. 1. The DVHs for these patients are presented in Fig. 2.

Figure 1. Representative dose distributions of brain stereotactic radiosurgery cases (patient #3 and #14): Dose distributions of volumetric modulated arc therapy plans created using RapidArc volumetric modulated arc therapy (VMATRA) (a) and HyperArc (VMATHA) (b) for patient #3. Dose distributions of VMATRA (c) and VMATHA (d) for patient #14. Doses are depicted by color wash with 2 Gy (the lowest dose) in blue and 30 Gy (the highest dose) in red.

Figure 2. Representative dose-volumetric histograms of brain stereotactic radiosurgery cases (patient #3 and #14). Volumetric modulated arc therapy plans generated using RapidArc (RA) (a) and HyperArc (HA) (b) plotted using solid and dashed lines, respectively, for the planning target volume (PTV) and normal brain.

2. Total MU, total beam-on time, and MCSv

Table 4 presents the average total MUs, beam-on times, and MCSv values. The data indicate that the VMATHA plans are more complex than the VMATRA plans, with statistically significant differences (MCSv values of 0.539 for VMATHA and 0.619 for VMATRA, P<0.001). This complexity is further emphasized by the mean total MUs and beam-on times. Specifically, the average total MUs for VMATHA significantly exceeded those for VMATRA (6,704.85 and 4,245.60, respectively, with P<0.0001). However, the total beam-on time for VMATRA exceeded that for VMATHA (424.56 seconds compared with 287.35 seconds, respectively, with P<0.0001). Notably, these beam-on times were calculated using different dose rates for each system: 600 MU/min for VMATRA and 1,400 MU/min for VMATHA. This variation in dose rates is a critical factor in understanding the differences in beam-on times between the two systems, reflecting the inherent complexities and operational characteristics of each technology.

Table 4 . Average total monitor units, beam-on time, and modulation complexity scores of volumetric modulated arc therapy plans for brain stereotactic radiosurgery.

VMATRAVMATHAP-value
MU4,245.60±750.586,704.85±1,497.61<0.0001
Beam-on time (s)424.56±75.06287.35±64.18<0.0001
MCSv0.619±0.0320.539±0.045<0.001

Data are presented as mean±standard deviation..

VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; MU, monitor unit; MCSv, modulation complexity score for volumetric modulated arc therapy generated by Masi et al. (2013) [23]..


Discussion

In this investigation, we compared the performance of HyperArc and RaipdArc in generating brain SRS VMAT plans. In particular, we focused on analyzing the DVH parameters for both the target volume and surrounding normal organs, total MUs, beam-on times, and MCSv values. Compared with VMATRA, VMATHA demonstrated a superior dose gradient from the PTV and a higher maximum dose in the PTV, while effectively minimizing dose exposure to the normal brain tissue. However, these improvements in plan dosimetric quality corresponded to an increase in the overall modulation complexity of the plan and total MUs, potentially affecting treatment deliverability and efficiency. Although VMATHA required approximately 1.6 times more total MUs than VMATRA, using TrueBeam STx with HyperArc, operating with 6 MV flattening filter free beams at a 1,400 MU/min dose-rate, could reduce the total beam-on time by a factor of 1.5 times compared with Clinac iX.

Similar studies, such as the study conducted by Vergalasova et al. [16], have explored the dosimetric quality of HyperArc at SRS treatment sites, further comparing it with the dosimetric qualities of other popular treatment plans such as GammaKnife and RapidArc. Their findings indicate that both RapidArc and HyperArc achieve conformality and low-dose brain spillage levels comparable to those of GammaKnife, while significantly reducing the beam-on time. Notably, the quality of the VMATRA plans varies substantially based on optimization constraint settings and the planner, while the VMATHA plans consistently demonstrate better dosimetric performance.

