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

Background and purpose: Extend 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 methods: First, 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. Results: For 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. Conclusion: The 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.