A prototype scintillator real-time beam monitor for ultra-high dose rate radiotherapy

Abstract

Background: FLASH Radiotherapy (RT) is an emergent cancer RT modality where an entire therapeutic dose is delivered at more than 1000 times higher dose rate than conventional RT. For clinical trials to be conducted safely, a precise and fast beam monitor that can generate out-of-tolerance beam interrupts is required. This paper describes the overall concept and provides results from a prototype ultra-fast, scintillator-based beam monitor for both proton and electron beam FLASH applications.

Purpose: A FLASH Beam Scintillator Monitor (FBSM) is being developed that employs a novel proprietary scintillator material. The FBSM has capabilities that conventional RT detector technologies are unable to simultaneously provide: (1) large area coverage; (2) a low mass profile; (3) a linear response over a broad dynamic range; (4) radiation hardness; (5) real-time analysis to provide an IEC-compliant fast beam-interrupt signal based on true two-dimensional beam imaging, radiation dosimetry and excellent spatial resolution.

Methods: The FBSM uses a proprietary low mass, less than 0.5 mm water equivalent, non-hygroscopic, radiation tolerant scintillator material (designated HM: hybrid material) that is viewed by high frame rate CMOS cameras. Folded optics using mirrors enable a thin monitor profile of ∼10 cm. A field programmable gate array (FPGA) data acquisition system generates real-time analysis on a time scale appropriate to the FLASH RT beam modality: 100-1000 Hz for pulsed electrons and 10-20 kHz for quasi-continuous scanning proton pencil beams. An ion beam monitor served as the initial development platform for this work and was tested in low energy heavy-ion beams (⁸⁶Kr⁺²⁶ and protons). A prototype FBSM was fabricated and then tested in various radiation beams that included FLASH level dose per pulse electron beams, and a hospital RT clinic with electron beams.

Results: Results presented in this report include image quality, response linearity, radiation hardness, spatial resolution, and real-time data processing. The HM scintillator was found to be highly radiation damage resistant. It exhibited a small 0.025%/kGy signal decrease from a 216 kGy cumulative dose resulting from continuous exposure for 15 min at a FLASH compatible dose rate of 237 Gy/s. Measurements of the signal amplitude versus beam fluence demonstrate linear response of the FBSM at FLASH compatible dose rates of >40 Gy/s. Comparison with commercial Gafchromic film indicates that the FBSM produces a high resolution 2D beam image and can reproduce a nearly identical beam profile, including primary beam tails. The spatial resolution was measured at 35-40 µm. Tests of the firmware beta version show successful operation at 20 000 Hz frame rate or 50 µs/frame, where the real-time analysis of the beam parameters is achieved in less than 1 µs.

Conclusions: The FBSM is designed to provide real-time beam profile monitoring over a large active area without significantly degrading the beam quality. A prototype device has been staged in particle beams at currents of single particles up to FLASH level dose rates, using both continuous ion beams and pulsed electron beams. Using a novel scintillator, beam profiling has been demonstrated for currents extending from single particles to 10 nA currents. Radiation damage is minimal and even under FLASH conditions would require ≥50 kGy of accumulated exposure in a single spot to result in a 1% decrease in signal output. Beam imaging is comparable to radiochromic films, and provides immediate images without hours of processing. Real-time data processing, taking less than 50 µs (combined data transfer and analysis times), has been implemented in firmware for 20 kHz frame rates for continuous proton beams.

Keywords: 2D beam imaging; FLASH Therapy; Radiation Therapy; fast real‐time beam monitor; radiation dosimetry.

FIGURE 1

Engineering drawing of prototype FBSM. The camera is shielded by a graphite-Pb radiation shield (not shown). The enclosure is light tight to minimize ambient light background. FBSM, FLASH Beam Scintillator Monitor.

FIGURE 2

Schematic of data flow in the FPGA based DAQ and analysis system. DAQ, data acquisition system; FPGA, field programmable gate array.

FIGURE 3

Left: Placement of the FBSM prototype device on the patient table, underneath the radiotherapy beam at UMH, radiation oncology. Right: Detail of the beam collimator structure used for the FBSM tests. UMH, University of Michigan Hospital.

FIGURE 4

Background subtracted images of a 3 mm collimated β⁻ particles on 1.25 mm thick CsI(Tl) crystal (left) and 0.43 mm thick HM scintillator (center). Projection histograms (right) shows the beam profile averaged over a 40 pixel wide band along the orthogonal direction. The Y-axis is the ADC count normalized to the scintillator thickness. ADC, analog-to-digital-converter; HM, Hybrid Material.

FIGURE 5

Left: reconstructed beam position in pixel units of a 3 mm beta source translated along the X-coordinate of the FBSM. The data points are in black, and the red line is a linear fit. Right: the residual distribution of the reconstructed positions.

FIGURE 6

Firmware beta version timing diagram from FPGA logic analyzer. The top timing chart shows a sequence of valid data blocks (DMA_VALID) comprising a single camera image frame. They are bookended by a start of image frame (DMA_SOP) and end of image frame (DMA_EOP). DATA_COUNTER registers the number of internal 4 ns FPGA clock cycles. The time between two sequential DMA_SOP markers is 12500 clock cycles corresponding to 50 µs. The bottom left inset shows the detailed timing of the DMA_SOP signal, which is used to reset DATA_COUNTER. The bottom right inset shows the detailed timing of the DMA_EOP which occurs approximately after 42.7 µs of the total of 50 µs between frames. The middle inset shows the details of the XDIV_DONE signal, marking the end of the frame analysis relative to the DMA_SOP, and that the analysis is completed in about 0.7 µs.

FIGURE 7

Left: The centroids, reconstructed in firmware, of a 1 cm LED emulated “beam”, translated along the X-coordinate of the FBSM operated at 20 000 fps. Data points are in black; the red line is a linear fit. Right: The residual distribution of the reconstructed positions.

FIGURE 8

16 MeV electron beam: (left) FBSM image, exposure = 1s (0.17 Gy), in sensor pixel coordinate system, original aspect ratio; (center) scintillator image after a homography transform; (right) Gafchromic film image for 20 Gy isocenter-equivalent dose exposure. Note that the 10 cm scale refers to total active area dimension. The 7 cm dimension of the Cu collimator shown in Figure 3 is outlined by the inner, darker tan region.

FIGURE 9

The projection of the beam along the X-axis for an averaged 10 pixel wide Y-axis band for the Gafchromic film and prototype FBSM image data. The double Gaussian fits of each curve are shown. Inset: residual difference histogram of the two curves. The RMS width is 0.5%.

FIGURE 10

Bremsstrahlung photon backgrounds for (A) and (B): 6 MeV and 16 MeV electrons, no shielding; (C) and (D) Same, but with the shielding enclosure surrounding the camera.

FIGURE 11

Left: image of a single pulse, 3.3 nC. Right: Beam profile along X-axis of the primary beam region.

FIGURE 12

HM scintillator normalized signal during extended irradiation. The linear fit slope line indicates an average signal loss of 0.025 % per kGy exposure.

FIGURE 13

Prototype FBSM average ADC signal of 8 MeV electron beam versus radiation equivalent dose per pulse. The line is a linear fit.