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

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