Spatial variation of dosimetric leaf gap and its impact on dose delivery

Self Cite

The Eclipse treatment planning system uses a single dosimetric leaf gap (DLG) value to retract all multileaf collimator leaf positions during dose calculation to model the rounded leaf ends. This study evaluates the dosimetric impact of the 2D variation of DLG on clinical treatment plans based on their degree of fluence modulation. In‐house software was developed to retrospectively apply the 2D variation of DLG to 61 clinically treated VMAT plans, as well as to several test plans. The level of modulation of the VMAT cases were determined by calculating their modulation complexity score (MCS). Dose measurements were done using the MapCHECK device at a depth of 5.0 cm for plans with and without the 2D DLG correction. Measurements were compared against predicted dose planes from the TPS using absolute 3%/3 mm and 2%/2 mm gamma criteria for test plans and for VMAT cases, respectively. The gamma pass rate for the 2 mm, 4 mm, and 6 mm sweep test plans increased by 23.2%, 28.7%, and 26.0%, respectively, when the measurements were corrected with 2D variation of DLG. The clinical anal VMAT cases, which had very high MLC modulation, showed the most improvement. The majority of the improvement occurred for doses created by the 1.0 cm width leaves for both the test plans and the VMAT cases. The gamma pass rates for the highly modulated head and neck (H&N) cases, moderately modulated prostate and esophageal cases, and minimally modulated brain cases improved only slightly when corrected with 2D variation of DLG. This is because these cases did not employ the 1.0 cm width leaves for dose calculation and delivery. These data suggest that, at the very least, the TPS plans with highly modulated fluences created by the 1.0 cm fields require 2D DLG correction. Incorporating the 2D variation of DLG for the highly modulated clinical treatment plans improves their planar dose gamma pass rates, especially for fields employing the outer 1.0 cm width MLC leaves. This is because there are differences in DLG between the true DLG exhibited by the 1.0 cm width outer leaves and the constant DLG value modeled by the TPS for dose calculation.PACS numbers: 87.55.D, 87.55.Qr, 87.56.Fc, 87.56.N, 87.56.nk

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Evaluation of fluence‐based dose delivery incorporating the spatial variation of dosimetric leaf gap (DLG)

Kumaraswamy

Bailey

et al. 2016

Self Cite

“…They evaluated PDD curves in the phantom by comparing between the dose calculated by MC method, by TPS with four algorithms, and experimental data according to the different field size from 1 × 1 cm 2 to 10 × 10 cm 2 . As the results of this study, the dose difference was increased up to about 40% in the region of lung‐tissue equivalent material comparing between MC and AAA for the 1 × 1 cm 2 field due to the lateral electronic disequilibrium effect . In our results, about 3% difference of the prescribed dose in the normal tissue could cause by a large ratio of LPs (<10mm) in the treatment plan, even though the patient case is not that heterogeneous case.…”

Section: Resultsmentioning

confidence: 49%

“…The dose of the TPS in the outer penumbra region of the x‐direction profiles was overestimated than that of the Geant4 and measurement because of the 1.7% leaf transmission set in TPS. We assumed that the reason for this overestimation is to compensate dosimetric difference at dose fall‐off region caused by the flat‐end shape of MLC . The second validation process demonstrated the accuracy of the current MC simulation with film dose measurements for two kinds of MLC field.…”

Section: Resultsmentioning

confidence: 86%

“…investigated the influence of lung heterogeneity on dose distribution in a soft tissue phantom. They evaluated PDD curves in the phantom by comparing between the dose calculated by MC method, by TPS with four algorithms, and experimental data according to the different field size from 1 × 1 cm 2 to 10 × 10 cm 2 . As the results of this study, the dose difference was increased up to about 40% in the region of lung‐tissue equivalent material comparing between MC and AAA for the 1 × 1 cm 2 field due to the lateral electronic disequilibrium effect .…”

Section: Resultsmentioning

confidence: 91%

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Development of a Geant4‐based independent patient dose validation system with an elaborate multileaf collimator simulation model

Choi

Park

Shin

et al. 2019

Despite the improvements in the dose calculation models of the commercial treatment planning systems (TPS), their ability to accurately predict patient dose is still limited. One of the limitations is caused by the simplified model of the multileaf collimator (MLC). The aim of this study was to develop a Monte Carlo (MC) method‐based independent patient dose validation system with an elaborate MLC model for more accurate dose evaluation. Varian Clinac 2300 IX was simulated using Geant4 toolkits, after which MC commissioning with measurements was performed to validate the simulation model. A DICOM‐RT interface was developed to obtain the beam delivery conditions including the hundreds of MLC motions. Finally, the TPS dose distributions were compared with the MC dose distributions for water phantom cases and a patient case. Our results show that the TPS overestimated the absolute abutting leakage dose in the closed MLC field, with about 20% more of the maximum dose than that of the MC calculation. For water phantom cases, the dose distributions inside the target region were almost identical with the dose difference of less than 2%, while the dose near the edge of the target shows difference about 10% between Geant4 and TPS due to geometrical differences in MLC model. For the patient analysis, the Geant4 and TPS doses of all organs were matched well within 1.4% of the prescribed dose. However, for organs located in areas with high ratio of leaf pairs with distances less than 10 mm leaf pair (LP (<10mm)), the maximum dose of TPS was overestimated by about 3% of the prescribed dose. These dose comparison results demonstrate that our system for calculating the patient dose is quite accurate. Furthermore, if the MLC sequences in treatment plan have a large ratio of LP (short), more than 3% dose difference in normal tissue could be seen.

Unlocking a closed system: dosimetric commissioning of a ring gantry linear accelerator in a multivendor environment

Saini

Tichacek

Johansson

et al. 2021

The Halcyon™ platform is self‐contained, combining a treatment planning (Eclipse) system TPS) with information management and radiation delivery components. The standard TPS beam model is configured and locked down by the vendor. A portal dosimetry‐based system for patient‐specific QA (PSQA) is also included. While ensuring consistency across the user base, this closed model may not be optimal for every department. We set out to commission independent TPS (RayStation 9B, RaySearch Laboratories) and PSQA (PerFraction, Sun Nuclear Corp.) systems for use with the Halcyon linac. The output factors and PDDs for very small fields (0.5 × 0.5 cm2) were collected to augment the standard Varian dataset. The MLC leaf‐end parameters were estimated based on the various static and dynamic tests with simple model fields and honed by minimizing the mean and standard deviation of dose difference between the ion chamber measurements and RayStation Monte Carlo calculations for 15 VMAT and IMRT test plans. Two chamber measurements were taken per plan, in the high (isocenter) and lower dose regions. The ratio of low to high doses ranged from 0.4 to 0.8. All percent dose differences were expressed relative to the local dose. The mean error was 0.0 ± 1.1% (TG119‐style confidence limit ± 2%). Gamma analysis with the helical diode array using the standard 3%Global/2mm criteria resulted in the average passing rate of 99.3 ± 0.5% (confidence limit 98.3%–100%). The average local dose error for all detectors across all plans was 0.2% ± 5.3%. The ion chamber results compared favorably with our recalculation with Eclipse and PerFraction, as well as with several published Eclipse reports. Dose distribution gamma analysis comparisons between RayStation and PerFraction with 2%Local/2mm criteria yielded an average passing rate of 98.5% ± 0.8% (confidence limit 96.9%–100%). It is feasible to use the Halcyon accelerator with independent planning and verification systems without sacrificing dosimetric accuracy.