Sensitivity of a helical diode array dosimeter to Volumetric Modulated Arc Therapy delivery errors
Purpose: To study the sensitivity of an ArcCHECK dosimeter in detecting delivery errors during the delivery of Volumetric Modulated Arc Therapy (VMAT). Methods: Three types of errors in Multi Leaf Collimator (MLC) position and dose delivery were simulated separately in the delivery of five prostate and five head and neck (H&N) VMAT plans: (i) Gantry independent: a systematic shift in MLC position and variation in output to the whole arc; (ii) Gantry dependent: sag in MLC position and output variation as a function of gantry angle; (iii) Control point specific MLC and output errors introduced to only a specific number of Control Points (CP). The difference in local and global gamma (γ) pass rate between the no-error and error-simulated measurements with 2%/2 mm and 3%/3 mm tolerances was calculated to assess the sensitivity of ArcCHECK. The clinical impact of these errors was also calculated. Results: ArcCHECK was able to detect a minimum 3 mm MLC error and 3% output error for Gantry independent errors using either local or global gamma with 2%/2 mm tolerance. For the Gantry dependent error scenario a minimum 3 mm MLC error and 3% dose error was identifiable by ArcCHECK using either global or local gamma with 2%/2 mm tolerance. In errors introduced to specific CPs a MLC error of 10 mm and dose error of 100% introduced to 4CPs were detected by ArcCHECK. Conclusion: ArcCHECK used with either local or global gamma analysis and 2%/2 mm criteria can be confidently used in the clinic to detect errors above the stated error values.