Feasibility Study of Measuring the Dose Per Pulse and Instantaneous Dose Rates Using a Silicon Photodiode Detector for a 10-MV Flattening Filter-Free Mode in Radiation Therapy

This study aimed at developing a pulse dosimeter for radiation therapy. The dose per pulse (DPP) and instantaneous dose rates of a commercially available medical linear accelerator were measured using a novel detector composed of a silicon photodiode (Si-PD) with a 0.4-mm-thick Cu filter, single operational amplifier, resistor, capacitor, and substrate. The pulse voltages from the detector were measured using an analog-to-digital converter. The dependence of 50, 100, 150, 200, and 250 monitor units (MU) was measured at a depth of 100 mm with a field size of 100 × 100 mm² and a constant distance of 1.0 m between the source and detector. Each MU was subjected to three measurements under a 10-MV flattening filter-free mode with a dose rate of 2400 MU/min. The DPP (mGy) was calibrated at 100 MUs using a Farmer-type ionization chamber, and the instantaneous dose rates were calculated using assumed values of a 2.8 ms pulse interval and a 4.5 μs pulse width. The depth dependence was also measured at 20 and 100 mm with a field size of 50 × 50 mm² for 100 MUs. The number of pulses was proportional to that of MUs with maximum values of DPP being 1.422 and 1.100 mGy/pulse at depths of 20 and 100 mm, respectively; the corresponding instantaneous dose rates were 0.508 and 0.393 Gy/s at 2.8 ms and 316.0 and 244.3 Gy/s at 4.5 μs. The results indicate that the developed Si-PD detector can efficiently measure DPP and instantaneous dose rates.