Feasibility of operating a millimeter‐scale graphite calorimeter for absolute dosimetry of small‐field photon beams in the clinic

Abstract

To characterize and build a cylindrically layered graphite calorimeter the size of a thimble ionization chamber for 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 absorbed dose calorimeter in quasi-adiabatic mode is assessed for high-energy acceleratorbased photon beams. Methods: This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter known as Aerrow. The new model was designed and developed using numerical methods. Medium conversion factors from graphite to water, small-field output correction factors, and layer perturbation factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system. A range of commercially available aerogel densities were studied for the insulating layers, and an optimal density was selected by minimizing the small-field output correction factors. Heat exchange within the detector was simulated using a five-body compartmental heat transfer model. In quasi-adiabatic mode, the sensitive volume (a 3 mm diameter cylindrical graphite core) experiences a temperature rise during irradiation on the order of 1.3 mK⋅Gy −1 . 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 (%dd(10) x = 63.5%) and 10 MV (%dd(10) x = 71.1%) flattening filter-free (FFF) photon beams for two field sizes,characterized by S clin dimensions of 2.16 and 11.0 cm. The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values are reported at d max in water. Results:The field output correction factors for this dosimeter design were computed for field sizes ranging from S clin = 0.54 to 11.0 cm. For all aerogel densities studied, these correction factors did not exceed 1.5%. The relative dose difference between the two dosimeters ranged between 0.3% and 0.7% for all beams and field sizes. The smallest field size experimentally investigated, S clin = 2.16 cm, which was irradiated with the 10 MV FFF beam, produced readings of 84.4 cGy (±1.3%) in the calorimeter and 84.5 cGy (±1.3%) in the ionization chamber. Conclusion:The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.6%. This preliminary experimental validation demonstrates that 7476

The probe‐format graphite calorimeter, Aerrow, for absolute dosimetry in ultrahigh pulse dose rate electron beams

“…The Monte Carlo dose ratio from Equation () was calculated to be 1.149(4) and was compared to the water‐to‐graphite stopping power ratio (SPR = 1.137(9)), a difference of 1.2(1)%. The difference in the two values is due to the volume averaging due to the non‐homogeneity of the beam and the radiation field perturbation caused by the detector 16 . The volume averaging can be roughly estimated to be 2.8% for the beam setup, which has a Gaussian shape with an FWHM of about 80 mm.…”

“…The difference in the two values is due to the volume averaging due to the non-homogeneity of the beam and the radiation field perturbation caused by the detector. 16 The volume averaging can be roughly estimated to be 2.8% for the beam setup, which has a Gaussian shape with an FWHM of about 80 mm. Therefore, it can be estimated that the net perturbation from the presence of the detector would be on the order of 4%.…”

“…Notably, it was also the first absorbed dose calorimeter to incorporate an aerogel‐based thermal insulation within its nested graphite structure. To date, the use of Aerrow to determine absolute dose in conventional, flattening filter free, small field, and MR‐guided MV photon beams has been shown to be comparably accurate to calibrated reference‐class ICs 15,16 …”

“…To date, the use of Aerrow to determine absolute dose in conventional, flattening filter free, small field, and MR-guided MV photon beams has been shown to be comparably accurate to calibrated reference-class ICs. 15,16 The purpose of this study is to evaluate the use of Aerrow as an absolute dosimeter of high-energy UHDR electron beams.In this paper,the calorimeter system is directly compared against an Advanced Markus IC with a dose calibration traceable to Physikalisch-Technische Bundesanstalt (PTB), Germany's national metrology institute, to investigate the potential influence of DPP up to 5.6 Gy. The finite element analysis of heat transfer corrections at various depths along the PDD of a 20-MeV electron beam and the Monte Carlo calculation of dose conversion factors necessary to calculate absorbed dose-to-water at a point from the measured dose-to-graphite are also presented.…”