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

Sterpin, Edmond [UCL] Hundertmark, Brian T Mackie, Thomas R Lu, Weiguo Olivera, Gustavo H Vynckier, Stefaan [UCL]

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 (<1cm). The method consisted in MC simulations simulations and measurements of the central dose in a water phantom for a 10cmxFW field scanned to deliver a 10x10cm(2) fluence. FW, the longitudinal field width, was varied from 0.4 to 5cm. 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 (5cm) 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 1.1mm 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 5cm phase-space. CONCLUSION: The designed model is able to simulate all jaw openings from the 5cm field phase-space file by applying a bi-dimensional analytical function accounting for the fluence and the angular distribution in the longitudinal direction.