TIARA: a fast gamma-ray detector for range monitoring in Proton Therapy

TL;DR

TIARA provides a fast, Cherenkov-based gamma-ray detection approach for real-time proton-range verification via Prompt Gamma Time Imaging (PGTI). By using PbF$_2$ Cherenkov radiators coupled to SiPMs in a compact module and coordinating with a fast beam monitor, the study demonstrates time resolutions in the few-hundred-ps range and a projected overall detection efficiency of $0.45\%$ for a full 30-module system. Optical Geant4 simulations calibrated with $^{60}$Co data indicate strong neutron insensitivity and favorable SNR, enabling reliable range localization with high timing precision. The measured range accuracy of $3.3 \pm 0.1$ mm at $2\sigma$ for a ~10$^{7}$ protons spot, along with the predicted performance at clinical intensities, support TIARA’s potential for real-time, in vivo range monitoring in proton therapy.

Abstract

We developed a novel gamma-ray detection system (TIARA) for range monitoring in Particle Therapy. The system employs Cherenkov-based gamma-ray detection modules arranged around the target or patient, operated in time coincidence with a fast plastic beam monitor (described in a separate paper). This work focuses on the design and comprehensive characterization of the gamma-ray detection module. It consists of a monolithic PbF$_2$ crystal (2 $\times$ 1.5 $\times$ 1.5 cm$^{3}$) coupled to a 2 $\times$ 2 SiPM matrix from Hamamatsu. A series of beam tests at different clinical facilities (MEDICYC and ProteusOne in France, CNAO in Italy) enabled the determination of the detector time resolution under various conditions, with values ranging from 222 to 283 ps FWHM (Full Width Half Maximum). Monte Carlo simulations including the optical response of PbF$_2$ allowed for the determination of the detection efficiency as a function of particle type and energy. For the final TIARA prototype, which will be composed of 30 modules, an overall detection efficiency of 0.45% is expected. Comparison with experimental data confirmed that the modules are effectively insensitive to neutrons, yielding an excellent signal-to-noise ratio (SNR), with an estimated SNR of 17 for a module placed at 25 cm from the 148 MeV proton beam axis. These features translate into high range accuracy: while the performance varies with beam energy and irradiation conditions, a range accuracy of 3.3 mm at 2$σ$ significance level was achieved at low intensity with 63 MeV protons at MEDICYC, for a small irradiation spot of $\mathbf{\sim}$10$\mathbf{^{7}}$ protons.

Figures (10)

• Figure 1: The TIARA block detector consists of a 2 $\times$ 1.5 $\times$ 1.5 cm$^{3}$ PbF$_2$ crystal covered by a white reflective paint and readout by a 2 $\times$ 2 matrix of 6 $\times$ 6 mm$^{2}$ SiPMs, multiplexed in a single readout channel.
• Figure 2: Experimental setup employed to measure the TOF between the proton arrival time in the beam monitor and the PG detection in the TIARA module. For all experiments, the gamma module was placed upstream the target, while for the ProteusOne campaign a second module was positioned downstream at 90° with respect to the Bragg peak. A thin target (10 mm PMMA or 1 mm copper) is used for time resolution measurements, while a thick (17 cm PMMA) target allows determining the SNR in more realistic conditions, where the beam is fully absorbed.
• Figure 3: TOF profiles obtained by irradiating a 1 mm copper target at MEDICYC (a) and a 10 mm PMMA slab at CNAO (b). For the CNAO result, raw data (in blue) are superimposed to the flat background (in orange) measured by removing the target. In each plot, the Gaussian fit (red) allows determining the system CTR.
• Figure 4: TOF profiles obtained by irradiating a thin target at the ProteusOne accelerator for TIARA modules placed upstream (left) and downstream (right) the target. On top, raw data are superimposed with the background corresponding to protons scattered in the beam monitor. The bottom plots represent the TOF profiles after background subtraction. The contribution of PGs emitted in the target is fitted in blue and, for the downstream module, the second contribution around 3 ns corresponds to protons scattered in the target.
• Figure 5: Visualisation of the optical simulation performed with Geant4. Two TIARA modules, each composed of a $2 \times 1.5 \times 1.5$ cm$^{3}$ PbF$_2$ crystal and four $6 \times 6$ mm$^{2}$ SiPMs interfaced by 0.1 mm of optical grease, are placed on either side of a cobalt source. The same set-up was realised experimentally to calibrate the optical parameters in the simulation.