SU-E-T-57: Estimation of Uncertainty in Dose Delivery Due to MLC Position Inaccuracies by Inverse Derivative Method during Volumetric Modulated Arc Therapy Delivery by Elekta Beam Modulator

Abstract

Purpose: To investigate uncertainties in dose delivery (UDD) due to MLC positional inaccuracies during Volumetric Modulated Arc therapy (VMAT) delivery by Elekta Beam Modulator LINAC. Methods: In Beam Modulator field size is defined by MLC's. In VMAT the field aperture χ is a function of the gantry angular speed ω and position of ith MLC Xi {=f (t)} at time t (χ=f(ω, dXi/dt)). The absorbed dose to EPID (DTW) is defined as a function of ω and dXi/dt; DTW=f(ω, dXi/dt). dDTW/dχ vs. χ calibration curve was generated, in Am-Si EPID, using higher than dmax buildup (2 cm) for a square field length of 0.8 to 3.2 cm for 6MV beam. MLC's were interrupted at some intermediate position (single or multiple time, in later case average error was considered) of the VMAT delivery and projection images were taken in EPID. Uncertainties in field definition were calculated by Δχ = Σ ΔXi * MLC width. UDD was calculated from dDTW/dx vs. χ calibrationcurve. Results: The MLCs move with linear, parabolic or circular velocity at 0.5, 1, 1.5, 2 and 2.5 cm/sec having dose rate 35, 70, 140, 280, 560 MU/min. UDD was determined for all combinations of these dose rate and MLC velocities for 101 MLC defined fields. Maximum and minimum UDD of 0.2 cGy/mu and 0.004cGy/mu were obtained for 560MU/min-1.5cm/sec and 35MU/min-2.0cm/sec respectively. Mean uncertainty in UDD was 0.15 cGy/MU with a standard deviation of 0.045. Conclusions: Mean UDD obtained was high (15%) and it's not the peripheral dose uncertainty. The UDD evaluated for either single or three interruptions, considering that in MLC movement will show similar behavior during their rest of the motion. Actually this fact was observed for multiple interruption cases. Nevertheless if it's possible to measure Δχ for each control point, by cine mode in EPID, better results can be obtained.