Figure 1 :The beam and cavity geometries used for the various Monte Carlo simulations in this study. (a) Single fi eld (SF), (b) Parallel opposed (POP) fi elds, (c) Four fi elds (4F)-small fi elds (2x2 and 10x2 cm2) and (d) 4F- large fi elds (5x5 or 7x7 cm2). Note: The centers of air cavities (1x1x1 or 3x3x3 cm3) coincide with the centre of the 20x20x20cm3 water phantom. All beams are symmetrical and their central axes pass through the isocenter. The “Dose points” indicated to represent the location of the interface dose calculation points (at 0.25, 0.75 and 1.25 mm from the interface (not to scale)).