| Résumé | Purpose: To characterize and build a cylindrically layered graphite calorimeterthe size of a thimble ionization chamber for absolute dosimetry of small fields.This detector has been designed in a familiar probe format to facilitate integra-tion into the clinical workflow. The feasibility of operating this absorbed dosecalorimeter 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 ver-sion of a previously validated aerogel-insulated graphite calorimeter known asAerrow.The new model was designed and developed using numerical methods.Medium conversion factors from graphite to water, small-field output correctionfactors, and layer perturbation factors for this dosimeter were calculated usingthe EGSnrc Monte Carlo code system.A range of commercially available aero-gel densities were studied for the insulating layers, and an optimal density wasselected by minimizing the small-field output correction factors. Heat exchangewithin the detector was simulated using a five-body compartmental heat transfermodel.In quasi-adiabatic mode, the sensitive volume (a 3 mm diameter cylindricalgraphite core) experiences a temperature rise during irradiation on the orderof 1.3 mK⋅Gy−1. The absorbed dose is obtained by calculating the product ofthis temperature rise with the specific heat capacity of the graphite.The detectorwas irradiated with 6 MV (%dd(10)x=63.5%) and 10 MV (%dd(10)x=71.1%)flattening filter-free (FFF) photon beams for two field sizes,characterized bySclindimensions of 2.16 and 11.0 cm. The dose readings were compared against acalibrated Exradin A1SL ionization chamber. All dose values are reported atdmaxin water.
Results: The field output correction factors for this dosimeter design were com-puted for field sizes ranging fromSclin=0.54 to 11.0 cm. For all aerogel densi-ties studied, these correction factors did not exceed 1.5%. The relative dosedifference between the two dosimeters ranged between 0.3% and 0.7% forall beams and field sizes. The smallest field size experimentally investigated,Sclin=2.16 cm, which was irradiated with the 10 MV FFF beam, producedreadings of 84.4 cGy (±1.3%) in the calorimeter and 84.5 cGy (±1.3%) in theionization chamber.
Conclusion: The median relative difference in absorbed dose values between acalibrated A1SL ionization chamber and the proposed novel graphite calorime-ter was 0.6%. This preliminary experimental validation demonstrates that Aerrow MK7 is capable of accurate and reproducible absorbed dose measure-ments in quasi-adiabatic mode. |
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