A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: a theoretical study on 10-MV X-ray dose calculations in thorax-like phantoms

Abstract

A convolution/superposition method is proposed for use with primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance. It should be noted that the number of energy bins is usually about ten, and that the reconstructed X-ray spectra can reasonably be applied to media with a wide range of effective Z numbers, ranging from water to lead. The study was carried out for 10-MV X-ray doses in water and thorax-like phantoms with the use of open-jaw-collimated fields. The dose calculations were made separately for primary, scatter, and electron contamination dose components, for which we used two extended radiation sources: one was on the X-ray target and the other on the flattening filter. To calculate the in-air beam intensities at points on the isocenter plane for a given jaw-collimated field, we introduced an in-air output factor (OPF(in-air)) expressed as the product of the off-center jaw-collimator scatter factor (off-center S (c)), the source off-center ratio factor (OCR(source)), and the jaw-collimator radiation reflection factor (RRF(c)). For more accurate dose calculations, we introduce an electron spread fluctuation factor (F (fwd)) to take into account the angular and spatial spread fluctuation for electrons traveling through different media.

10-MV X-ray dose calculation in water for MLC and wedge fields using a convolution method with X-ray spectra reconstructed as a function of off-axis distance

“…Instead, it treats the radiation reflected from the jaws as a small increase in the in-air beam intensity using a jaw radiation reflection factor [6] that lies outside the jaw field, as described by a Monte Carlo simulation model [12] stating that the photons scattered from the jaws can be ignored when estimating the in-air beam intensity within the jaw field.…”

“…where express the beam water collision kerma at the corresponding phantom surface element (∆S) (equation 43); ∆S is defined as the size of the area element on the phantom surface, which faces the source (S) without interception by the phantom; θ S is the angle between the normal vector line on the ∆S surface and the negative vector of ; G(A jaw ) expresses the electron contamination factor as a function of the jaw field (A jaw ) [6,7]. and are introduced to improve the G function, which can apply only to open jaw fields and only to electrons streaming along the ray lines emanating from the source (S).…”

“…For phantoms constructed of water-like media, we express as , (6) where is the relative electron density averaged between point P and the ∆S center (however, it has been found [13][14][15] that the contamination dose does not vary simply in proportion to the beam water collision kerma of ).…”

“…where is evaluated along the line connecting P and the effective point within the ∆V element; and F hetero is a correction factor [6,7] for phantom heterogeneity. This correction factor is simply used only for forward primary dose calculations, not as a function of E N .…”

“…We have not yet dealt with the condition described in (a), Iwasaki, et al [6] and Kimura, et al [7] dealt with the condition described in (b) using inhomogeneous phantoms, proposing a correction factor for calculation of the primary dose within thoraxlike phantoms, and also dealt with the condition described in (c) using X-ray spectra reconstructed as a function of the off-axis distance. In the present paper, we proposed a special correction factor for nonuniformity of the incident beam intensity described in the above (d) using multileaf collimator (MLC) and/or wedge fields.…”