Dual-layer Multileaf Collimator Research Articles

A leaf sequencing algorithm for an orthogonal dual-layer multileaf collimator.

Dual layer MLC (DMLC) has have been adopted in several commercial products and one major challenge in DMLC usage is leaf sequencing for intensity-modulated radiation therapy (IMRT). In this study we developed a leaf sequencing algorithm for IMRT with an orthogonal DMLC. 
Methods and Materials: This new algorithm is inspired by the algorithm proposed by Dai and Zhu for IMRT with single layer MLC (SMLC). It iterately determines a delivery segment intensity and corresponding segment shape for a given fluence matrix and leaves residual fluence matrix to following iterations. The segment intensity is determined according to complexities of residual fluence matrix when segment intensity varies from one to highest level in the matrix. The segment intensity and corresponding segment shape that result least complexity was selected. This algorithm has been evaluated with 9 groups of randomly generated fluence matrices with various dimensions and intensity levels. Sixteen fluence matrices generated in Pinnacle system for two clinical IMRT examples were also used for evaluation. Statistical information of leaf sequences generated with this algorithm for both the random and clinical matrices were compared to the results of two typical algorithms for SMLC: that proposed by Dai and Zhu and that proposed by Bortfled. 
Results: Compared to the SMLC delivery sequences generated with Dai and Zhu's algorithm, the proposed algorithm for orthogonal DMLC delivery reduces the average number of segments by 27.7%~41.8% for 9 groups of randomly generated fluence matrices and 10.5%~41.7% for clinical ones. When comparing MU efficiency between different algorithms, it is observed that the proposed algorithm performs better than the optimal efficiency of SMLC delivery when dealing with simple fluence matrices, but slightly worse when handling complex ones.&#xD.

The delivery assessment for small targets on Halcyon radiotherapy system - Measured and calculated dose comparison.

With the ever-increasing requirements of accuracy and personalization of radiotherapy treatments, stereotactic radiotherapy (SRT) with volumetric modulated arc therapy (VMAT) on O-ring Halcyon radiotherapy system could potentially provide a fast, safe, and feasible treatmentoption. The purpose of this study was to assess the delivery of Halcyon VMAT plans for small targets. Well-defined VMAT-SRT plans were created on Halcyon radiotherapy system with the stacked and staggered dual-layer MLC design for the film measurement set-up and the target sizes and shapes designed to emulate the targets of the stereotactic treatments. The planar dose distributions were acquired with film measurements and compared to a current clinical reference dose calculation with AcurosXB (v18.0, Varian Medical Systems) and to Monte Carlo simulations. With the collapsed arc versions of the VMAT-SRT plans, the uncertainty in dose delivery due to the multileaf collimator (MLC) without the gantry rotation could be separated andanalyzed. The target size was mainly limited by the resolution originated from the design of the MLC leaves. The results of the collapsed arc versions of the plans show good consistency among measured, calculated, and simulated dose distributions. With the full VMAT plans, the agreement between calculated and simulated dose distributions was consistent with the collapsed arc versions. The measured dose distribution agreed with the calculated and simulated dose distributions within the target regions, but considerable local differences were observed in the margins of the target. The largest differences located in the steep gradient regions presumably originating from the deviation of the isocenter. The potential of the Halcyon radiotherapy system for VMAT-SRT delivery was evaluated and the study revealed valuable insights on the machine characteristics with thedelivery.

Characterization of a novel VenusX orthogonal dual-layer multileaf collimator.