Our study highlights that using a next-generation treatment planning system for VMATHA produces a significantly higher HI than VMATRA plans, particularly if a dose constraint for the maximum dose is not established during optimization. Plans with higher HI values—traditionally considered less desirable for conventional fractionated radiotherapy—may be acceptable for hypo-fractionated therapies such as stereotactic body radiation therapy (SBRT) and SRS, particularly if the maximum dose is constrained within the GTV. This concept aligns with GammaKnife units, often prescribed for PTV with a 50% isodose line, implying a maximum dose within the target volume that is twice the prescription dose. The VMATHA planning approach facilitated further improvements in plan quality, including sharp dose gradients and reduced exposure to surrounding brain regions. This technique also opens up possibilities for dose escalations in treating larger tumors while maintaining normal tissue tolerance. Studies, such as those by Dong et al. [24] and others, have demonstrated the feasibility of dose escalations in treating various cancers using noncoplanar dose delivery techniques such as 4π radiotherapy [25,26]. Future studies could explore the viability of dose escalations using VMATHA planning for brain metastases and other tumor sites.

Both the 4π and VMATHA planning approaches represent noncoplanar planning techniques on conventional C-arm-type Linacs. The 4π optimization method begins with a vast array of more than 1,162 beams distributed across the 4π angle space with a 6° gap between adjacent beams [26], requiring the machine to traverse several noncoplanar beams. In contrast, the VMATHA optimization integrates noncoplanar beams with VMAT using four couch angles and one isocentric irradiation, potentially achieving shorter dose delivery times for multiple targets compared to the 4π technique.

An essential aspect of VMATHA planning is the SRS normal tissue objective, aimed at achieving the most closely packed dose possible while minimizing the dose spread across targets. In this regard, our findings indicated that compared to the VMATRA plans, the VMATHA plans involved more intricate MLC patterns with smaller segments (lower MCSv values). However, as reported by Ohira et al. [27], lower MCSv values in VMAT plans could lead to reduced dosimetric accuracy, as evidenced by gamma pass rates, highlighting the need for meticulous dosimetric validation before clinically implementing VMATHA plans.

In this study, we utilized two different MLC systems: high-definition 120TM MLCs and Millennium MLCs from TrueBeam STx and Clinac iX, respectively. Notably, these MLCs exhibit evident differences in their geometric and dosimetric characteristics. Numerous studies have explored the effects of different MLC systems on planning and deliverability. In planning comparisons, the dosimetric impacts of different MLC systems on plan quality have been observed to be minimal [28-30]. Subramanian et al. [30] demonstrated that MLCs with a 5 mm width completely satisfied the RTOG-0813 treatment planning criteria for lung SBRT. The dosimetric superiority of VMATHA resulted from the delivery technique and optimization algorithm for planning rather than the different types of MLC systems.

Increased exposure of nontarget tissues to low radiation doses represents a crucial concern when using single-isocentric VMATHA for multiple targets. This is attributed to shared MLC leaf pairs between two or more targets, preventing the effective blocking of radiation exposure to normal tissues surrounding multiple metastases [31]. Wu et al. [32] demonstrated that employing a collimator optimization algorithm could significantly reduce the dose exposure to normal brain tissues while maintaining similar target coverages. This algorithm, developed for VMATHA planning, solves the “island blocking” issue, and our results demonstrate that compared to multi-isocentric irradiation, VMATHA provides a lower volume of brain tissues receiving a dose >4 Gy. Despite the slightly higher V2Gy of the VMATHA plan, this low-dose spread is not deemed clinically significant, specifically considering that it remains below the levels typically used for whole-brain irradiation in a single fraction.

Conclusions

Compared with VMATRA, VMATHA demonstrated superior performance in achieving rapid dose fall-offs (as indicated by the GI) and a higher HI at the PTV, while more effectively sparing normal tissues in brain SRS. The enhanced dosimetric plan quality achieved by HyperArc highlights its potential as a new and preferred option for delivering SRS doses, particularly in high-precision treatments for brain SRS. This advancement presents HyperArc as a significant development in the field of radiation therapy, offering optimized treatment outcomes for patients undergoing brain SRS.

Funding

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. RS-2023-00253604).

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: So-Yeon Park, Na Young Jang. Data curation: So-Yeon Park, Noorie Choi. Formal analysis: So-Yeon Park. Funding acquisition: So-Yeon Park. Investigation: So-Yeon Park. Methodology: So-Yeon Park. Na Young Jang. Project administration: So-Yeon Park. Resources: Noorie Choi, Na Young Jang. Software: So-Yeon Park. Supervision: Na Young Jang. Validation: Noorie Choi. Visualization: So-Yeon Park, Na Young Jang. Writing – original draft: So-Yeon Park. Writing – review & editing: So-Yeon Park, Na Young Jang.

