Purpose: To present commissioning data for the Mobetron electron radiation therapy system (IntraOp) at ultra-high dose rate (UHDR) using a plastic scintillation detector (PSD).
Methods: The dosimetry was evaluated for nominal electron energies of 6 MeV and 9 MeV using the 10 cm UHDR collimator. Percent depth-dose (PDD) and reference dosimetry were evaluated using Solid Water slabs. Measurements were performed using Gafchromic EBT-XD film (Ashland) and PSD coupled to HYPERSCINT research platform RP-200 (MedScint). The Exradin A17 ion chamber (Standard Imaging) was placed under the stack of Solid Water for relative beam output monitoring. Measurements were repeated using the PSD at the depth of maximum dose for variable pulse width (PW) ranging from 1-4 µs and pulse repetition frequency (PRF) ranging from 10-90 Hz. The number of pulses varied between 2 and 15 pulses.
Results: The reference dosimetry showed good agreement (within 1%) between PSD and film. The dose per pulse (DPP) at PW = 4 µs and PRF = 60 Hz was 1.6 Gy/pulse and 1.7 Gy/pulse at 6 MeV and 9 MeV, respectively. The agreement in beam penetration (R80, R50) was within 0.5 mm and 1 mm for 6 MeV and 9 MeV, respectively. The linearity with number of pulses was within 1% between 2 and 10 pulses, while the DPP decreased with greater number of pulses. The linearity with PW was within 3% except for 4 µs (5%), considering that the nominal PW of 4 µs corresponds to an actual PW of 3.7 µs. Varying the PRF did not affect the dose by more than 2% over the investigated range except for 10 Hz (6%). The short-term output stability was within 1%.
Conclusion: We demonstrated the suitability of using a PSD as an active dosimeter for commissioning measurements of an UHDR electron beam.
Funding Support, Disclosures, and Conflict of Interest: This project is funded by the TransMedTech Institute and its main financial partner, the Apogee Canada First Research Excellence Fund