A new method for the measurement of bremsstrahlung spectra

Abstract

The X-ray energy spectrum from a 15 MeV linear accelerator (Siemens Mevatron 77/20) was determined combining attenuation and photoactivation analysis. The X-ray flux of the therapy linac is produced in a thick target and filtered through several materials. The theoretical expression for a thin-target spectrum was corrected for absorption in the target and materials in the accelerator head. The low-energy part of the spectrum was calculated from attenuation measurements and the high-energy part of the spectrum was determined by photoactivation.

Monte Carlo calculation of nine megavoltage photon beam spectra using the BEAM code

“…There have been various experimental methods used to derive such spectra. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, the Monte Carlo method remains the most comprehensive and potentially the most accurate method of obtaining such spectra. Almost two decades ago, Mohan et al 20 provided a series of megavoltage photon beam spectra for various energies of the Varian linacs using the Monte Carlo technique.…”

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

“…In previous transmission analysis studies, the ion-chamber energy response was roughly approximated by one of the following three methods: (1) assuming that the material of the chamber wall and buildup cap are air-equivalent, thus the aircavity dose, D cav , equals the collision air-kerma, K air col , and consequently, R(d, E) is proportional to ðl en =qÞ air , the mass energy absorption coefficient for air; 16,17,23,24,26,31,32 (2) measuring the response at a few known low energies (e.g., 137 Cs and 60 Co) then interpolating or extrapolating to other energies; [4][5][6][11][12][13] or, (3) developing approximate expressions to take into account the deviation from air equivalence caused by the wall and/or cap materials. 8,14,27 To the authors' knowledge, no previous study on transmission analysis used Monte Carlo for detailed detector modeling.…”

“…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.…”

Is attenuation technique sensitive enough to be used to make distinction between FF and FFF spectrum of therapy linac

“…Di rect (in beam) mea sure ment of the pho ton spec tra of med i cal lin ear ac cel er a tors with NaI(Tl) or HpGe is not pos si ble mostly due to high in ten si ties of pho ton beams and dif fi cul ties in de ter min ing de tec tor ef ficiency in high en ergy re gions. There fore, sev eral in direct meth ods were de vel oped to de ter mine the pho ton spec tra: at ten u a tion anal y sis [1][2][3][4][5][6][7][8], Compton spec troscopy [9][10][11], photoactivation anal y sis [3,4], spec tral un fold ing from depth-dose mea sure ment [12] and Monte Carlo sim u la tion [13]. Most of the de vel oped in di rect tech niques are quite com pli cated to be routinely per formed in clin i cal en vi ron ment and they also have some dis ad van tages.…”