SU-E-T-141: Leaf Position Verification during Dynamic Beam Delivery of Volumetric Modulated Arc Therapy Using Electronic Portal Imaging Device

Abstract

Purpose: Volumetric‐modulated arc therapy is a technique for delivering intensity‐modulated radiation therapy using rotational beams. During delivery of the rotational beam, the field shape, created by a multileaf collimator(MLC), changes continuously. The objectives of this study were to verify the MLC positional error at some arbitrary intermediate point during delivery for different dose rates and MLC speeds. Methods: The Elekta Synergy‐S has a discreet dose rate, forty pairs of MLC.Dose rate, gantry speed, MLC speed is constant between two Control Points. The MLC Positions were verified with AmSi portal imaging. The leaf speed depends on three factors: ‐ dose rate, delivered MU and distance traveled by MLC. The leaf positions were verified for all combinations of available dose rates 35, 70, 140, 280, 560 MU/min and MLC speed of 0.5, 1, 1.5, 2, 2.5 cm/sec for a dynamically varying field size from 20×16 cm2 to 10×16 cm2 for a 0– 10 degree gantry rotation. Results: The largest MLC Leaf positional error found for 2.5 cm/sec speed for 35MU/min was −1.1±0.26 mm (95% Confidence Intervals‐CI). The minimum leaf positional error encountered for 1.5cm/ sec and 140MU/min was 0.03± 0.16 mm (95% CI). The single highest positional error was −1.31 mm. The maximum and minimum leaf positional error of all 25 sets results mean error of 0.36±0.72mm and − 0.08±0.67mm respectively with 95% CI. The MLC positional error depends on the MLC speed as well as dose rate also. Conclusions: 3.4% tested leaf positional errors found more than tolerance limit (±1 mm). It can be concluded that MLC positional error is higher in case of lower dose rate and highest MLC leaf Speed. The largest three errors (in descending order) encountered for highest speed 2.5 cm/sec and dose rate 35, 70 and 140 MU/min respectively. This clearly indicates lower delivered MU increases the Leaf positional error. No Funding, support and no conflict of interest