The Progress in Medical Radiation Physics series presents in-depth reviews of many of the significant developments resulting from the application of physics to medicine. This series is intended to span the gap between research papers published in scientific journals, which tend to lack details, and complete textbooks or theses, which are usually far more detailed than necessary to provide a working knowledge of the subject. Each chapter in this series is designed to provide just enough information to enable readers to both fully understand the development described and apply the technique or concept, if they so desire. Thorough references are provided for those who wish to consider the original literature. In this way, it is hoped that the Progress in Medical Radiation Physics series will be a catalyst encouraging medical physicists to apply new techniques and developments in their daily practice. Colin G. Orton ix Contents 1-1. The Tracking Cobalt Project: From Moving-Beam Therapy to Three-Dimensional Programmed Irradiation W. A. Jennings 1. Introduction 2. Establishing Moving-Beam Techniques at the Royal Northern Hospital, 1945-1955 4 2.1. Alternative Moving-Beam Techniques 4 2.2.