A Monte Carlo study on internal wedges using BEAM

To do calculations for wedged photon beams with the NRC Monte Carlo simulation package BEAM, a new Component Module for wedges called WEDGE has been designed and built. After an initial series of benchmarks using monoenergetic photon beams as well as realistic 6 MV and 10 MV beams, it was found, that the new CM did work fine for the large wedge (maximum field size 30 x 40 cm2) of the Elekta SL-linac. The next step was to calculate dose distributions and output factors for a range of wedged fields with field size from 3 x 3 cm2 to 30 x 30 cm2. Results from these simulations have been compared to measurements. Calculated values for the reference wedge transmission factor and the relative wedge transmission factors were within 1.5% from the measured data. Dose distributions showed an identical behavior; both depth-dose curves as well as cross profiles were within 1.5% from measured data, usually even better. Despite the increased mean energy, there was no indication that, as a result, the phantom scatter output factors will change for a 10 MV photon beam. It was found that by adding a wedge the contributions for the different sources of head scatter changed considerably as compared to the open fields, apart from the additional scatter from the wedge. Another consequence of inserting a wedge was an increase in the mean energy of both primary and scattered radiation with 0.3 MV and 0.7 MV, respectively, for all wedged fields with respect to the corresponding open fields. Despite the statistical uncertainty in the calculated data, which is in the same order of magnitude as the effect to be determined, it was possible to derive reliable data for the beam hardening from the calculated dose distributions. Only for the smallest field (field size 3 x 3 cm2) a large difference between the measured and calculated beam hardening factor was found due to the relative large voxel size of 1 x 1 x 1 cm3 compared to the field size. For a description of the influence of a wedge on a photon beam, the results of this study strongly support the use of a reference wedge transmission factor (determined under reference conditions) in combination with a relative wedge transmission factor. The product of these variables should replace the collimator scatter output factor used in open fields. The influence on the dose distribution should be incorporated by using the (field size dependent) beam hardening. The ultimate solution will be to make this beam hardening depending on the actual position in the radiation field, as the photon energy varies over the field (holds also for open fields).