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 multileaf 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 True-BeamSTx. 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 planspecific 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.

Evaluating Mobius3D Dose Calculation Accuracy for Small-Field Flattening- Filter-Free Photon Beams

“…However, in recent years, comparative studies for an overall range of clinically relevant plans, including intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT), have been performed. 20 – 22 , 29 , 30 Moreover, detailed characteristics of Mobius3D/FX, such as the dosimetric leaf gap (DLG) correction factor, have been explored for optimal performance. 20 – 22 , 30 , 31 In summary, in-depth evaluations of dosimetric accuracy of Mobius3D/FX for IMRT and VMAT plans have been actively performed.…”

“… 20 – 22 , 29 , 30 Moreover, detailed characteristics of Mobius3D/FX, such as the dosimetric leaf gap (DLG) correction factor, have been explored for optimal performance. 20 – 22 , 30 , 31 In summary, in-depth evaluations of dosimetric accuracy of Mobius3D/FX for IMRT and VMAT plans have been actively performed. Yet, dosimetric accuracy for SBRT plans that mostly use flattening-filter-free (FFF) x-ray beams with small fields has not been intensively studied.…”

“…In general software, because the DVH-based method provides more relevant comparisons of dose distributions, various commercial products such as Mobius3D/FX (Varian Medical Systems, Inc., CA, USA), RadCalc (LAP, Lüneburg, Germany), PerFRACTION (Sun Nuclear Corp, FL, USA), and Dosimetry Check (Math Resolution, MD, USA) have been released and clinically implemented. [20][21][22][23][24][25][26][27][28] Mobius3D/FX, a DVH-based patient-specific QA tool, has been employed at multiple institutions for clinical use.…”