Characteristics and limitations of a secondary dose check software for VMAT plan calculation

Abstract

Purpose: To assess the implementation, accuracy, and validity of the dosimetric leaf gap correction (DLGC) in Mobius3D VMAT plan calculations.

Methods: The optimal Mobius3D DLGC was determined for both a TrueBeam with a Millennium multi-leaf collimator and a TrueBeamSTx with a high-definition multi-leaf collimator. By analyzing a broad series of seven VMAT plans and comparing the calculated to the measured dose delivered to a cylindrical phantom, optimal DLGC values were determined by minimizing the dose difference for both the collection of all plans, as well as for each plan individually. The effects of plan removal from the optimization of the collective DLGC value, as well as plan-specific DLGC values, were explored to determine the impact of plan suite design on the final DLGC determination.

Results: Optimal collective DLGC values across all energies were between -0.71 and 0.89 mm for the TrueBeam, and between 0.35 and 1.85 mm for the TrueBeamSTx. The dose differences ranged between -6.1% and 2.6% across all plans when the optimal collective DLGC values were used. On a per-plan basis, the plan-specific optimal DLGC values ranged from -4.36 to 2.35 mm for the TrueBeam, and between -1.83 and 2.62 mm for the TrueBeamSTx. Comparing the plan-specific optimal DLGC to the average absolute leaf position from the central axis for each plan, a negative correlation was observed.

Conclusions: The optimal DLGC determination depends on the plans investigated, making it essential for users to utilize a suite of test plans that encompasses the full range of expected clinical plans when determining the optimal DLGC value. Validation of the secondary dose calculation should always be based on measurements, and not a comparison with the primary TPS. Varying disagreement with measurements across plans for a single DLGC value indicates potential limitations in the Mobius3D MLC model.

Keywords: MLC model; Mobius3D; dosimetric leaf gap; leaf offset; secondary dose check.

FIG. 1

Tomotherapy “cheese” phantom measurement setup. The phantom was positioned with the ion chamber inserts located laterally across the central slice. Ion chambers were adjusted on a per‐plan basis such that they were in the target location for each plan.

FIG. 2

Average Dose Difference vs. Collective DLGC. The average dose difference relative to measurements across all seven test plans for a given collective DLGC, machine, and energy. The optimal collective DLGC for each machine/energy was determined by fitting the data linearly and identifying the value for 0.0% absolute average dose difference. Each data point represents the percent difference averaged across the seven test plans.

FIG. 3

Plan Specific Dose Deviation at Optimal Collective DLGC. The dose deviation between Mobius3D calculated dose and measurement at the optimal collective DLGC for each plan and energy on the (a) TrueBeam with MMLC and (b) TrueBeamSTx with HDMLC is presented. Measurements for the lateral cylinder plan was noted to be consistently lower than calculations, while measurements for the lung SBRT plan was consistently higher than calculations; others show mixed agreement.

FIG. 4

Plan‐Specific DLGC Analysis. The dose difference between the calculated and measured dose at varying DLGC values for each plan as calculated for the 6 MV flattened beam model on the TrueBeam. The points indicate explicit calculations that were performed, while the solid line indicates the line of best fit for each plan.

FIG. 5

Optimal DLGC vs. Plan. The optimal plan‐specific DLGC across all energies for a given machine is presented for each plan. The error bars represent the standard deviation of the dose difference for all energies for a given plan.

FIG. 6

Optimal DLGC vs. Average Leaf Position off Central Axis. The optimal DLGC averaged across all energies is compared to the MLC positioning characteristics of the plan. As the average leaf position off the central became larger, the optimal DLGC became more negative. DLGC error bars are representative of a 1% dosimetric uncertainty and ALPCA error bars are given by the standard deviation across all energies.

FIG. 7

Effects of Plan Exclusion. The change in the optimal DLGC while excluding the lung SBRT or lateral cylinder plan as compared to the analysis including all plans is provided in (a). The dosimetric effect of applying the DLGC determined while excluding the given plan from the analysis is shown in (b).