An iterative algorithm for reconstructing incident beam distributions from transmission measurements using electronic portal imaging

Abstract

The problem of reconstructing incident radiotherapy beam profiles from electronic portal images recorded behind a phantom is addressed. To this end an iterative algorithm is presented, which is able to extract the input beam profile from a portal image by compensating for the attenuation of the beam and subtracting the amount of scatter emitted by the phantom. The algorithm requires only a thickness map of the phantom. Scatter is estimated using a superposition method based on precalculated Monte Carlo scatter kernels. The method is tested for a homogeneous water-equivalent slab phantom for simple rectangular and complex multileaf collimated fields. It is shown that the method produces a stable result within four iterations yielding an accuracy for the incident beam distribution of better than 3%.

Enhancement of image quality with a fast iterative scatter and beam hardening correction method for kV CBCT

“…The geometry for the scatter kernel calculations in the former MV CBCT [49][50][51][52] was chosen in such a way that the bottom of the upright standing cylinder was always at a constant distance to the detector, namely the bottom of the cylinder was placed at the isocenter. In contrast to the former study, the kV kernels in this work were calculated based on the real distance (isocentric placement) of the phantom to the detector.…”

3D In Vivo Dosimetry Using Megavoltage Cone-Beam CT and EPID Dosimetry

“…The earlier steps (i.e., Eq. 2 and 3) were repeated three times, which was necessary to ensure convergence of the iterative scheme (11,14). The scattered dose distribution was set to zero for the first loop.…”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…More rigorous treatments of scatter from a patient or a phantom to the EPID, especially for smaller air gaps, are described in the literature. 46,74,75,[112][113][114]119 …”

“…This approach has been applied using an algorithm to back-project the PDI to the mid-plane 11,14,65 or to reconstruct the patient dose based on convolution / superposition methods, back-projecting the fluence measured with the EPID. 79 Applications of portal dosimetry have been reported mainly for the type of EPIDs based on a liquid-filled (Li-Fi) ionisation chamber matrix 11,14,36,59,65,123 59,122 the CCD camera-based EPID 28,48,60,96,114,130 and a solid-state based EPID. 45 These studies compared the PDI with various combinations of film and ionisation chamber measurements.…”