Technical Note: Single‐pulse beam characterization for FLASH‐RT using optical imaging in a water tank

Abstract

High dose rate conditions, coupled with problems related to small field dosimetry, make dose characterization for FLASH-RT challenging. Most conventional dosimeters show significant dependence on dose rate at ultra-high dose rate conditions or fail to provide sufficiently fast temporal data for pulse to pulse dosimetry. Here fast 2D imaging of radioluminescence from a water and quinine phantom was tested for dosimetry of individual 4 μs linac pulses. Methods: A modified clinical linac delivered an electron FLASH beam of >50 Gy/s to clinical isocenter. This modification removed the x-ray target and flattening filter, leading to a beam that was symmetric and gaussian, as verified with GafChromic EBT-XD film. Lateral projected 2D dose distributions for each linac pulse were imaged in a quinine-doped water tank using a gated intensified camera, and an inverse Abel transform reconstruction provided 3D images for on-axis depth dose values. A total of 20 pulses were delivered with a 10 MeV, 1.5 cm circular beam, and beam with jaws wide open (40 × 40 cm 2 ), and a 3D dose distribution was recovered for each pulse. Beam output was analyzed on a pulse by pulse basis. Results: The R p , D max , and the R 50 measured with film and optical methods agreed to within 1 mm for the 1.5 cm circular beam and the beam with jaws wide open. Cross beam profiles for both beams agreed with film data with >95% passing rate (2%/2 mm gamma criteria). The optical central axis depth dose agreed with film data, except for near the surface. A temporal pulse analysis revealed a ramp-up period where the dose per pulse increased for the first few pulses and then stabilized. Conclusions: Optical imaging of radioluminescence was presented as a valuable tool for establishing a baseline for the recently initiated electron FLASH beam at our institution.

Technical note: Characterization and practical applications of a novel plastic scintillator for online dosimetry for an ultrahigh dose rate (FLASH)

“…Rahman et al. used a fast photomultiplier tube‐based Cherenkov detector with a 2‐ns sampling rate, two orders of magnitude above that presently achieved using the Hyperscint 23,45,46 . This detector allows the resolution of single linac pulses and the measurement of 2‐D dose distributions through a water tank delivered at 60 Hz (100 MU/min using photons).…”

“…Rahman et al used a fast photomultiplier tubebased Cherenkov detector with a 2-ns sampling rate, two orders of magnitude above that presently achieved using the Hyperscint. 23,45,46 This detector allows the resolution of single linac pulses and the measurement of 2-D dose distributions through a water tank delivered at 60 Hz (100 MU/min using photons). By using a small region of interest in their 2-D dosimeter, the authors were also able to resolve signals acquired at 360 Hz (600 MU/min) to monitor beam output.…”

Image guidance for FLASH radiotherapy

“…Images from a time‐gated camera for acquisition of each Linac pulse (a), from emission in a water phantom for depth dose, 72 and on the surface of a board for lateral profile 73 . The ability to image this light signal during each pulse (c) shows the value for pulse‐to‐pulse verification of delivery.…”

“…Lateral and depth profile images are possible to fully quantify the 3D dose distributions in water tanks, 23,70 while limited to surface images in applications to patients. Applicability to protons and light ions, for which Cherenkov and luminescence emissions have recently been reported, interestingly below the expected theoretical threshold limits, 71 could further benefit from F I G U R E 2 Images from a time-gated camera for acquisition of each Linac pulse (a), from emission in a water phantom for depth dose, 72 and on the surface of a board for lateral profile. 73 The ability to image this light signal during each pulse (c) shows the value for pulse-to-pulse verification of delivery.…”

A Diamond-Based Dose-per-Pulse X-ray Detector for Radiation Therapy

“…The literature on pulse-by-pulse detection systems is scarce, and mainly focused on scintillator- and fiber-optics-based dosimetry [ 28 , 29 ]. As for diamond-based solutions, Velthuis et al [ 30 ] reported on a prototypal system for single-pulse measurements; however, this exhibited a response time of several hundreds of μs and required relatively complex front-end analogue electronics.…”