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

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= 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.