Session Item

There are strong indications that ultra-high dose rates (UHDR) used for FLASH-radiation therapy (RT) strongly reduce the adverse effects of conventional RT. The radiobiological mechanisms and exact conditions for obtaining the FLASH-effect are, however, under investigation. Radiochromic film is generally used for dosimetry in this field, but is subject to drawbacks such as non-linear calibration and a cumbersome read-out procedure. Optically stimulated luminescence (OSL) dosimetry is therefore investigated as alternative.

Measurements were performed using the ElectronFlash linac (1) which generates pulsed electron beams and allows large systematic variation of the important parameters for FLASH-RT research. The linear response to dose per pulse (DPP) and total dose, and the stability with pulse repetition frequency (PRF) were investigated for a phosphor coated OSL film optimized for radiation therapy (2) and compared with EBT-XD GafChromic film (3). The DPP ranged from 0.0-9.6 Gy by variation of pulse length and/or applicator diameter. Total dose ranged from 0-20 Gy by variation of the number of pulses, both at conventional- and UHDR. The PRF was varied from 1-245 Hz. The OSL film was irradiated in a dark environment and read out by a computed radiography (CR) digitizer, adapted for conventional RT. The radiochromic films were scanned 48 hours after irradiation using an EPSON 10000XL (4) transmission scanner and their calibration curve was obtained against an Advanced Markus (5) ionization chamber in a conventional electron beam.

Radiochromic film and the OSL systems showed good linearity with increasing DPP up to 12 Gy/pulse (R² > 0.995) and with dose (R²>0.999). For the OSL system linearity was maintained up to 20 Gy. Radiochromic film has a nominal dynamic range up to 60 Gy. No difference between conventional- and UHDR was observed. PRF stability was examined as the percentage difference from the 1 Hz signal. The OSL system showed maximal deviation of -2.2 % at 245 Hz. Radiochromic film showed maximal deviation of 2.5 % at 5 Hz.

With the good linearity with DPP and total dose, and good PRF stability, the OSL system has the potential to be a valid alternative to radiochromic film as reference dosimeter in UHDR electron beams. However, further investigation of the important characteristics is needed and currently ongoing.  It has strong benefits with the straight forward read-out procedure, linear calibration and reusability of the films. Modification of the coating and digitizer optimization are being investigated to further improve the OSL characteristics in UHDR electron beams. The need for irradiation in dark conditions is currently a drawback of the system. Also a dark fading curve is required to correct for time differences between irradiation and read-out. This work is part of the 18HLT04 UHDpulse project which received funding from the EMPIR programme. The first author is funded via the young potential PhD grant from SCK CEN.