SU-E-T-02: 3D Dose Reconstruction of Volumetric Modulated Arc Therapy From Delivered EPID Dose Image.
PURPOSE
To evaluate dose reconstruction of volumetric modulated arc therapy (VMAT) from EPID image prediction.


METHODS
A test VMAT plan with dynamic MLCs was developed on a water equivalent cubic phantom with dimension of 31.4×34×34 cm3 . From the initial MLC opening of 5×5 cm2 at gantry 0°, they moved with a constant speed as the gantry rotates, until they reached X1 at -2.5 cm and X2 at = -2.237 cm at gantry 90°. The plan was delivered while cine EPID images were acquired. For the 84 gantry angle segments, doses to the phantom were calculated by XVMC and reconstructed from EPID based on fast density-scaled model and non-iterative dose reconstruction method we have previously developed. The reconstructed dose was compared with the forwardly calculated dose individually over 84 gantry angle segments and cumulatively in terms of gamma evaluation and dose volume histogram for PTV (3×3×3 cm3 ) positioned at the center of the phantom.


RESULTS
3D-gamma pass rate was 97.0 % cumulatively with 3 %/3 mm criteria and maximum 100% individually. The DVHs of PTV were indistinguishable except in the region of high dose (>95% of prescribed dose) where the difference of a few percent in volume was found.


CONCLUSION
Showing good agreement with the forwardly calculated dose, we have extended our model of dose reconstruction successfully to the verification of VMAT. Although more clinical demonstration is to be followed, our study presents a promising tool for time-resolved and cumulative VMAT validation.