Tomotherapy has the potential to provide much better radiation therapy because it combines optimized radiotherapy treatment planning, intensity modulated radiation delivery and tomographic verification. TomoTherapy, Inc. is developing a commercial tomotherapy system, but we are missing a key enabling technology: a detector for optimized megavoltage computed tomography. This technology will allow more conformal dose distributions to be verifiably delivered. A key enabling technology for tomotherapy is megavoltage computed tomography. Existing computed tomography imaging systems do not have optimal performance for tomotherapy applications. This proposal explores the use of thin film amorphous selenium (a-Se) imaging for use in tomotherapy. Critical issues to be investigated in Phase I are feasibility, linearity, stability and temporal response of the system. A preliminary design for a prototype will be developed. In Phase II we will design the thin film arrays and readout electronics, fabricate a prototype detector and evaluate its performance for tomotherapy. This is the key enabling technology to allow tomotherapy to enter the marketplace. PROPOSED COMMERCIAL APPLICATIONS: Tomotherapy is a promising new method to verifiably deliver highly conformal dose distributions for cancer therapy. The total market for radiotherapy delivery equipment is hundreds of millions of dollars per year. One of the key technologies for enabling tomotherapy is a Computed Tomography-like megavoltage detector. Amorphous selenium imaging technology is highly promising for this application.