(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To examine the capabilities of plastic scintillator dosimeters (PSDs) to accurately measure FLASH radiotherapy dose rates delivered with an x-ray tube.
Methods: The output of three PSDs with various geometries was measured as a function of x-ray tube current at two separate distances from the source, achieving dose rates of up to 17 Gy/s on the surface of the x-ray source and 340 Gy/s directly next to the beryllium window. One PSD was made out of BCF-10 with a scintillator volume of 3.56 mm³ (Probe 1) and two PSDs were made of BCF-12 with scintillator volumes of 0.15mm³ (Probe 2) and 0.19mm³(Probe 3). Each scintillator was irradiated for 30 s at each 1-25 mA tube current values. The irradiations resulted in 510 Gy and 10,200 Gy per irradiation at maximum tube current on the surface of the x-ray source and next to the beryllium window, respectively. Each session consisted of 5 to 8 irradiations with varying rest periods between irradiations of 1 to 10 min. To study energy dependence, the probes were irradiated with 80- 120 kVp x-rays.
Results: For dose rates of 17 Gy/s, each PSD exhibited exponential decrease in output at high delivered doses. The rate of output decrease ranged from 0.11%/kGy (Probe 2) to 0.35%/kGy (Probe 1). For dose rates of 340 Gy/s, a PSD output decrease 0.39%/kGy (Probe 3) was observed. The output decrease of Probe 3 was more prominent for the 120 kVp beam (0.39 %/kGy) than for the 80 kVp beam (0.28 %/kGy).
Conclusion: We have shown that PSDs are capable of measuring FLASH dose rates provided they are periodically calibrated. Given that FLASH treatment doses are expected to be 20-80 Gy, the observed decrease in output should not negatively affect PSD FLASH dose measurements.
Funding Support, Disclosures, and Conflict of Interest: F T-P is a founder of MedScint.
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