VMAT testing for an Elekta accelerator

Darryl G.L Kaurin,Saikanth Mahendra,Edward I Marshall,Larry E Sweeney

doi:10.1120/jacmp.v13i2.3725

Darryl G.L Kaurin, Saikanth Mahendra + Show 2 more

Open Access

https://doi.org/10.1120/jacmp.v13i2.3725

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Abstract

Volumetric‐modulated arc therapy (VMAT) has been shown to be able to deliver plans equivalent to intensity‐modulated radiation therapy (IMRT) in a fraction of the treatment time. This improvement is important for patient immobilization/ localization compliance due to comfort and treatment duration, as well as patient throughput. Previous authors have suggested commissioning methods for this modality. Here, we extend the methods reported for the Varian RapidArc system (which tested individual system components) to the Elekta linear accelerator, using custom files built using the Elekta iComCAT software. We also extend the method reported for VMAT commissioning of the Elekta accelerator by verifying maximum values of parameters (gantry speed, multileaf collimator (MLC) speed, and backup jaw speed), investigating: 1) beam profiles as a function of dose rate during an arc, 2) over/under dosing due to MLC reversals, and 3) over/under dosing at changing dose rate junctions. Equations for construction of the iComCAT files are given. Results indicate that the beam profile for lower dose rates varies less than 3% from that of the maximum dose rate, with no difference during an arc. The gantry, MLC, and backup jaw maximum speed are internally consistent. The monitor unit chamber is stable over the MUs and gantry movement conditions expected. MLC movement and position during VMAT delivery are within IMRT tolerances. Dose rate, gantry speed, and MLC speed are accurately controlled. Over/under dosing at junctions of MLC reversals or dose rate changes are within clinical acceptability.PACS numbers: 87.55.de, 87.55.Qr, 87.56.bd

Highlights

56 Kaurin et al.: Volumetric-modulated arc therapy (VMAT) tests for Elekta parameters specific to Elekta, do not test the individual components of the treatment modality directly, as Ling et al(6) did
While this is a necessary component for commissioning a system, we thought this to be inadequate for catching errors that would be noticed by measuring individual components. (For example, a Picket Fence multileaf collimators (MLCs) test will catch 1 to 2 mm errors that will not be seen in patient specific intensity-modulated radiation therapy (IMRT) QA)
C.2.3 MLC reversals Bedford and Warrington(7) investigated if the MLCs could reverse direction accurately via a treatment plan, which was not considered by Ling et al(6) We investigated this by making an iComCat file with five 4 cm wide strips with dose rates (ΔMU/Δt)max/2 with differing numbers of traversals at MLC speeds of 1.0, 0.95, 0.90, and 0.18 (Δx/Δt)max

Summary

Introduction

56 Kaurin et al.: VMAT tests for Elekta parameters specific to Elekta ( e.g., dose rate, in particular), do not test the individual components of the treatment modality directly, as Ling et al(6) did (e.g., multicollimator leaf control, gantry speed). Most Elekta VMAT physicists have commissioned their systems by comparing measured dose profiles of VMAT-planned patient cases. While this is a necessary component for commissioning a system, we thought this to be inadequate for catching errors that would be noticed by measuring individual components. The differences between our approach and that of Bedford and Warrington is an extension of some of their tests, excluding reliance on the treatment planning system (TPS), as well as additional tests (open beam profiles of differing dose rates during arcs, fluence errors during rapid MLC reversals, fluence errors at dose rate change junctions, verifying internal consistency of maximum gantry, MLC speed, and jaw speeds, and covering a range of gantry and MLC speeds up to and exceeding the maximum values). The TPS with end-to-end testing is necessary for commissioning, but not investigated here

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