Spot scanning with a rapid cycling proton beam: Dose delivery algorithms

Abstract

A rapid cycling proton beam has several distinct characteristics that are superior to a slow extraction synchrotron: the beam energy and energy spread, beam intensity and spot size can be controlled and varied spot by spot. In this study, we demonstrate the feasibility of using a spot scanning beam from a rapid-cycling-medical-synchrotron (RCMS) at 10 Hz repetition frequency in proton therapy. The versatility of the RCMS beam in proton therapy is illustrated by two examples: (1) a cylindrical volume embedded in a water phantom irradiated by a single field and (2) a spherical volume in a water phantom irradiated by two parallel opposed fields. A uniform dose distribution is to be delivered to the volumes. The Monte Carlo code, Geant4, is used to validate the dose distributions in each example. Monte Carlo codes can also be used in actual treatment plans to optimize beam spot distributions. Transverse delivery algorithms are developed to produce uniform distributions in each transverse plane in the two examples. Longitudinally, different proton energies are used in successive transverse planes to produce the SOBP required to cover the volumes. In general, uniformity of dose distribution within 3% is obtained for the cylinder and 3.5% for the sphere. The transverse algorithms require only few hundred beam spots for each plane. The algorithms may be applied to larger volumes by increasing the intensity spot by spot for the same delivery time of the same dose. The treatment time can be made shorter than 1 minute for any field configuration and tumor shape. The unique beam characteristics of a spot scanning beam from a RCMS at 10 Hz repetition frequency are used to design transverse and longitudinal algorithms to produce uniform distribution for any arbitrary shape and size of targets. The proposed spot scanning beam is more versatile than the existing spot scanning beams in proton therapy with better beam control and lower neutron dose.