To investigate and characterize the performance of a novel orthogonal dual-layer alpha multileaf collimator (αMLC) mounted on the LinaTech VenusX linac. We evaluated leaf positioning accuracy and reproducibility using an electronic portal imaging device through the picket fence test. The average, interleaf, intraleaf, and leaf tip transmissions of the single and dual layers were measured using an ionization chamber. Square and rhombus fields were used to evaluate the leaf penumbra of αMLC. To investigate the advantages of the orthogonal dual-layer multileaf collimator (MLC) in field shaping, right triangular and circular pattern fields were formed using both the dual layers and single layers of the αMLC. The average maximum positioning deviations of the upper and lower αMLC over 1 year were 0.76±0.09mm and 0.62±0.07mm, respectively. The average transmissions were 1.87%, 1.83%, and 0.03% for the upper-, lower- and dual-layer αMLC, respectively. The maximum interleaf transmissions of the lower- and dual-layer were 2.43% and 0.17%, respectively. The leaf tip transmissions were 9.34% and 0.25%, respectively. The penumbra of the square field was 6.2mm in the X direction and 8.0mm in the Y direction. The average penumbras of the rhombus fields with side lengths of 5 and 10cm were 3.6 and 4.9mm, respectively. For the right triangular and circular fields, the fields shaped by the dual-layer leaves were much closer to the set field than those shaped by single-layer leaves. The dose undulation amplitude of the 50% isodose lines and leaf stepping angle change of the dual-layer leaves were smaller than those of the single-layer leaves. The αMLC benefits from its orthogonal dual-layer design. Leaf transmission, dose undulations at the field edge, and MLC field dependence of the leaf stepping angle of the dual-layer αMLC were remarkably reduced.

Examination of Conversion Method of Dose Distribution of Lung Cancer IMRT Using Fluence Reversible Calculation Function in O-Ring Type Linac and C Type Linac

<h3>Purpose/Objective(s)</h3> An O-ring type Linear Accelerator (O-Linac) has a dual-layer multi-leaf collimator (MLC) with flattening filter-free (FFF). Generally, when Linac fails, the converted irradiation between C-arm typed Linac (C-Linac) has the same mechanical structure, so it is possible to perform converted irradiation without re-calculating the dose distribution by the radiation therapy planning system (TPS). The O-Linac differs from C-Linac in the process of forming the dose distribution. Therefore, if O-type Linac becomes unusable due to a breakdown or inspection, it will be necessary to formulate a treatment plan from scratch. It is of great benefit to patients to clarify the consistency of treatment plans between these lilacs and to establish a simpler converted plan creation method. In this study, we investigate a method for more easily performing converted irradiation from O-Linac to C-Linac. <h3>Materials/Methods</h3> Thirty patients with lung cancer planned with VMAT using O-Linac are included. The O-lilac's dose distribution is converted into energy fluence by the function of TPS. The alternative Linac MLC is then optimized to achieve energy fluence. The time required to convert one plan is about 20-30 minutes. The HI, CI, and PTV (D95%, D2%) of the converted plan were compared with the original plan. For OARs, the DVH of lung, esophagus, heart, and spinal Cord were evaluated. The shapes of the iso-dose curves for each 10% in each plan were compared using the dice similarity coefficient (DSC). <h3>Results</h3> The statistical results of PTV and OARs are reported in Table. There was no significant difference for PTV (<i>P</i> > 0.05). The lung V5 (%) and Dmean (Gy) of the converted plan were slightly higher than those of the original plan. The lung V10 (%) and V20 (%), heart V50 (Gy), esophageal V60 (Gy), and spinal cord Dmax (Gy) of the converted plans were slightly lower than the original plans. However, these differences were not statistically significant (<i>P</i> > 0.05). The mean DSCs of the 30% to 100% iso-dose curves of the prescribed dose are above 0.8, and the DSCs on the iso-dose curves of ≥ 105% and ≤ 20% were less than 0.8. <h3>Conclusion</h3> Due to the structural differences of lilacs, it is considered that the dose-volume and the generation position were different in the dose range of ≥ 105% and ≤ 20%, and as a result, DSCs have decreased. However, no statistically significant difference in DVH has been identified for both treatment plans. From this result, it is considered possible to convert the dose distribution planned by O-Linac into C-lilac's dose distribution. It was proposed a simple replanning method.

Dosimetric leaf gap and leaf trailing effect in a double-stacked multileaf collimator.

