A revised IAEA standard redefines calibration practices for ionization chambers and absorbed dose to water in modern radiotherapy. This revised edition of the IAEA TRS-398 Code of Practice fulfils the need for a systematic and internationally unified approach to the calibration of ionization chambers in terms of absorbed dose to water and to the use of these detectors in determining the absorbed dose to water for the radiation beams used in radiotherapy. It is based on new key data for radiation dosimetry published by the International Commission on Radiation Units and Measurements (ICRU). It contains updated information on new commercially available ionization chambers and addresses the needs of professionals working with newer radiotherapy technologies.