Po-Thur Eve General-14: 3rd Generation Co-60 based Megavoltage Computed Tomography

Abstract

Patient set up verification is a necessary step for high precision radiation therapy. There has been considerable work to develop such verification using megavoltage (MV) imaging on Electronic Portal Imaging Devices (EPIDs). Recently, MV image guidance has been advanced with the extension to MV computed tomography (MVCT). At the CCSEO we have been investigating MVCT with a Cobalt-60 unit, in the context of developing Cobalt-60 tomotherapy. The system is comprised of an automated rotary stage added to a Theratronics T780C Co-60 unit; recently our imaging has advanced with the development of a 3rd generation imaging system using a Varian PortalVision LC250 EPID. The EPID was calibrated for relative dose-response using a parallel-plate ion chamber; the calibration curve follows a quadratic form. The image reconstruction is based on a generalized Feldkamp technique; executed in MATLAB. Consistent with previous 1st generation studies, results on various phantoms show that Co-60 based EPID — MVCT is free from beam hardening and metallic artifacts. Phantoms included a variety of orthopaedic hip implants, Rando head phantom with stainless steel and tungsten inserts, and a plug phantom containing materials of known atomic number. EPID determined MVCT numbers also correspond linearly with relative electron density. The work will summarize the differences between kVp and MV CT images, while highlighting the improvements of EPID based imaging over 1st generation Cobalt MVCT.