A 100 ps TOF Detection System for On-Line Range-Monitoring in Hadrontherapy

The accuracy of hadrontherapy treatment is currently limited by ion-range uncertainties. In order to fully exploit the potential of this technique, we propose the development of a novel system for online control of particle therapy, based on TOF-resolved (time-of-flight) Prompt Gamma (PG) imaging with 100 ps time resolution. Our aim is to detect a possible deviation of the proton range with respect to treatment planning within the first few irradiation spots at the beginning of the session. The system consists of a diamond-based beam hodoscope for single proton tagging, operated in time coincidence with one or more gamma detectors placed downstream of the patient. The TOF between the proton time of arrival in the hodoscope and the PG detection time provides an indirect measurement of the proton range in the patient with a precision strictly related to the system time resolution. With a single ~38 cm
³
BaF
₂
detector placed at 15 cm from a heterogeneous PMMA target, we obtained a coincidence time resolution of 101 ps (rms). This system allowed us to measure the thickness and position of an air cavity within a PMMA target, and the associated proton range shift: a 3 mm shift can be detected at 2s confidence level within a single large irradiation spot (~10
⁸
protons). We are currently conceiving a multi-channel PG timing [1],[2] detector with 3D target coverage. Each pixel of about 5×5 mm
²
detection surface will provide the PG detection time and its hit position, that can be used to reconstruct the longitudinal distribution of PG vertices in the patient. The number of PG detected in each channel is used to reconstruct the vertex in the transverse plane. Our approach does not require collimation and allows to dramatically increase the detection efficiency. Since both signal detection and background rejection are based on TOF, the constraints on energy resolution can be relaxed to further improve time resolution. The pixel detector technology is currently under test and will be based on Cherenkov radiators coupled to Silicon Photomultipliers (SiPM).