To investigate (i) the dosimetric leaf gap (DLG) and the effect of the "trailing distance" between leaves from different multileaf collimator (MLC) layers in Halcyon systems and (ii) the ability of the currently available treatment planning systems (TPSs) to approximate this effect. DICOM plans with transmission beams and sweeping gap tests were created in Python for measuring the DLG for each MLC layer independently and for both layers combined. In clinical Halcyon plans both MLC layers are interchangeably used and leaves from different layers are offset, thus forming a trailing pattern. To characterize the impact of such configuration, new tests called "trailing sweeping gaps" were designed and created where the leaves from one layer follow the leaves from the other layer at a fixed "trailing distance" between the tips. Measurements were carried out on five Halcyons SX2 from different institutions and calculations from both the Eclipse and RayStation TPSs were compared with measurements. The dose accumulated during a sweeping gap delivery progressively increased with the trailing distance . We call this "the trailing effect." It is most pronounced for between 0 and 5mm, although some changes were obtained up to 20mm. The dose variation was independent of the gap size. The measured DLG values also increased with up to 20mm, again with the steepest variation between 0 and 5mm. Measured DLG values were negative at =0 (the leaves from both layers at the same position) but changed sign for ≥1mm, in line with the positive DLG sign usually observed with single-layer rounded-end MLCs. The Eclipse TPS does not explicitly model the leaf tip and, as a consequence, could not predict the dose reduction due to the trailing effect. This resulted in dose discrepancies up to +10% and -8% for the 5mm sweeping gap and up to ±5% for the 10mm one depending on the distance . RayStation implements a simple model of the leaf tip that was able to approximate the trailing effect and improved the agreement with measured doses. In particular, with a prototype version of RayStation that assigned a higher transmission at the leaf tip the agreement with measured doses was within ±3% even for the 5mm gap. The five Halcyon systems behaved very similarly but differences in the DLG around 0.2mm were found across different treatment units and between MLC layers from the same system. The DLG for the proximal layer was consistently higher than for the distal layer, with differences ranging between 0.10mm and 0.24mm. The trailing distance between the leaves from different layers substantially affected the doses delivered by sweeping gaps and the measured DLG values. Stacked MLCs introduce a new level of complexity in TPSs, which ideally need to implement an explicit model of the leaf tip in order to reproduce the trailing effect. Dynamic tests called "trailing sweeping gaps" were designed that are useful for characterizing and commissioning dual-layer MLC systems.

Effect of treatment planning system parameters on beam modulation complexity for treatment plans with single-layer multi-leaf collimator and dual-layer stacked multi-leaf collimator.

High levels of beam modulation complexity (MC) and monitor units (MU) can compromise the plan deliverability of intensity-modulated radiotherapy treatments. Our study evaluates the effect of three treatment planning system (TPS) parameters on MC and MU using different multi-leaf collimator (MLC) architectures. 192 volumetric modulated arc therapy plans were calculated using one virtual prostate phantom considering three main settings: (1) three TPS-parameters (Convergence; Aperture Shape Controller, ASC; and Dose Calculation Resolution, DCR) selected from Eclipse v15.6, (2) four levels of dose-sparing priority for organs at risk (OAR), and (3) two treatment units with same nominal conformity resolution and different MLC architectures (Halcyon-v2 dual-layer MLC, DL-MLC & TrueBeam single-layer MLC, SL-MLC). We use seven complexity metrics to evaluate the MC, including two new metrics for DL-MLC, assessed by their correlation with γ passing rate (GPR) analysis. DL-MLC plans demonstrated lower dose-sparing values than SL-MLC plans (p<0.05). TPS-parameters did not change significantly the complexity metrics for either MLC architectures. However, for SL-MLC, significant variations of MU, target volume dose-homogeneity, and dose spillage were associated with ASC and DCR (p<0.05). MU were found to be correlated (highly or moderately) with all complexity metrics (p<0.05) for both MLC plans. Additionally, our new complexity metrics presented a moderate correlation with GPR (r<0.65). An important correlation was demonstrated between MC (plan deliverability) and dose-sparing priority level for DL-MLC. TPS-parameters selected do not change MC for DL-MLC architecture, but they might have a potential use to control the MU, PTV homogeneity or dose spillage for SL-MLC. Our new DL-MLC complexity metrics presented important information to be considered in future pre-treatment quality assurance programs. Finally, the prominent dependence between plan deliverability and priority applied to OAR dose sparing for DL-MLC needs to be analyzed and considered as an additional predictor of GPRs in further studies. Dose-sparing priority might influence in modulation complexity of DL-MLC.

