Room: Track 1
Purpose: To build and characterize a graphite calorimeter the size of a thimble ion chamber for accurate and absolute dosimetry of small fields. This detector has been designed in a familiar probe format to facilitate integration into the clinical workflow. The feasibility of operating this detector in quasi-adiabatic mode is assessed for high-energy accelerator-based photon beams.
Methods: This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter. In quasi-adiabatic mode, the sensitive volume of Aerrow MK7 (a 3 mm-diameter cylindrical graphite core) experiences a temperature rise during irradiation. The absorbed dose is obtained by calculating the product of this temperature rise with the specific heat capacity of the graphite.
The detector was irradiated with 6 MV FFF and 10 MV FFF photon beams, for field sizes ranging from 2 × 2 cm² to 10 × 10 cm². The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values reported are for 100 cGy delivered at d(m)??.
Dose conversion factors from graphite to water, clinical output factors for small fields and mass impurity correction factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system.
Results: The relative dose difference between the two dosimeters ranged between 0.1 - 0.5 % for all beams and field sizes. The smallest field size (2 × 2 cm²) produced readings of 84.4 cGy (± 1.3 %) in Aerrow MK7 and 84.5 cGy (± 1.3 %) in the A1SL chamber when irradiated with a 10 MV FFF beam.
Conclusion: The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.4%, demonstrating that Aerrow MK7 is indeed capable of accurate and reproducible absorbed dose measurements in quasi-adiabatic mode.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Institutes of Health Research (CIHR). Absorbed dose calorimetry research at McGill University is supported in part by Sun Nuclear Corporation.