Possibilities for tailoring dose distributions through the manipulation of electron beam characteristics

The influence of the properties of an electron beam on resulting dose distributions, and the potential benefits for dose conformity and optimizing dose distribution characteristics by electron beam manipulation, are theoretically examined. A simulated annealing routine is used to weight electron pencil beams of discrete energies incident at discrete locations and angles on one side of a phantom. The resulting optimal electron phase space provides a dose distribution which most closely approaches a desired distribution on the basis of a physical comparison. For simple desired distributions, intuitive results are obtained such as the benefits of energy modulation for distributing dose with depth, of angular and spatial modulation for overcoming disequilibrium effects and their combination in boosting surface doses. For a complex desired dose distribution, the optimization routine instigates a complex interplay of energy, angular and spatial modulation in attempting to achieve dose conformity. A significant result shows that, for a suitably selected beam energy, angular modulation can compensate for the variation in the depth of the distal edge of a superficial target. The effects of varying just energy for normally incident electrons are compared with those of varying the distribution of incidence angle (for monoenergetic electrons) and the combination of both, indicating the relative merits of the manipulation of available degrees of freedom.