Scattered radiation in portal images: A Monte Carlo simulation and a simple physical model

Abstract

The scattered radiation in 6 MV radiotherapy portal images is analyzed. First, a quantity SPR* is studied, by means of Monte Carlo (MC) modeling. SPR* is defined as the ratio, on the central axis, of the signal due to scattered radiation to that due to the primary radiation. The detector model mimics a high-energy photon detector in the context of transit dosimetry. Second, a physical model of SPR* has been developed from first principles. For a cylindrical phantom, placed symmetrically about the isocenter, it predicts that SPR* depends on the area A at the isocenter of the circular field and the phantom thickness d as follows. SPR* = k0Ad(1 + k1d)(1 + k2A), where k0 = 0.0266(L1 + L2)2/(L1L2)2, k2 = - [L1(-2) + L2(-2) + (L1(-1) + L2(-1))2((2/3) + (3 kappa/2))]/2pi, L1 is the source-to-isocenter distance, L2 is the isocenter-to-detector distance, and kappa is the mean energy of the radiation beam (MeV/0.511). Constant k1, for which there is no simple expression, depends on L2. Comparison to the MC data shows that for 60 <or= L2 <or= 100 cm the dependence is weak and k1 approximately equal to 2 x 10(-3) cm-1. The root mean square (rms) agreement between the MC-derived values of SPR* and the physical model is better than 0.001 over a wide range of A and d values likely to be encountered in clinical practice for L2 >or= 50 cm. Third, experimental measurements of the scatter-to-primary ratio were obtained using our custom built imaging system mounted on a Philips SL25 linear accelerator. In the first experiment, A was varied from 40 to 400 cm2 with L1 = L2 = 100 cm with d = 20 cm. In the second experiment water depth d was varied from 0 to 28 cm with L1 = L2 = 100 cm and A = 200 cm2. The rms agreements between the MC data and the experiments were 0.0015 and 0.0045, respectively.

Enhancement of megavoltage electronic portal images for markerless tumor tracking

“…However, electronic portal images are noisy, blurred, and show poor contrast in identifying patient's anatomy in detail 13, 14, 15. Image enhancements have been used to obtain the optimal readability of EPID images, including contrast improvement, edge deblurring, and noise reduction 16, 17, 18.…”

Deep Scatter Estimation (DSE): Accurate Real-Time Scatter Estimation for X-Ray CT Using a Deep Convolutional Neural Network

“…While scatter suppression approaches try to reduce the amount of scattered X-rays reaching the detector using anti-scatter grids or collimators B Joscha Maier joscha.maier@dkfz.de 1 German Cancer Research Center, Im Neuenheimer Feld 280, 69120 Heidelberg, Germany [26], scatter estimation approaches aim at deriving an estimate of the scatter distribution that is used to correct the acquired projection data [27]. Thereby, the scatter estimate can either be derived using dedicated hardware such as beam blockers or primary modulation grids [3,7,8,20,24,28,38,39] or using software-based approaches that rely on physical or empirical models to predict X-ray scattering [1,2,11,17,18,21,22,30,[32][33][34]36,37]. Among these methods the gold standard is to use a Monte Carlo (MC) photon transport code [27].…”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…In the analytical scatter model of Swindell and Evans, 119 the scatter-toprimary ratio is for vanishingly small scattering volumes proportional to the irradiated volume, i.e. to the field area and the thickness of the patient or phantom.…”

“…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 …”