The Quality Assurance Team for Radiation Oncology (QUATRO) provides independent quality audits both proactive (comprehensive reviews of the radiotherapy practices) and reactive (focused investigations in response to suspected or actual incidents during radiotherapy). The QUATRO methodology for proactive audit is described in the IAEA publication “Comprehensive Audits of Radiotherapy Practices: A Tool for Quality Improvement”. This publication describes the audit technique for medical physics aspects of the operation of radiotherapy hospitals with the purpose of helping to identify and rectify problems in the area of radiotherapy physics. It includes the follow-up of inconsistent results detected with the thermoluminescent dosimetry postal service operated jointly by the IAEA and WHO as well as problems found during the radiation treatment planning process for both tele- and brachytherapy. Reactive QUATRO audits help hospitals at a very early stage in the problem solving process, focusing on the prevention of incidents or accidents in radiotherapy.