Investigation of intra-fractionated range guided adaptive proton therapy: (Part I) on-line PET imaging and range measurement.

Develop a prototype on-line PET scanner and evaluate its capability of on-line imaging and intra-fractionated proton-induced radioactivity range measurement.
Approach: Each detector consists of 32×32 array of 2×2×30 mm3 Lutetium-Yttrium Oxyorthosilicate scintillators with single-scintillator-end readout through a 20×20 array of 3×3 mm2 Silicon Photomultipliers. The PET can be configurated with a full-ring of 20 detectors for conventional PET imaging or a partial-ring of 18 detectors for on-line imaging and range measurement. All detector-level readout and processing electronics are attached to the backside of the system gantry and their output signals are transferred to a Field-Programable-Gate-Array based system electronics and data acquisition that can be placed 2 meters away from the gantry. The PET imaging performance and radioactivity range measurement capability were evaluated by both the offline study that placed a radioactive source with known intensity and distribution within a phantom and the online study that irradiated a phantom with proton beams under different radiation and imaging conditions.
Main results: The PET has 32 cm diameter and 6.5 cm axial length field-of-view (FOV), ~2.3 to 5.0 mm spatial resolution within FOV, 3% sensitivity at the FOV center, 18% to 30% energy resolution, and ~9 ns coincidence time resolution. The offline study shows the PET can determine the shift of distal falloff edge position of a known radioactivity distribution with the accuracy of 0.30.3 mm even without attenuation and scatter corrections, and online study shows the PET can measure the shift of proton-induced positron radioactive range with the accuracy of 0.60.3 mm from the data acquired with a short-acquisition (60 second) and low-dose (5 MU) proton radiation to a human head phantom.
Significance: This study demonstrated the capability of intra-fractionated PET imaging and radioactivity range measurement and will enable the investigation on the feasibility of intra-fractionated, range-shift compensated adaptive proton therapy.