Beam quality and dose perturbation of 6 MV flattening-filter-free linac

Abstract

The aim of this study is twofold: (a) determination of the spectral differences for flattening-filter-free (FFF) versus standard (STD) linac under various clinical conditions, (b) based on an extensive list of clinically important beam configurations, identification of clinical scenarios that lead to higher macroscopic dose perturbations due to the presence of high-Z material. The focus is on dose enhancement due to contrast agents including high-Z elements such as gold or gadolinium. EGSnrc was used to simulate clinical beams under various irradiation conditions: open/IMRT/spit-IMRT fields, in/out-off-field areas, different depths and field sizes. Spectra were calculated and analyzed for about 80 beams and for a total of 480 regions. Quantitative differential effects in beam quality were characterized using energy-dependent and cumulative dose perturbation metrics. Analysis of the spectral database showed that even though the general trends for both linacs (FFF/STD) were the same, there were crucial differences. In general, the relative changes between different conditions were smaller for FFF spectra. This was because of the higher component of low-energy photons of the FFF linac, which already lead to higher dose enhancement than for the STD linac (photon energies were more "uniformly" distributed for FFF spectra and henceforth their perturbation resulted in lesser relative changes). For out-of-field FFF spectra and split-IMRT fields the strongest enhancement were observed (∼25 and ∼5 respectively). Different spectral scenarios lead to different dose enhancements, however, they scale with the higher effective-Z of the materials and were directly related to the lower range of the spectra (<200 keV).

Nanoscale radiation transport and clinical beam modeling for gold nanoparticle dose enhanced radiotherapy (GNPT) using X-rays

“…For instance, the spectrum at a given point in the patient depends on the depth and off-axis location, field size, MLC transmission and scatter, inside or out-of field location, anatomy and other irradiation details. 53,55 For instance, it has been shown that split IMRT fields, which contain a relatively large proportion of out-of-field dose, gives rise to significantly larger DER than the non-split beams. For these reasons, DER may vary substantially and has to be computed for the specific type of irradiation technique, field size and location inside patient.…”

“…Simulation of radiation transport and energy deposition as well as the associated radiobiological quantities have been performed in various ways [46][47][48][49][50][51][52][53][54][55][56][57][58] using a plethora of radiation transport codes, simulation beams GNP geometries, GNP sizes and concentrations, X-ray sources and energies, as well as normalization schemes and metrics. Table 1 summarizes the literature on simulations using nanoscale MC and deterministic radiation transport.…”

“…For convenience, we arrange the literature according to the following order: general radiation transport aspects relevant for nanoscale simulations, [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] macroscopic dose enhancement in contrast agents and dose perturbation at high-Z material interfaces, radiation transport for nanoscopic dose enhancement of GNP, [46][47][48][49][50][51][52][53][54][55][56][57][58] radiobiological and clinical aspects of GNPT, [59][60][61][62][63][64][65][66][67] application of Monte Carlo (MC) simulations to cellular environment, [68][69][70] modification of linear accelerator spectra to maximize dose enhancement 71 and GNPT using other than X-ray therapeutic beams (proton, electron). …”

Low‐Z linac targets for low‐MV gold nanoparticle radiation therapy

“…Spectra were obtained from the phase spaces for a 2 × 2 cm 2 area at the central axis (CAX) of the beam for each case and depth. 12 Spectral bins were 2 keV wide up to 100 keV, and they were 24, 41, and 64 keV for the rest of the spectrum up to the maximum energy of 2.5, 4, and 6.5 MeV, respectively.…”

“…12,13 Three materials were selected for the targets: beryllium (Be, density 1.85 g/cm 3 ), diamond (density of 3.5 g/cm 3 ), and tungsten with copper backing (W, density of 18 g/cm 3 and Cu density 7.93 g/cm 3 ), the latter according to the manufacturer of the standard linac. For Be and diamond targets different thicknesses were simulated: 20%, 60%, 70%, and 80% of the continuous slowing down approximation (CSDA) electron range for each nominal energy considered.…”