Absorbed dose calorimetry

This article reviews the development and summarizes the state-of-the-art in absorbed dose calorimetry for all the common clinical beam modalities covered in reference dosimetry codes of practice, as well as for small and nonstandard fields, and brachytherapy. It focuses primarily on work performed in the last ten years by national laboratories and research institutions and is not restricted to primary standard instruments. The most recent absorbed dose calorimetry review article was published over twenty years ago by Ross and Klassen (1996 Phys. Med. Biol. 41 1-29), and even then, its scope was limited to water calorimeters. Since the application of calorimetry to the measurement of radiation has a long and often overlooked history, a brief introduction into its origins is provided, along with a summary of some of the landmark research that have shaped the current landscape of absorbed dose calorimeters. Technical descriptions of water and graphite calorimetry are kept general, as these have been detailed extensively in relatively recent review articles (e.g. McEwen and DuSautoy (2009 Metrologia 46 S59-79) and Seuntjens and Duane (2009 Metrologia 46 S39-58). The review categorizes calorimeters by the radiation type for which they are applied; from the widely established standards for Co-60 and high-energy x-rays, to the prototype calorimeters used in high-energy electrons and hadron therapy. In each case, focus is placed on the issues and constraints affecting dose measurement in that beam type, and the innovations developed to meet these requirements. For photons, electrons, proton and carbon ion beams, a summary of the ionization chamber beam quality conversion factors (k _Q ) determined using said calorimeters is also provided. The article closes with a look forward to some of the most promising new techniques and areas of research and speculates about the future clinical role of absorbed dose calorimetry.