Ethics Approval and Consent to Participate

This study was approved by the Institutional Review Board of Veterans Health Service Medical Center (IRB No. 2023-01-001). The requirement for written informed consent was waived by the IRB due to the retrospective nature of the study.

Fig 1.

Figure 1.Representative dose distributions of brain stereotactic radiosurgery cases (patient #3 and #14): Dose distributions of volumetric modulated arc therapy plans created using RapidArc volumetric modulated arc therapy (VMATRA) (a) and HyperArc (VMATHA) (b) for patient #3. Dose distributions of VMATRA (c) and VMATHA (d) for patient #14. Doses are depicted by color wash with 2 Gy (the lowest dose) in blue and 30 Gy (the highest dose) in red.
Progress in Medical Physics 2024; 35: 36-44https://doi.org/10.14316/pmp.2024.35.2.36

Fig 2.

Figure 2.Representative dose-volumetric histograms of brain stereotactic radiosurgery cases (patient #3 and #14). Volumetric modulated arc therapy plans generated using RapidArc (RA) (a) and HyperArc (HA) (b) plotted using solid and dashed lines, respectively, for the planning target volume (PTV) and normal brain.
Progress in Medical Physics 2024; 35: 36-44https://doi.org/10.14316/pmp.2024.35.2.36

Table 1 Planning constraints for the planning target volume, normal brain tissue, and ring structures surrounding the PTV at distances of 3 and 13 mm in brain stereotactic radiosurgery plans

StructurePlanning constraintsPriority
PTVV100% >95%150
D100% >100% of the prescription dose150
Normal brainV2Gy <20%100
Ring3mmDmax <70% of the prescription dose100
Ring13mmDmax <30% of the prescription dose100

PTV, planning target volume; Vn%, volume receiving n% of the prescription dose; Dn%, dose received by n% of the target volume; VnGy, volume receiving n Gy; Dmax, maximum dose; Ringnmm, ring structure surrounding the PTV at a distance of n mm.


Table 2 Average DVH parameters for planning target volumes in brain stereotactic radiosurgery plans

DVH parameterVMATRAVMATHAP-value
Volume (cm3)9.77±7.42-
Minimum dose (%)95.28±1.9790.81±4.08<0.0001
Maximum dose (%)112.77±11.12142.60±6.60<0.0001
Mean dose (%)105.94±5.39119.33±4.57<0.001
CIpaddick1.10±0.130.99±0.09-
HI1.13±1.111.43±0.66<0.0001
GI3.79±0.762.56±0.17<0.001

Data are presented as mean±standard deviation.

DVH, dose-volumetric histogram; VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; CIpaddick, conformity index suggested by Paddick et al.; HI, homogeneity index; GI, gradient index.


Table 3 Average DVH parameters for normal brain regions in brain stereotactic radiosurgery plans

DVH parameterVMATRAVMATHAP-value
Volume (cm3)1,460.82±296.47-
V100% (cm3)11.03±9.329.40±7.05<0.001
V50% (cm3)37.29±25.4323.37±16.70<0.0001
V18Gy (%)0.28±0.320.08±0.080.021
V12Gy (%)1.04±0.920.46±0.460.002
V2Gy (%)17.82±11.1811.28±9.49<0.0001

Data are presented as mean±standard deviation.

DVH, dose-volumetric histogram; VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; Vn%, volume receiving n% of the prescription dose; VnGy, volume receiving at least n Gy.


Table 4 Average total monitor units, beam-on time, and modulation complexity scores of volumetric modulated arc therapy plans for brain stereotactic radiosurgery

VMATRAVMATHAP-value
MU4,245.60±750.586,704.85±1,497.61<0.0001
Beam-on time (s)424.56±75.06287.35±64.18<0.0001
MCSv0.619±0.0320.539±0.045<0.001

Data are presented as mean±standard deviation.

VMATRA, volumetric modulated arc therapy with RapidArc; VMATHA, volumetric modulated arc therapy with HyperArc; MU, monitor unit; MCSv, modulation complexity score for volumetric modulated arc therapy generated by Masi et al. (2013) [23].


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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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