Quality assurance of dynamic parameters in volumetric modulated arc therapy

The purpose of this study was to demonstrate quality assurance checks for accuracy of gantry speed and position, dose rate and multileaf collimator (MLC) speed and position for a volumetric modulated arc treatment (VMAT) modality (Synergy S; Elekta, Stockholm, Sweden), and to check that all the necessary variables and parameters were synchronous.

Three tests (for gantry position-dose delivery synchronisation, gantry speed-dose delivery synchronisation and MLC leaf speed and positions) were performed.

The average error in gantry position was 0.5° and the average difference was 3 MU for a linear and a parabolic relationship between gantry position and delivered dose. In the third part of this test (sawtooth variation), the maximum difference was 9.3 MU, with a gantry position difference of 1.2°. In the sweeping field method test, a linear relationship was observed between recorded doses and distance from the central axis, as expected. In the open field method, errors were encountered at the beginning and at the end of the delivery arc, termed the "beginning" and "end" errors. For MLC position verification, the maximum error was -2.46 mm and the mean error was 0.0153 ±0.4668 mm, and 3.4% of leaves analysed showed errors of >±1 mm.

This experiment demonstrates that the variables and parameters of the Synergy S are synchronous and that the system is suitable for delivering VMAT using a dynamic MLC.