PO-1381: Performance in clinical conditions of the dual-layer multileaf collimator from a Halcyon 2.0 linac

PO-1381: Performance in clinical conditions of the dual-layer multileaf collimator from a Halcyon 2.0 linac

Plan complexity quantification of dual-layer multi-leaf collimator for volumetric modulated arc therapy with Halcyon linac.

This study characterized the plan complexity of the dual-layer multi-leaf collimator (MLC) performance for volumetric modulated arc therapy (VMAT) in Halcyon. Plan complexity metrics were computed independently for each MLC layer (proximal and distal): modulation complexity score (MCS), plan averaged beam area, irregularity, and modulation (PA, PI, and PM) were obtained. These were investigated when weighted by proportions of the effective proximal MLCand distal MLC (MCSw, PAw, PIw, and PMw). Conventional plan complexity metrics for the effective 5.0 mm resolution MLC (MCS5, PA5, PI5, PM5), small aperture score (SAS), effective distal MLC score (EDS), and MU were also evaluated. Forty-five consecutive VMAT plans were analyzed retrospectively for treatment sites of the prostate, head and neck (HN), and other parts. The mean values of the metrics for proximal MLCand distal MLC were 0.310 and 0.245 (MCS), 71.09 cm2 and 55.92 cm2 (PA), 6.24 and 10.19 (PI), and 0.593 and 0.645 (PM), respectively. The degree of plan complexity in the distal MLC was larger than in the proximal MLC. The percentage of the effective distal MLC was higher than that of the proximal MLC (mean EDS = 0.555). The MCSw, PAw, PIw, and PMw showed significant differences between the prostate and HN groups like the PA5, PI5, and PM5; however, the MCS5 did not. Additionally, the MCS5 correlated with MU and PM5, while the MCSw correlated with SAS and PA5. The plan complexity for each MLC layer and subsequently combining these complexities might be useful to better understand the dual-layer MLC performance.

Intracranial Stereotactic Radiation Therapy With a Jawless Ring Gantry Linear Accelerator Equipped With New Dual Layer Multileaf Collimator

PurposeTo test the feasibility of a simplified, robust, workflow for intracranial stereotactic radiation therapy (SRT) using a ring gantry linear accelerator (RGLA) equipped with a dual-layer stacked, staggered, and interdigitating multileaf collimator. Materials and MethodsTwenty recent clinical SRT cases treated using a radiosurgery c-arm linear accelerator were anonymized. From these data sets, a new planning workflow was developed and used to replan these cases, which then were compared to their clinical counterparts. Population-based dose-volume histograms were analyzed for target coverage and sparing of healthy brain. All plans underwent plan review and quality assurance and were delivered on an end-to-end verification phantom using image guidance to simulate treatment. ResultsThe RGLA plans were able to meet departmental standards for target coverage and organ-at-risk sparing and showed plan quality similar to the clinical plans. RGLA plans showed increases in the 50% isodose in the axial plane but decreases in the sagittal and coronal planes. There were no statistically significant differences in the homogeneity index or number of monitor units between the 2 systems. There were statistically significant increases in conformity and gradient indices, with median values of 1.09 versus 1.11 and 2.82 versus 3.13, respectively, for the c-arm versus RGLA plans. These differences were not believed to be clinically significant because they met clinical goals. The population-based dose-volume histograms showed target coverage and organ-at-risk sparing similar to that of the clinical plans. All plans were able to meet the departmental quality assurance requirements and were delivered under image guidance on an end-to-end phantom with measurements agreeing within 3% of the expected value. RGLA plans showed a median reduction in delivery time of ≈50%. ConclusionsThis work describes a simplified and efficient workflow that could reduce treatment times and expand access to SRT to centers using an RGLA.

A Method of High-Resolution Radiotherapy Delivery Fluences with a Pair of Fields with Orthogonal Collimator Settings: A Study on Ten Head-and-Neck Cancer Patients.

