Scan pattern optimization for uniform proton beam scanning

Magnetic beam scanning allows one to spread proton beam over the desired radiation field area, improving beam utilization and conformity to the target area. This article discusses generic scan forms for generating uniform circular and rectangular fields and establishes criteria that can be applied to optimize selected scan patterns. During construction of the Midwest Proton Radiotherapy Institute (MPRI), Indiana University developed a magnetically scanned beam spreading system for the 3 m long gantry nozzle. Based on the commissioning experience, criteria for optimizing the scan patterns were derived. A numerical integration model was used to perform initial optimization of the resulting dose distribution. The selected scan patterns were then experimentally validated via test irradiation of Gafchromic films. Generic spiral and linear scan forms are proposed capable of delivering uniform circular and rectangular fields in continuous scanning mode. The test irradiations performed indicate that dose uniformity is within +/- 3% for both scan forms and that penumbra of the uncollimated field can approach the radius of the pristine beam spot. A well designed uniform scanning system can have a large library of uniform circular and rectangular fields of different sizes, which would increase beam utilization and minimize out-of-field dose to the patient.