Purpose: To investigate the workload limitation for proton beam FLASH delivery based on survey results.
Methods: The radiation survey was performed with a Ludlum 42-38 WENDY-2 neutron detector and a Ludlum 9DP ion chamber survey meter around a rotational gantry room using an ultra-high dose rate proton beam (FLASH). A couple of 250 MeV beams were delivered with multiple cyclotron currents ranging from 30nA to 250nA (~10-80nA at the nozzle), which provided a peak dose rate of 40-300 Gy/s at isocenter. A water block was placed at the isocenter as a beam stopper.
Results: The highest readings from the FLASH beam were along the beam path in the Image Suite of the adjacent treatment room (FBR) and the neutron dose reading is 59.1mRem/h on WENDY-2 and the photon dose rate was 14.8mRem/h on 9DP ion chamber meter for 80nA nozzle current at the isocenter. The scatter dose from the FLASH beam is relatively small, around 1mRem/hr combined under the same condition. From the survey results, a person in the FBR image suite will receive 2mRem in 7790nAs if the gantry angle is at 90 degrees and will receive 10mRem in 39000nAs. These numbers could be used as a guideline for hourly and weekly workload limits. When gantry angle is 0 degrees, the reading of the surrounding area is insignificant.
Conclusion: During the current stage of FLASH beam delivery, the hourly(2mRem) and weekly(10mRem) workload are limited to the exposure to the surrounding area, and it also heavily depends on the gantry angle. Understanding what beyond the beams should be important during the delivery of the FLASH beam to avoid unnecessary activations.