Context:Introduction of dual-layer multileaf collimator (MLC) radiotherapy linear accelerators into clinical practice is an important development in advanced external beam radiotherapy. A method of delivering comparable high-resolution fluences with a single-layer MLC is presented.Aims:The aims of this study are to present new algorithms and approaches to define high-resolution hypermodulated fluences, obtain orthogonal decomposition of fluences, and deliver them on a linear accelerator with single MLC from two perpendicular collimator settings.Materials and Methods:High-resolution fluences were defined using Monte Carlo (MC) calculation. A novel use of a limited-memory, bounded, Broyden–Fletcher–Goldfarb–Shanno algorithm was used to decompose such fluences to ones deliverable with a pair of fields with mutually orthogonal collimator settings. Such a technique, here named cross motion leaf calculator (XMLC), is compared against single sliding window (SSW) technique typically used in intensity-modulated radiation therapy (IMRT). An electronic portal imaging device (EPID) is used, and the results were compared with gamma analysis. Furthermore, MC was used to determine dose distributions for computed tomography images of ten head-and-neck cancer patients.Results:Gamma analysis (3%, 3 mm) against ideal fluence is considerably more favorable to XMLC (94% ± 4%) versus SSW (76% ± 5%). Furthermore, the dose–volume histogram (DVH) analysis showed that XMLC enables delivery of fluences superior to that of IMRT and these results in clinically relevant enhancements in DVH results.Conclusions:At the time of writing of this study, there were more than 12,000 medical linear accelerators in clinical use, and XMLC can prove itself useful wherever linac is equipped with MLC but cannot delivery latest techniques, such as volumetric modulated arc therapy.

Single-arc VMAT optimization for dual-layer MLC

Dual-layer multi-leaf collimator (DLMLC) has recently attracted renewed interest due to its good balance among resolution, low leakage, and high fabricability. However, existing progressive sampling based volumetric modulated arc therapy (VMAT) algorithm is ineffective for DLMLC, requiring more arcs to achieve dosimetry comparable to VMAT plans with higher resolution single-layer MLC (SLMLC). In this study, we develop a novel single-arc VMAT optimization framework to take advantage of the unique DLMLC characteristics fully. Direct aperture optimization (DAO) for single-arc DLMLC VMAT was formulated as a least square dose fidelity objective, along with an anisotropic total variation term to regulate the fluence smoothness and a single segment term for forming simple apertures. The DAO was solved through alternating optimization approach. The DLMLC deliverability constraint and the MLC leaf speed constraint were formulated as the optimization constraints and solved using a graph optimization algorithm. Feasibility of the proposed framework was tested on a brain, a lung, and a prostate cancer patient. The framework was further adapted for a simultaneous integrated boost (SIB) case. The single-arc DLMLC-10 mm (leaf width) plan was compared against single-arc SLMLC VMAT plans including SLMLC-5mm, SLMLC-10mm, and SLMLC with 10 mm leaf width and 5 mm leaf step size (SLMLC-10mm-5mm). Compared with the SLMLC-10mm plan and the SLMLC-10mm-5mm plan, with the same target coverage, the DLMLC-10 mm plan reduced R50 by 30.7% and 10.0%, the average max OAR dose by 5.79% and 3.7% of the prescription dose, and the average mean OAR dose by 4.18% and 2.1% of the prescription dose, respectively. The plan quality is comparable to that of the SLMLC-5mm plan. The novel single-arc VMAT optimization framework for DLMLC utilizes two MLC layers to improve the effective modulation resolution and afford more sophisticated modulation. Consequently, DLMLC VMAT achieves superior dosimetry to SLMLC VMAT with the same leaf width.

Application of TG-100 risk analysis methods to the acceptance testing and commissioning process of a Halcyon linear accelerator.

