On the practical use of calorimetry for routine absolute dosimetry in the radiotherapy clinic

The aim of this work is to present the development and modes of operation of a small-scale graphite calorimeter probe (GPC) conceived for absolute dosimetry in the radiotherapy clinic (Patent No. PCT/CA2013/000523). Based on a numerically optimized design, a functioning prototype with embedded active thermal control capable of quasiadiabatic and isothermal operation was constructed in-house. In quasi-adiabatic mode, the radiation-induced temperature rise is measured in the sensitive volume while the device is only partially controlled. In isothermal mode, the entire device is subject to active thermal control and the quantity of interest in the electrical power necessary to maintain a stable temperature while irradiated. An experimental characterization of the GPC repeatability, linearity, and dose rate dependence while operated in quasi-adiabatic mode was carried out in a 6 MV photon field. Similarly, a characterization study of the GPC's isothermal mode was conducted by simulating various radiation fields by means of electrical power dissipation. For the quasi-adiabatic mode, a repeatability of 0.4% was established (n = 64), linearity was characterized by an adjusted R-2 value of 0.9996, and no statistically-significant dose rate dependence was observed. For isothermal mode, differences between expected and actual GPC response ranged between 0.3 - 1.4%, and in general, agreement was superior for longer irradiations with dose rates greater than 2 Gy/min. By establishing the feasibility of constructing a GPC capable of both modes of operation, this work lays the foundation for further device enhancements. The next step is the development of the GPC is to experimentally demonstrate self-consistency between the two independent modes of operation. Following this, a validation study comparing the GPC's absolute dose measurements against calibrated reference class dosimeters in both standard and non-standard radiation fields.