Dosimetric modelling of a scanned pencil beam apparatus for evaluation of cobalt-60 tomotherapy

Tomotherapy is a special application of intensity modulated radiation therapy based on a geometry similar to computed tomography (CT) imaging. Recently, the viability of using cobalt-60 as the radiation source for tomotherapy has been demonstrated. To further investigate this application an experimental system consisting of a computer controlled first generation translate/rotate CT jig has been constructed. The intensity is modulated by control of the velocity of motion of the phantom through a narrow (1×1 cm²) beam. By comparison, a commercial system developed for linac tomotherapy utilizes the Mimic, a 2D multileaf collimator. The goal of this work is to assess the agreement between an intensity modulated beam generated by these two methods. Beam profiles for a single beam portal are modelled using a pencil-beam based treatment planning system. Beam profiles from a simple wedged beam to more complex profiles are calculated for the multileaf collimator and then an optimal match generated for the scanning pencil beam. Agreement between the profiles is better than 3% in the high dose region. Therefore, beam delivery assessment with our experimental system will be relevant for the investigation of a potential clinical device