A new type of linear accelerator (linac) was recently introduced into the market by a major manufacturer. Our institution is one of the early users of this preassembled and preconfigured dual-layer multileaf collimator (MLC), ring-gantry linac - Halcyon™ (1st version). We performed a set of full acceptance testing and commissioning (ATC) measurements for three Halcyon machines and compared the measured data with the standard beam model provided by the manufacturer. The ATC measurements were performed following the guidelines given in different AAPM protocols as well as guidelines provided by the manufacturer. The purpose of the present work was to perform a risk assessment of the ATC process for this new type of linac and investigate whether the results obtained from this analysis could potentially be used as a guideline for improving the design features of this type of linac. AAPM's TG100 risk assessment methodology was applied to the ATC process. The acceptance testing process relied heavily on the use of a manufacturer-supplied phantom and the automated analysis of electronic portal imaging device (EPID) images. For the commissioning process, a conventional measurement setup and process (e.g., use of water tank for scanning) was largely used. ATC was performed using guidelines recommended in various AAPM protocols (e.g., TG-106, TG-51) as well as guidelines provided by the manufacturer. Six medical physicists were involved in this study. Process maps, process steps, and failure modes (FMs) were generated for the ATC procedures. Failure modes and effects analysis (FMEA) were performed following the guidelines given in AAPM TG-100 protocol. The top 5 and top 10 highest-ranked FMs were identified for the acceptance and commissioning procedures, respectively. Quality control measures were suggested to mitigate these FMs. A total of 38 steps and 88 FMs were identified for the entire ATC process. Fourteen steps and 34 FMs arose from acceptance testing. The top 5 FMs that were identified could potentially be mitigated by the manufacturer. For commissioning, a total of 24 steps and 54 potential FMs were identified. The use of separate measurement tools that are not machine-integrated has been identified as a cause for the higher number of steps and FMs generated from the conventional ATC approach. More than half of the quality control measures recommended for both acceptance and commissioning could potentially be incorporated by the manufacturer in the design of the Halcyon machine. This paper presents the results of FMEA and quality control measures to mitigate the FMs for the ATC process for Halcyon machine. Unique FMs that result from the differences in the ATC guidelines provided by the vendor and current conventional protocols, and the challenges of performing the ATC due to the new linac features and ring-gantry design were highlighted for the first time. The FMs identified in the present work along with the suggested quality control measures, could potentially be used to improve the design features of future ring-gantry type of linacs that arelikely to be preassembled, preconfigured, and heavily reliant on automation and image guidance.

Impact of Multi-leaf Collimator Parameters on Head and Neck Plan Quality and Delivery: A Comparison between Halcyon™ and Truebeam® Treatment Delivery Systems.

PurposeA new dual-layer multi-leaf collimator (MLC) system with several improved characteristics was introduced with the Varian Halcyon™ treatment platform. This study evaluated this new MLC’s impact on head and neck plan quality and delivery efficiency.MethodsNine patients were retrospectively studied with Institutional Review Board (IRB) approval. To compare plan quality between the Halcyon dual-layer MLC and Truebeam® MLC, all patients were replanned with the same prescription and target coverage following the institutional clinical protocol for both platforms and using both intensity modulated radiation therapy (IMRT) or volumetrically modulated arc therapy (VMAT) techniques. Organs-at-risk (OAR) dose-volume histogram (DVH) statistics were compared along with total plan monitor units (MU). To evaluate delivery efficiency, actual beam-on time for five patients’ plans were recorded and compared. To evaluate the impact of MLC performance parameters on plan quality, virtual MLC models were generated by matching Truebeam MLC’s parameters to those of the Halcyon dual-layer MLC both individually and combined. OAR doses were then compared between these virtual MLCs, the Truebeam MLC, and the actual Halcyon MLC.ResultsOverall the Halcyon dual-layer MLC provided similar plan quality compared to Truebeam MLC for VMAT plans, and improved sparing for majority of the OARs when using IMRT. Paired comparison showed median dose differences in mean doses to the parotids, cochlea, esophagus, and larynx ranged from -0.83 Gy to 0.37 Gy for VMAT, and from -4.79 Gy to -0.04 Gy for IMRT, with negative values indicating improved performance by Halcyon. Despite a slight increase in plan MU, the Halcyon reduced the total beam-on time by 42.8 ± 8.5%. Virtual MLC simulations demonstrated that matching MLC transmission accounted for nearly half of the total dose difference between Halcyon and Truebeam IMRT plans.ConclusionWhen compared to the Truebeam, the Halcyon’s dual-layer MLC achieved similar plan quality using VMAT, and improved OAR sparing using IMRT, while providing nearly twice as fast treatment delivery. Reduction in MLC transmission is the dominating factor contributing to dosimetric differences in OAR sparing.

