[en] The IAEA has a long standing history of providing support and assistance for radiotherapy dosimetry audits in Member States, for educating and training radiotherapy professionals, and for reviewing the radiotherapy process in a variety of situations. Since 1969, and in collaboration with the World Health Organization (WHO), the IAEA has implemented a dosimetry audit service using mailed thermoluminescent dosimeters (TLD) to verify the calibration of radiotherapy beams in hospitals in Member States. The IAEA/WHO TLD service aims at improving the accuracy and consistency of clinical radiotherapy dosimetry worldwide. Detailed follow-up procedures have been implemented for correcting incorrect beam calibrations. When necessary, on-site visits by IAEA experts in radiotherapy physics are organized to identify and rectify dosimetry problems in hospitals. The IAEA has also been requested to organize expert missions in response to problems found during the radiation treatment planning process. Assessment of the doses received by affected patients and a medical assessment were undertaken when appropriate. Although vital for the radiotherapy process, accurate beam dosimetry and treatment planning alone cannot guarantee the successful treatment of a patient. The quality assurance (QA) of the entire radiotherapy process has to be taken into account. Hence, a new approach has been developed and named 'Quality Assurance Team for Radiation Oncology (QUATRO)'. The principal aim of QUATRO is to review the radiotherapy process, including the organization, infrastructure, clinical and medical physics aspects of the radiotherapy services. It also includes reviewing the hospital's professional competence, with a view to quality improvement. The QUATRO methodology is described in the IAEA publication Comprehensive Audits of Radiotherapy Practices: A Tool for Quality Improvement. QUATRO, in addition, offers assistance in the resolution of suspected or actual dose misadministrations (over and under-exposures) in radiotherapy. It includes the follow-up of inconsistent results detected with the IAEA/WHO TLD postal service and helps Member States at a very early stage in the problem-solving process, focusing on prevention of incidents or accidents in radiotherapy. The structure and systematic approach of QUATRO combined with its low-key problem-solving mode provide a complement to the operations of the IAEA Response and Assistance Network which deals with nuclear and radiological accidents and emergencies through the Convention on Assistance in the Case of a Nuclear Accident or Radiological Emergency. QUATRO involves audits both pro-active, i.e. comprehensive reviews of the radiotherapy practice, and reactive, i.e. focused investigations in response to suspected or actual incidents during radiotherapy. This publication describes the audit technique for medical physics aspects of the operation of radiotherapy hospitals in Member States. The audit methodology was developed by a group of international experts through a series of IAEA consultants meetings conducted 1999-2005