Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The mission of the Radiological Physics Center (RPC) is to assure NCI and the Cooperative Groups that institutions participating in clinical trials deliver prescribed radiation doses that are clinically comparable and consistent. We do this by assessing the institution''s radiotherapy programs, helping the institutions implement remedial actions, assisting the study groups in developing protocols and QA procedures, and informing the community of our finding.