Monte Carlo-based analytical model for small and variable fields delivered by TomoTherapy

Background and purposeExtend to very small fields the validity of a Monte Carlo (MC) based model of TomoTherapy called TomoPen for future implementation of the dynamic jaws feature for helical TomoTherapy.Materials and methodsFirst, the modelling of the electron source was revisited using a new method to measure source obscuration for very small fields (<1 cm). The method consisted in MC simulations simulations and measurements of the central dose in a water phantom for a 10 cm × FW field scanned to deliver a 10 × 10 cm2 fluence. FW, the longitudinal field width, was varied from 0.4 to 5 cm. The second part of the work consisted of adapting TomoPen to account for any configuration of the jaws in a fast and efficient way by using routinely only the phase-space file of the largest field (5 cm) and interpolated analytical information of phase-space files of smaller field widths.ResultsFor the electron source fine tuning, it was shown that the best results were obtained for a 1.1 mm wide spot. Our single phase-space method showed no significant differences compared to MC simulations of various field widths even though only longitudinal intensity and angular analytical functions were applied to the 5 cm phase-space.ConclusionThe designed model is able to simulate all jaw openings from the 5 cm field phase-space file by applying a bi-dimensional analytical function accounting for the fluence and the angular distribution in the longitudinal direction.