The effect of compensator-induced scatter on external beam dose calculations

[摘要] English: In this study the effect of compensator-induced scatter on external beam dose calculationswere studied for compensators of wax, aluminum, brass, copper and lead for 6, 8 and 15MV parallel x-ray beams. An outline is given of the necessity for the inclusion ofcompensator induced scatter in the design of compensators for their use in IMRTapplications. A method is described for deriving effective attenuation coefficients(EACs), as calculated by the DOSXYZ Monte Carlo (MC) code. Various properties ofthe EACs were studied, among which their dependence on small beam (beamlet) sizes aswell as their depth dependence in water. These EACs are used for the initial approximatedesign of a compensator. In conjuction with these EACs, scatter and beam hardening isincluded in the compensator modeling process. Compensator-induced scatter and beamhardening properties were studied in some detail. The EGSnrc based DOSRZnrc MCcode was used to study the evolution of a pencil beam (PB) as it traverses differentthicknesses of a compensator material. It was found that the relative dose profiles of thePB could be adjusted for scatter and beam hardening using empirically derived functions,and that these adjustments were proportional to the thickness of the compensatormaterial. A compensator planning system (CPS) is described, used in the design of acompensator. Dose calculations are performed with this CPS using the superpositionmethod for cartesian PBs. An algorithm is described that transforms the cylindrical PB asobtained with the DOSRZnrc MC code, into a cartesian PB. The CPS was tested for astep wedge shaped compensator over square field sizes with side lengths of 5, 10 and 20cm. A correction function was introduced to account for side penetration in thecompensator. It was found that the relative dose profiles calculated with the CPS at adepth of 10 cm in water was within 1.5 percent of similar dose profile data derived fromDOSXYZ MC dose calculations for a 5x5 cm2 field. For the 20x20 cm2 field, theaccuracy was within 3 percent in most cases and beam energies. It is also demonstratedhow an aluminum compensator can be designed by an iterative method with the CPS toyield a dose profile that conforms accurately to a pre-determined dose profile, such aswould be produced by an inverse planning system for IMRT treatments.

[发布机构] University of the Free State