Quality assurance of dynamic parameters in volumetric modulated arc therapy

Abstract

Objectives: 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.

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

Results: 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.

Conclusion: 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.

Figure 1

Difference between predicted and measured gantry angle and predicted and measured monitor unit (MU) level for a linear variation of gantry angle with monitor unit level.

Figure 2

Difference between predicted and measured gantry angle and predicted and measured monitor unit (MU) level for a parabolic variation of gantry angle with monitor unit level.

Figure 3

Difference between predicted and measured gantry angle and predicted and measured monitor unit (MU) level for a zigzag (sawtooth) variation of gantry angle with monitor unit level.

Figure 4

Transition from (a) a regular to (c) an irregular shape meant for 50 MU. (b) A multileaf collimator configuration at the interrupted position (43.2 MU).

Figure 5

(a) and (c) show a multileaf collimator configuration for initial and destination images for an anticlockwise rotation meant for 50 MU; (b) shows an arbitrary interruption point at 34.5 MU.

Figure 6

Gamma analysis of the profiles with and without interruptions using I’mRT MatriXX (IBA Dosimetry GmbH, Schwarzenbruck, Germany). The gamma analysis showed two strips of higher error at the two ends, confirming the “beginning” and “end” errors.