Therapeutic step and shoot proton beam spot-scanning with a multi-leaf collimator: a Monte Carlo study.

In step and shoot spot-scanning, a small-diameter proton beam is magnetically swept and varied in energy in order to cover the tumour. Initial estimates of the beam size indicate that additional collimating hardware will be needed for lower energy proton beams in order to achieve a clinically acceptable lateral dose falloff at the edge of the proton beam. In this report, we present dosimetric data from Monte Carlo simulations with a model of a simple multileaf collimator which indicate that such a device may be used to improve the lateral dose falloff. The dosimetric quantities relevant to the clinical usefulness of the device are studied, including lateral penumbra, leaf transmission and scalloping effect. Multileaf collimation is compared with a differential spot-weighting technique of sharpening the lateral dose falloff.