Reference conditions for ion-chamber based HDR brachytherapy dosimetry and for the calibration of high-resolution solid detectors

The aim of this study has been to develop a two-step method of in-phantom dosimetry around a brachytherapy Ir-192 photon source. The first step is to measure the absorbed dose rate to water with a calibrated ionization chamber under reference conditions, the second to cross-calibrate, under these conditions, small solid-state detectors such as silicon diodes, synthetic diamond or scintillation detectors suited for spatially resolved dose rate measurements at other, particularly at smaller source axis distances in the water phantom. This two-step approach constitutes a method for inphantom dosimetry in brachytherapy, analogous to the "small calibration field" commonly used in teletherapy to provide the reference conditions for the cross-calibration of high-resolution detectors. Under reference conditions, all known corrections for radiation quality, volume averaging and position of the chamber's effective point of measurement (EPOM) have to be applied. The study is therefore particularly devoted to (1) the experimental determination of the position of the source axis, (2) a general formulation for the volume averaging correction factor of small ionization chambers and (3) the experimental determination of the EPOM positions for the PinPoint chamber 31014 and the 3D-PinPoint chamber PTW 31022 (both PTW Freiburg, Germany). The distance of 30 mm from the source axis was chosen as the reference condition for cross calibrations. This concept is realized with the instrumentation available in a hospital, a scanning-type water phantom, a software package for small field dosimetry and detectors typically used in clinical routine dosimetry. The present development of a method of in-phantom dose measurement under Ir-192 brachytherapy conditions was performed in recognition of the primary role of dose calculations, e.g. according to the AAPM TG43 recommendations. But in addition, the methodology tested here is paving a practicable way for the experimental check of typical dose values under clinical conditions, should the need arise.