Analytical scatter kernels for portal imaging at 6 MV

Abstract

X-ray photon scatter kernels for 6 MV electronic portal imaging are investigated using an analytical and a semi-analytical model. The models are tested on homogeneous phantoms for a range of uniform circular fields and scatterer-to-detector air gaps relevant for clinical use. It is found that a fully analytical model based on an exact treatment of photons undergoing a single Compton scatter event and an approximate treatment of second and higher order scatter events, assuming a multiple-scatter source at the center of the scatter volume, is accurate within 1% (i.e., the residual scatter signal is less than 1% of the primary signal) for field sizes up to 100 cm2 and air gaps over 30 cm, but shows significant discrepancies for larger field sizes. Monte Carlo results are presented showing that the effective multiple-scatter source is located toward the exit surface of the scatterer, rather than at its center. A second model is therefore investigated where second and higher-order scattering is instead modeled by fitting an analytical function describing a nonstationary isotropic point-scatter source to Monte Carlo generated data. This second model is shown to be accurate to within 1% for air gaps down to 20 cm, for field sizes up to 900 cm2 and phantom thicknesses up to 50 cm.

Experimental verification of a portal dose prediction model

“…Another possibility is to take an effective scatter source point into account that is not located in the midplane of the phantom. We did, however, not compare our method with the analytical model of Spies et al 25,30 in a quantitative way. The ultimate comparison for portal dose predictions models is with measured portal dose values, which was the purpose of this study.…”

“…Monte Carlo simulations of the scatter kernels 24,29 are also presented in the literature but this approach requires complex calculations, while in addition such a Monte Carlo method is not available in our department. Analytically derived scatter kernels 25,30 have also been presented but these kernels are only an estimate of the photon fluence or do not represent the detector response. We developed a simple method that describes the pencil beam scatter kernel with a pre-defined kernel function.…”

Experimental study of a single-pixel prototype anti-scatter detector for megavoltage x-ray imaging

“…It is well known that scattered x rays are detrimental to the image quality of x-ray transmission radiography (Yaffe and Johns, 1983;Kanamori and Matsumoto, 1987;Swindell et al, 1991;Jaffray et al, 1994;Swindell and Evans, 1996;Spies and Bortfeld, 2001). The use of an anti-scatter grid is an effective way to significantly reduce the amount of scattered radiation reaching the x-ray detector and thereby improve the image quality for diagnostic x-ray imaging (Sandborg et al, 1993;Fetterly and Schueler, 2009;Mizuta et al, 2012).…”

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 …”