RESUMO
A range compensator (abbreviated as a RC hereafter) is used to form a conformal dose distribution for heavy-charged-particle therapy. However, it induces distortion of the dose distribution. The induced inhomogeneity may result in a calibration error of a monitor unit (MU) assigned to a transmission ionization chamber. By using a bi-material RC made from a low-Z material and a high-Z material instead of the regular RC, the dose inhomogeneity has been obviously reduced by equalizing the lateral dose distributions formed by pencil beams traversing elements of the RC with different base thicknesses at the same water-equivalent depth. We designed and manufactured a 4 x 4 matrix-shaped single-material RC and a bi-material RC with the same range losses at corresponding elements of the RCs. The bi-material RC is made from chemical wood (the main chemical component is an ABS resin) as a low-Z material and from brass as a high-Z material. Sixteen segments of the RC are designed so that the range-loss differences of the adjacent segments of the RC range from 0 to 50 mm in steps of 5 mm. We measured dose distributions in water formed by a 160 MeV proton beam traversing the single-material RC or the bi-material RC, using the HIMAC biology beam port. Large dips and bumps were observed in the dose distribution formed by the use of the single-material RC; the dose uniformity has been significantly improved in the target region by the use of the bi-material RC. The improvement has been obtained at the expense of blurring lateral penumbra. For clinical application of this method to a patient with large density inhomogeneity, a simple modification method of the original calculation model has been given.
