Prompt Gamma Timing in Carbon Therapy: First Experimental Results with the TIARA Detector

TL;DR

The results demonstrate that PGT-based range monitoring remains viable for carbon-ion beams, although increased background from secondary protons indicates that detector configuration adaptations are required.

Abstract

In the context of range monitoring for particle therapy, this study presents the first experimental results obtained with the TIARA detector using carbon-ion beams at the CNAO clinical center in Pavia, Italy. TIARA is based on the Prompt Gamma Timing (PGT) technique, which measures the time of flight (TOF) between incident ions and prompt gamma rays (PGs) emitted during nuclear interactions in the target. While the TIARA prototype has previously been validated with protons, carbons present a more challenging scenario due to their higher linear energy transfer, nuclear fragmentation products, and the continuous beam time structure of synchrotron accelerators. Experiments were performed by irradiating PMMA targets of different thicknesses with 200 MeV/u carbon beams. A coincidence time resolution of 279$\pm$35 ps FWHM was achieved, outperforming results previously obtained with protons at the same facility. A range accuracy of 4.74$\pm$0.36 mm at a 2$σ$ confidence level was measured at clinical intensity, when considering 5600 detected PGs, corresponding to the grouping of four irradiation spots of 2.4$\cdot$10$^6$ ions each. Overall, the results demonstrate that PGT-based range monitoring remains viable for carbon-ion beams, although increased background from secondary protons indicates that detector configuration adaptations are required.

Figures (7)

• Figure 1: (a) Photo of the TIARA prototype setup at CNAO, showing the thin PMMA calibration target, the beam monitor, and three PG modules. (b) Schematic of the experimental setup: the beam (in red) goes through a 0.5 mm plastic beam monitor, followed by a PMMA target (either 1 cm or 20 cm thick, depending on the experiment). Three gamma modules are placed upstream the target at approximately 20 cm from the beam axis.
• Figure 2: (a) Response of the plastic scintillator beam monitor to 200 MeV/u carbon ions delivered at the CNAO synchrotron at two different intensities: two bunches are visible at this time scale. (b) Histogram of the time between two consecutive pulses obtained for the $6\cdot10^{6}$ ions/s dataset. Error bars represent the statistical uncertainty. The fit is a constrained multi-gaussian with an unique value for the standard deviation, and the centroid values multiples of $\mu$.
• Figure 3: TOF distributions obtained from the irradiation of the 1 cm thick PMMA target with a 200 MeV/u carbon beam. Results are shown for the three gamma modules at the intensity of 2$\cdot$10$^{6}$ ions/s (top row, and bottom left), and for module 3 at the intensity of 2$\cdot$10$^{7}$ ions/s (bottom, right). In each plot, the uncorrected datasets correspond to raw data, the background datasets are obtained by removing the PMMA target at acquisition, while the corrected datasets represent the raw data once the background is subtracted.
• Figure 4: Uncorrected TOF distributions, background TOF distributions, corrected TOF distributions for a 200.61 MeV/u beam at 2 $10^{6}$ ions/s impinging on the thick target. The PG signal from the target is centered around 6 ns. In this region, SNRs are 10.5, 14.1 and 8.8, for PG modules 1, 2 and 3 respectively.
• Figure 5: Left: background-subtracted TOF distributions for three of the five investigated energies. Right: schematic illustration of the algorithm used to determine the ion range. The vertical black lines indicate the integration window, while the orange curve represents the normalized cumulative TOF distribution. The green arrow denotes the calculated TOF width metric employed in the bootstrap procedure, defined as the difference between the 90th and 10th percentile times of the cumulated TOF distribution. This calculation is performed for each sub-sample.