Optimization of Multi-leaf Collimator Leaf Offset Table for a Treatment Planning System

Objective: High levels of beam modulation complexity (MC) and monitor units (μ) 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 μ using different multi leaf collimator (MLC) architectures. Methods: 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. Results: DL-MLC plans demonstrated lower dose-sparing values than SL-MLC plans (p < 0.05). TPS-parameters didn’t change significantly the complexity metrics for either MLC architectures. However, for SL-MLC, significant variations of μ, target volume dose-homogeneity, and dose-spillage were associated with ASC and DCR (p < 0.05). μ 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. Conclusions: TPS-parameters selected do not change MC for DL-MLC architecture, but they might have a potential use to control the μ, 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 analysed and considered as an additional predictor of GPRs in further studies. Advances in knowledge: Dose-sparing priority might influence in modulation complexity of DL-MLC.

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Inter-unit variability of multi-leaf collimator parameters for IMRT and VMAT treatment planning: a multi-institutional survey

Journal of Radiation Research ◽

10.1093/jrr/rrz082 ◽

2020 ◽

Vol 61 (2) ◽

pp. 307-313 ◽

Cited By ~ 1

Author(s):

Masaru Isono ◽

Yuichi Akino ◽

Hirokazu Mizuno ◽

Yoshihiro Tanaka ◽

Norihisa Masai ◽

...

Keyword(s):

Treatment Planning ◽

Intensity Modulated Radiotherapy ◽

Planning System ◽

Treatment Planning System ◽

Transmission Factor ◽

Source Surface ◽

Sensitive Volume ◽

Medical Systems ◽

Multi Leaf Collimator ◽

Dosimetric Leaf Gap

Abstract Modern treatment machines have shown small inter-unit variability regarding beam data. Recently, vendor-provided average beam data, such as the Representative Beam Data (RBD) of the TrueBeam (Varian Medical Systems, Palo Alto, CA, USA), has been used for modeling of the Eclipse (Varian Medical Systems) treatment planning system. However, RBD does not provide multi-leaf collimator (MLC) parameters, such as MLC leaf transmission factor (LTF) and dosimetric leaf gap (DLG). We performed a web-based multi-institutional survey to investigate these parameters as well as the measurement protocols and customization of the parameters for intensity-modulated radiotherapy (IMRT) and/or volumetric modulated radiotherapy (VMAT) commissioning. We collected 69 sets of linear accelerator (linac) data from 58 institutions. In order to measure MLC parameters, most institutions used farmer-type ionization chambers with a sensitive volume of 0.6 cm3, water phantoms, source surface distance of 90 cm with 10 cm depth, and a vendor-provided plan. The LTF showed small inter-unit variabilities, although the DLG showed large variations. For optimization of the parameters for IMRT/VMAT calculations, DLG values were upwardly adjusted at many institutions, whereas the LTF values were modestly changed. We clarified that MLC parameters were measured under the same conditions at more than half of the facilities. Most institutions customized parameters in a similar manner for IMRT/VMAT. The median measured and customized values obtained in our study will be valuable to verify MLC installation accuracy and to shorten the iterative processes of finding the optimal values.

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