Applications of Monte Carlo Techniques to Ionization Chamber Dosimetry in External Photon Beams

Over the last decades, developments in Monte Carlo radiation transport algorithms have had a tremendous impact in different areas of radiation dosimetry. During the experimental determination of absorbed dose, several quantities are often difficult (or impossible) to estimate accurately without numerical models. In the context of radiation dosimetry, detectors are most of the time constituted of several components, whose material differs substantially from the medium where absorbed dose is to be known. This situation induces a well-known problem that can be characterized in terms of perturbation factors and changes in response to energy (or energy dependence). Over the different generations of radiation dosimetry protocols, progress in Monte Carlo techniques has affected the improvement in accuracy during the determination of absorbed dose. While air kerma-based protocols of the 1980s, e.g., AAPM’s TG-21 ( AAPM, 1983 ), were the first ones to consider the ionization chamber design in detail, some quantities such as the replacement and the wall perturbation factors (i.e., P repl and P wall , respectively) were estimated with approximate measurements, analytical models or simply taken as unity based on judicious assumptions. Nowadays, such factors can be calculated with Monte Carlo methods without the need for such approximations, improving the accuracy of absorbed dose determination with radiation detectors.