Study of formation of flat field by particle radiations

A range compensator (abbr. as RC hereafter) distorts dose distribution formed by heavy-charged-particle beams. The induced inhomogeneity may result in a calibration error of MU (Monitor Unit) assigned to the transmission ionization chamber. Using a bi-material RC made of a low-Z material and a high-Z material, the dose inhomogeneity has been reduced by controlling the lateral spread of pencil beams at the same water-equivalent depth. We measured dose distributions in water formed by a 290 MeV/u carbon beam and a 160 MeV proton beam traversing a single-material RC or a bi-material RC. We found that dose transition corresponding to boundaries of the RC elements became smooth without a dip and a bump when we used a bi-material RC. (author)