Verification of IMRT dose distributions using a water beam imaging system

A water beam imaging system (WBIS) was developed and used to verify dose distributions for intensity modulated radiotherapy (IMRT) using dynamic multileaf collimator (MLC). The WBIS consists of a water container, a scintillator screen, a charge-coupled device (CCD) camera, and a portable personal computer (Wellhofer Dosimetrie, Schwarzenbruck, Germany). The scintillation image is captured by the CCD camera. The pixel value in this image indicates the dose value in the scintillation screen. The verification is performed by comparing the WBIS image achieved from the measurement with dose distribution from the IMRT plan. Because of Light scattering in the WBIS the image is blurred. An iterative reconstruction algorithm is proposed to remove the blurring effect of light scattering. From the measured image of a 10 cm x 10 cm x-ray beam and the simulation result of the dose distribution using the Monte Carlo method, the blur function can be achieved. Based on this function, the proposed algorithm is applied to reconstruct the true dose distribution for an IMRT plan from the measured WBIS image. The reconstructed dose distributions are compared with Monte Carlo simulation results. Reasonable agreement can be observed from the comparison. The proposed approach makes it possible to carry out real-time quality assurance tasks for IMRT dose verification.