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* = k(0)Ad(1 + k(1)d)(1 + k(2)A), where k(0) = 0.0266(L(1) + L(2))(2)/(L(1)L(2))(2), k(2) = -[L(1)(-2) + L(2)(-1))(2)((2/3) + (3 kappa/2))]/2 pi, L(1) is the source-to-isocenter distance, L(2) is the isocenter-to-detector distance, and kappa is the mean energy of the radiation beam (MeV/0.511). Constant k(1) for which there is no simple expression, depends on L(2). Comparison to the MC data shows that for 60 less than or equal to L(2) less than or equal to 100 cm the dependence is weak and k(1) congruent to 2 x 10(-3) cm(-1). The root mean square (rms) agreement between the MC-derived values of SPR* cm 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 L(2) greater than or equal to 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 cm(2) with L(1) = L(2) = 100 cm with d = 20 cm. In the second experiment water depth d was varied from 0 to 28 cm with L(1) = L(2) = 100 cm and A = 200 cm(2). The rms agreements between the MC data and the experiments were 0.0015 and 0.0045, respectively. (C) 1996 American Association of Physicists in Medicine.