SU-E-T-718: Modeling a Fast Neutron Therapy Beam with a Convolution/superposition Algorithm

Abstract

Purpose: To determine if a photon convolution/superposition algorithm could be used to model a fast neutron therapy beam in a commercial treatment planning system. Methods: The beam to be modeled was the Clinical Neutron Therapy System (CNTS) fast neutron beam produced by 50 MeV protons on a Be target at the University of Washington(UW) Medical Center. The dose calculation model was that implemented in Pinnacle, 3</sup> (Philips Medical Systems). Measured neutron dose data were acquired with an IC30 ion chamber with tissue equivalent gas. The neutron beam was modeled using the auto-modeling tools available in the Pinnacle system for photon beams. Doses were then computed using a 100 MU beam incident on a water equivalent phantom for open and wedged square fields as well as MLC shaped irregular fields. Pinnacle generated profiles and central axis dose points were compared to two sets of doses: 1) doses computed with the UW PRISM treatment planning system in the same geometry as the Pinnacle setup and 2) doses measured in a water tank. Results: The Pinnacle photon model incorporates most of the important dosimetric features of the neutron beam. Computed doses compared well to both the Prism TPS and measured data. We found that calculated dose points among open and wedged square fields were within 2% of both Prism and measured doses along the central axis, and within 5% of measurement in the penumbra region. Dose point calculations using irregular treatment type fields were within 3% of measured dose points. Conclusion: The Pinnacle TPS has sufficient computational modeling ability to adequately produce a viable neutron model for potential use in clinical treatment planning. Further testing of irregular fields must be performed and results analyzed prior to implementation in the clinic.