Determination of radiotherapy X-ray spectra using a screen-film system

Abstract

A method to determine the X-ray spectrum delivered by a medical linear accelerator is presented. This method consists of an analytical calculation of the primary spectrum using the Schiff bremsstrahlung cross-section formula. A correction factor that accounts for the scatter component of the spectrum is estimated by comparing the signal in two screen-film systems to a theoretical prediction using a model of energy deposition in such detectors. The model makes use of the quantum absorption efficiency and the average energy deposited per interacting photon concepts. These two quantities are calculated by means of Monte Carlo simulations of the screen-film systems used. This method is capable of determining the spectrum as a function of the spatial position across a plane perpendicular to the beam central axis. It does not, however, render information about the direction cosines of the X-ray fluence crossing such a plane, a requirement in order to produce a full phase-space file that can be used in conjunction with a Monte Carlo dose calculation engine.

An improved physics-based approach for unfolding megavoltage bremsstrahlung spectra using transmission analysis

“…Answers to this question include the methods of Laplace transform pairs, [4][5][6] direct matrix inversion, 7,8 neural networks, 9 and iterative unfolding (including least-squares, expectationmaximization, and simulated annealing) with: (a) a priori spectral knowledge and smoothing constraints, [10][11][12][13][14][15][16][17] (b) regularization, [18][19][20][21] or (c) spectrum parameterization. [22][23][24][25][26][27][28][29][30][31][32][33] In this study, before approaching the unfolding issue, we first try to answer the more fundamental question: how can we make the transmission data themselves contain more (and accurate) spectral information? To this end, EGSnrc (Refs.…”

Functional forms for photon spectra of clinical linacs

“…Thicktarget formulae use reasonable approximations to account for the spreading and slowing down of electrons in the target (Nordell and Brahme 1984, Findlay 1989, Desobry and Boyer 1991. A number of parameterized functional forms which are based on thick-target formulae have been proposed (Ahnesjö and Andreo 1989, Baker 1993, Harrison et al 1993, Garnica-Garza 2008, an example of which is function 3, which was proposed in the context of spectral unfolding from depth-dose curves. In this function, dσ br /dE| (x,E e ) is the bremsstrahlung cross-section for electrons of kinetic energy E e , where E e is the mean kinetic energy of the electron spectrum at depth x in the target.…”