Electron beam response corrections for an ultra-high-dose-rate capable diode dosimeter.

BACKGROUND: Ultra-high-dose-rate (UHDR) electron beams have been commonly utilized in FLASH studies and the translation of FLASH Radiotherapy (RT) to the clinic. The EDGE diode detector has potential use for UHDR dosimetry albeit with a beam energy dependency observed. PURPOSE: The purpose is to present the electron beam response for an EDGE detector in dependence on beam energy, to characterize the EDGE detector's response under UHDR conditions, and to validate correction factors derived from the first detailed Monte Carlo model of the EDGE diode against measurements, particularly under UHDR conditions. METHODS: Percentage depth doses (PDDs) for the UHDR Mobetron were measured with both EDGE detectors and films. A detailed Monte Carlo (MC) model of the EDGE detector has been configured according to the blueprint provided by the manufacturer under an NDA agreement. Water/silicon dose ratios of EDGE detector for a series of mono-energetic electron beams have been calculated. The dependence of the water/silicon dose ratio on depth for a FLASH relevant electron beam was also studied. An analytical approach for the correction of PDD measured with EDGE detectors was established. RESULTS: Water/silicon dose ratio decreased with decreasing electron beam energy. For the Mobetron 9 MeV UHDR electron beam, the ratio decreased from 1.09 to 1.03 in the build-up region, maintained in range of 0.98-1.02 at the fall-off region and raised to a plateau in value of 1.08 at the tail. By applying the corrections, good agreement between the PDDs measured by the EDGE detector and those measured with film was achieved. CONCLUSIONS: Electron beam response of an UHDR capable EDGE detector was derived from first principles utilizing a sophisticated MC model. An analytical approach was validated for the PDDs of UHDR electron beams. The results demonstrated the capability of EDGE detector in measuring PDDs of UHDR electron beams.

Characterization of a diode dosimeter for UHDR FLASH radiotherapy.

BACKGROUND: Ultra-high dose rate (UHDR) FLASH beams typically deliver dose at rates of  >40 Gy/sec. Characterization of these beams with respect to dose, mean dose rate, and dose per pulse requires dosimeters which exhibit high temporal resolution and fast readout capabilities. PURPOSE: A diode EDGE Detector with a newly designed electrometer has been characterized for use in an UHDR electron beam and demonstrated appropriateness for UHDR FLASH radiotherapy dosimetry. METHODS: Dose linearity, mean dose rate, and dose per pulse dependencies of the EDGE Detector were quantified and compared with dosimeters including a W1 scintillator detector, radiochromic film, and ionization chamber that were irradiated with a 10 MeV UHDR beam. The dose, dose rate, and dose per pulse were controlled via an in-house developed scintillation-based feedback mechanism, repetition rate of the linear accelerator, and source-to-surface distance, respectively. Depth-dose profiles and temporal profiles at individual pulse resolution were compared to the film and scintillation measurements, respectively. The radiation-induced change in response sensitivity was quantified via irradiation of ∼5kGy. RESULTS: The EDGE Detector agreed with film measurements in the measured range with varying dose (up to 70 Gy), dose rate (nearly 200 Gy/s), and dose per pulse (up to 0.63 Gy/pulse) on average to within 2%, 5%, and 1%, respectively. The detector also agreed with W1 scintillation detector on average to within 2% for dose per pulse (up to 0.78 Gy/pulse). The EDGE Detector signal was proportional to ion chamber (IC) measured dose, and mean dose rate in the bremsstrahlung tail to within 0.4% and 0.2% respectively. The EDGE Detector measured percent depth dose (PDD) agreed with film to within 3% and per pulse output agreed with W1 scintillator to within -6% to +5%. The radiation-induced response decrease was 0.4% per kGy. CONCLUSIONS: The EDGE Detector demonstrated dose linearity, mean dose rate independence, and dose per pulse independence for UHDR electron beams. It can quantify the beam spatially, and temporally at sub millisecond resolution. It's robustness and individual pulse detectability of treatment deliveries can potentially lead to its implementation for in vivo FLASH dosimetry, and dose monitoring.

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

PURPOSE: The purpose of this investigation is to evaluate the use of a probe-format graphite calorimeter, Aerrow, as an absolute and relative dosimeter of high-energy pulse dose rate (UHPDR) electron beams for in-water reference and depth-dose-type measurements, respectively. METHODS: In this paper, the calorimeter system is used to investigate the potential influence of dose per pulses delivered up to 5.6 Gy, the number of pulses delivered per measurement, and its potential for relative measurement (depth-dose curve measurement). The calorimeter system is directly compared against an Advanced Markus ion chamber. The finite element method was used to calculate heat transfer corrections along the percentage depth dose of a 20-MeV electron beam. Monte Carlo-calculated dose conversion factors necessary to calculate absorbed dose-to-water at a point from the measured dose-to-graphite are also presented. RESULTS: The comparison of Aerrow against a fully calibrated Advanced Markus chamber, corrected for the saturation effect, has shown consistent results in terms of dose-to-water determination. The measured reference depth is within 0.5 mm from the expected value from Monte Carlo simulation. The relative standard uncertainty estimated for Aerrow was 1.06%, which is larger compared to alanine dosimetry (McEwen et al. https://doi.org/10.1088/0026-1394/52/2/272) but has the advantage of being a real-time detector. CONCLUSION: In this investigation, it was demonstrated that the Aerrow probe-type graphite calorimeter can be used for relative and absolute dosimetries in water in an UHPDR electron beam. To the author's knowledge, this is the first reported use of an absorbed dose calorimeter for an in-water percentage depth-dose curve measurement. The use of the Aerrow in quasi-adiabatic mode has greatly simplified the signal readout, compared to isothermal mode, as the resistance was directly measured with a high-stability digital multimeter.