Purpose: To assess the suitability of the 3D PRESAGE phantom for relative dose measurements in a high dose per pulse FLASH electron beam.
Methods: Irradiations were performed at the Radiological Research Accelerator Facility (RARAF) using a Varian iX linear accelerator, modified to deliver a high dose per pulse 9 MeV electron beam. By varying the pulse frequency and SSD, PRESAGE phantoms were irradiated at four dose rates: 6 Gy/min, 40 Gy/s, 86 Gy/s, and ~900 Gy/s . At each position, except for the exit window (~900 Gy/s), the dose rate was determined using an advanced Markus chamber. A dose of 5-7.1 Gy was delivered to each phantom. The dose to the phantom was then evaluated using an in-house optical CT scanner. Due to the expected uncertainty in measuring absolute dosimetry at such a high pulse rate, we compared the percentage depth dose curves for this study. The resultant PDD curves were compared with each other and to the PDD curve measured using EBT3 Gafchromic film which has been shown to have no dose rate dependence.
Results: PDD curves measured for the four PRESAGE phantoms were very similar over dose rates ranging from 6 Gy/min used in conventional radiation therapy to an ultra-high FLASH dose rate of about 900 Gy/s. No dose rate dependency was seen. Comparisons to EBT3 Gafchromic film were similar but some differences were observed. Discrepancies among between PDD are possibly due to difficulties in setup and alignment.
Conclusion: PDD curves measured in PRESAGE 3D phantoms for a wide range of dose rates were comparable to those obtained using Gafchromic film, indicating the PRESAGE may be useful for relative dose measurements in FLASH electron beams. Further studies should be performed to investigate differences observed between film and 3D phantom and to investigate PRESAGE for absolute dosimetry in FLASH.