Poster — Wed Eve—34: Design of a Primary Collimator for Cone Beam CT Imaging

Abstract

Intensity Modulated Radiation Therapy (IMRT) is becoming the standard of care for a number of cancer sites. Our research work has focused on developing Co‐60 based IMRT as an alternative for those areas in the world with limited infrastructure for supporting LINAC based systems. We have, to date, considered Co‐60 IMRT based on the tomotherapy approach because the full rotational delivery properties of tomotherapy helps to overcome the problems associated with the limited penetration of Co‐60 beams. However, with the recent introduction of arc based broad beam IMRT technology such as RapidArc (Varian Medical Systems, Palo Alto, CA), there is a potential for Co‐60 to be used in this approach. One potential advantage of broad beam versus fan beam Co‐60 IMRT delivery would be a reduction in the beam‐on time; a feature particularly important for a source that decays. Cone beam CT (CBCT) imaging is widely used for image guidance of broad‐beam IMRT and could likely play a role in a Co‐60 based broad beam IMRT system. The simplest and least complex design for such a system would consist of a single Co‐60 source for therapy and CBCT imaging. In this report we describe and analyze a new multileaf variable aperture primary collimator that permits a single Co‐60 source to be “switched” from therapy to a lower dose rate imaging mode. Monte carlo simulations of this design show that with a combination of altered aperture shape and an attenuator, an acceptable imaging dose rate without loss of spatial resolution is achieved.