CANCER IMAGING: Instrumentation and Application. Volume 2. Edited by M.A. Hayat. Page 3-16. ELSEVIER: Academic Press is an imprint of Elsevier, New York, 2008 1 of 39 CHAPTER 1 Proton Computed Tomography Jerome Zhengrong Liang, Tianfang Li, Reinhard W. Schulte, Todd J. Satogata, David C. Williams, and Hartnut F.-W. Sadrozinski 1. INTRODUCTION This chapter presents the principles of proton computed tomography (pCT) and reviews its clinical applications. The emphasis is on image reconstruction from projected data along proton paths which may not necessarily be straight lines through the object to be imaged. The potential of pCT in medicine relies mainly on its role in improving proton beam therapy. Proton beams have distinct advantages compared to other radiation therapy options, such as X- rays, electron beams, etc, because they deliver radiation energy in a quite precise manner while leaving the normal tissues around a targeted tumor mostly unharmed or undamaged. This is possible due to the characteristics of the dose distribution along the proton path inside the body: a relatively low dose along the entrance toward the path end and a high dose peak at the end, called high-dose Bragg peak. Beyond the Bragg peak the dose falls off rapidly, i.e., from 90% to 20% of the peak dose within a few millimeters. Positioning the peak inside the target delivers a maximum dose to the tumor with minimal damage to the surrounding tissues. By contrast, when an X-ray beam of therapeutic energy (at MeV level) traverses the whole body, it delivers radiation energy along its entire path in a relatively uniform manner (see the chapter by Meeks in this volume: Megavoltage Computed Tomography Imaging). If a tumor is