Calculation of electron beam depth-dose curves and output factors for arbitrary field shapes

A previously presented method to calculate depth-dose curves and output factors for arbitrarily shaped electron beams is evaluated. The method employs a Gaussian pencil model for direct incident and applicator scattered electrons; the parameter values are derived from measured central axis depth-dose distributions. In addition, an empirical model is used to compute the dose due to electrons scattered by field-defining frames. In this way, the properties of the clinical electron beams are taken into account. In this paper, calculations and measurements for electron beams with energies between 6 and 20 MeV, treatment field dimensions between 3 and 14 cm, and various applicator sizes are compared. The results demonstrate the importance of irregular field dose calculations and the scope of the present method. Agreement better than 3% in dose and 0.2 cm in depth is achieved. For electron beams without applicators, the calculations show the same accuracy. Another method in electron treatment planning that derives values for the radial width parameter of the pencil beam from measured broad beam profiles is also investigated. This method gives good results for dose calculations in beams without applicator scatter. It should be used with care, however, for beams that contain such a scatter component. When electrons scattered by the applicator walls and field-defining frames are neglected, differences between measured and calculated dose up to 8% are found.