Normal Brain Dose and Treatment Efficiency of Coplanar-Only IMRT/VMAT plans for Glioblastoma Multiforme using a Novel Ring Gantry Linac Delivery System

A novel ring gantry linac delivery system (RDS) with a 6MV flattening filter free (FFF) photon beam, a fast gantry rotation, and a jawless, fast dual-layer multileaf collimator (MLC) is now available commercially. The new RDS provides only the coplanar beam arrangements for IMRT or VMAT to treat glioblastoma multiforme (GBM). The purpose of this study was to compare the normal brain dose and delivery efficiency of treatment plans using the RDS with those obtained from the clinically treated plans using conventional linacs. Ten clinical IMRT or VMAT plans (4 coplanar and 6 non-coplanar, 46 Gy/23 fx) used previously for GBM treatment were retrospectively selected. The corresponding coplanar RDS plans were created with the same technique and number of beams/arcs using a treatment planning system. All RDS plans were optimized and normalized such that target coverages, average dose (Dmean) to PTV1-46Gy and Conformity index (CI) were comparable between the RDS plans and clinical plans. The RDS plans were also optimized to meet the same OAR dose constraints as the clinical plans. The R50 and D2cm from the isocenter in transverse and superior-inferior directions were compared. The normal brain dose (Brain minus PTV1) was evaluated with V10Gy and V30Gy. Treatment efficiency of the RDS was compared by recording the delivery time of all beams without including CBCT imaging/registration time. All RDS treatment plans were dosimetrically comparable to clinically treated plans; these also fulfilled the prescribed constraints and priorities: Dmean (47.7 ± 0.6 Gy (RDS) vs. 47.3 ± 0.6 Gy (linacs)) and CI (0.97 ± 0.09 (RDS) vs. 0.96 ± 0.09 (linacs)). When compared with non-coplanar linac plans, the average beam delivery times of the RDS plans are significantly reduced depending on the number of arcs/angles (2.1 ± 0.5 min (RDS) vs. 9.7 ± 2.9 min (linacs), p=0.007)). The median R50 of the RDS plans is about 7.7 % larger than the median R50 of the clinical linac plans (2.8 ± 0.7 (RDS) vs. 2.6 ± 0.5 (linacs). D2cm of the RDS plans in the transverse orientation is 13.8 % higher than the coplanar clinical linac plans (30.6 ± 3.2 Gy (RDS) vs. 26.9 ± 2.6 Gy (linacs)). It is noteworthy that the transverse D2cm becomes elevated to + 65 % when compared only with the non-coplanar linac plans. The V10Gy and V30Gy of two groups (RDS vs. non-coplanar linacs) are 73.3% vs. 68.9% and 22.9% vs. 21.1%, respectively. Our results show that the new RDS system provides coplanar IMRT/VMAT plans for the GBM with significantly improved treatment efficiency with no collision risk. When using the same technique and # of beams/arcs with non-coplanar linac plans, the R50, V10Gy and V30Gy, and transverse D2cm of the RDS plans become significantly elevated. Proper addition of arcs or static beams can adjust the parameters (data not shown).

Dosimetric Performance of a New Ring-Gantry Linear Accelerator for IMRT and VMAT Prostate Plans

