A beam model applicable to small fields: development and validation

Abstract

This study presents the development and evaluation of a beam model that can accurately predict dose distributions measured in water with a diode for a broad range of field sizes, from 40 cm down to a few millimeters. This model takes into consideration the two main effects influencing the dose distributions for narrow beams: (i) the fact that the source is not point-like, but extended in space and (ii) the nonequilibrium conditions within the field. The size and shape of the radiation source is determined by a combination of a “slitmethod” reconstruction and a collimator factor fitting procedure. The non-equilibrium conditions are taken into account by the use of a polyenergetic pencil beam kernel. We have validated this model in a 6 MV beam by the comparison of measured and calculated output factors (OFs), and planar dose profiles in water for circular and squared fields. The validation tests show good agreement (2%, 2 mm) between modeled and measured distributions for all the studied cases. OFs were measured using two independent methods. The approach presented in this study can be used as a performance check for the linac, serving as a basis for independent dose verification of systems using small fields and, furthermore, as an alternative to conventional dosimetry of small fields.