Aerrow-Mini: A Probe-Format Graphite Calorimeter for Absolute Dosimetry of Small High-Energy Photon Fields

Absorbed dose calorimetry is an absolute and direct method of measuring radiation doses. Calorimeters, used in various designs, are applied in radiation dosimetry and driven primarily by national metrology institutes with a principle aim of achieving the minimum uncertainty on the dose measurement. With the newest specialized and non conformal radiation modalities (MR-linacs, RelfeXion, Gamma Knife, Cyberknife, etc.), that cannot produce a standard reference field (10 × 10 cm² ), dosimetry has to be adapted for smaller fields. Small calorimeters have been developed and rely on vacuum pump systems to achieve pressures of typically less than 10^-3 Pa thereby limiting heat transfer between the absorbing bodies. Although, effective at thermal isolation, the challenges and time required to establish a partial vacuum and maintaining it within a volume of porous graphite, make these an impractical solution for routine clinical use. The aim of the Aerrow project was to develop a probe-format graphite calorimetry system specifically designed for routine use in the clinical environment. Originally constructed at McGill University by Renaud et al., the calorimeter, referred to herein as the Aerrow, has to-date been used to determine the absorbed dose to a small sensitive volume in high-energy clinical accelerator-based photon beams under reference conditions. The aim of this work is to complete the prototyping of a miniature version of the Aerrow. The mini-version is designed especially for a small-field use. Because of the reduced size of the sensitive volume, the feasibility of absorbed dose measurement down to a field size of 0.5 × 0.5 cm² will be investigated in this study.