We aimed to evaluate dosimetric performance and delivery efficiency of a new ring-gantry linac characterized by a staggered dual-layer multi-leaf collimator (MLC). Performance of treatment plans generated using a cloud-based planning system is reported here for the first time. Using IMRT & VMAT prostate plans, we compared performance of the ring gantry linac with that of a C-arm based linac. We hypothesized that both IMRT and VMAT treatments on the new system would yield similar target coverage and dose conformity, albeit with differences in treatment time and organ-at-risk (OAR) sparing. Ten prostate cancer patients previously planned using a treatment planning system (TPS) and treated with VMAT on a conventional C-arm linac (2300EX) at a dose rate of 300 MU/min were retrospectively re-planned for the new linac using a newer version of the TPS. Each plan was re-planned for IMRT (7 and 9 fields) and VMAT (2 and 3 arcs). Prescription dose for the target was standardized to 66.6 Gy. Plans were normalized to deliver 100% prescription dose to 95% of the target volume. Due to limitations in the current version of the ring gantry system, plans were generated using a single layer MLC (leaf width of 1 cm). Metrics of plan quality such as dose conformity, OAR sparing, and monitor units (MU) for new IMRT & VMAT plans were compared with those of the clinically delivered plans. Wilcoxon signed-rank test was used to compare differences in plan quality. Compared to the clinically delivered plans, rectum/bladder sparing and dose conformity were comparable for the new 9-field IMRT & VMAT (2- and 3-arc) plans, but were inferior for the 7-field IMRT plan (p <0.05). Both new IMRT plans achieved significantly better femoral head sparing than the delivered plans and the new VMAT plans (p <0.05). A significantly higher MU was required for all new plans (P < 0.05), to compensate for the wider leaf width. However, delivery time for the new VMAT plans is about 3 times shorter on the new system due to faster gantry and MLC motion. The 9-field IMRT and 2- and 3-arc VMAT prostate plans generated for the new ring-gantry linac offer dose coverage and OAR sparing that is comparable to the VMAT plans delivered on the conventional linac. When comparing ring-gantry IMRT and VMAT plans, the 9-field IMRT plans achieved better OAR sparing while maintaining similar dose coverage.Abstract TU_28_3176; Table 1Dosimetric comparison between clinically treated plans vs. plans for the ring-gantry linacClinically DeliveredPlans for new ring-gantry linac7-field IMRT9-field IMRT2-arc VMAT3-arc VMATCI1.02 ± 0.021.10 ± 0.051.06 ± 0.031.05 ± 0.031.04 ± 0.02HI1.05 ± 0.011.06 ± 0.011.05 ± 0.011.06 ± 0.011.05 ± 0.01MU666 ± 179931 ± 216948 ± 2321123 ± 4651151 ± 271Rectum V70 (%)33.3 ± 11.435.8 ± 14.931.5 ± 11.631.7 ± 10.133.1 ± 13.4Bladder V70 (%)32.2 ± 21.433.9 ± 20.828.8 ± 20.030.6 ± 18.431.1 ± 20.3Left Femoral Head V30 (%)19.1 ± 15.27.6 ± 10.49.7 ± 9.419.0 ± 14.317.9 ± 15.0Right Femoral Head V30 (%)18.3 ± 17.98.83 ± 15.54.5 ± 7.921.7 ± 11.916.0 ± 11.4 Open table in a new tab

SU-FF-T-28: A New Concept of Multileaf Collimator (dual-Layer MLC)

Multileaf collimator (MLC) is widely used dose delivery device for intensity-modulated radiation therapy (IMRT). The MLC is convenience to use and to adjust the field shape and beam fluence. However, there are some limitations to improve. One of them is longer beam-on time. As beam-on time is longer, patient's integrated dose is unnecessarily increased because of inter-leaf leakage. In this study, a new concept of MLC (dual-layer MLC) was proposed to solve this limitation in the aspects of total delivery intensity or monitor unit. The dual MLC consists of independently movable two MLC layer; upper and lower. Their thicknesses are 1 half value layer (HVL) for upper MLC and 5 HVL for lower. The initial intensity (I) of photon beam is attenuated half intensity (0.5I) if the beam is passing through the upper MLC only. Therefore the intensities are able to be delivered three steps; 0, 0.5, and 1. The sequencing algorithm was modified to dual-layer MLC in step and shot technique. To evaluate the usefulness of a new concept, the total delivery intensity and number of segments were compared with conventional MLC with by three field sizes (5×5, 10×10, 15×15, and 20×20) and four intensity levels (5, 8, 10, and 16). 10000 intensity maps were randomly created for each field size and intensity level. As a result, the total delivery intensity was reduced by 9% to 22% in all field size and number of segment was reduced by 25% to 45%. The unnecessary patient' dose is reduced effetely with dual-layer MLC because the required total delivery intensity was successfully reduced. The treatment time is also reduced.