Novel in-room proton range verification using mobile PET: Potential for proton therapy monitoring

Objectives PET imaging of proton induced activity (¹¹C,¹⁵O, ¹³N) allows in vivo range verification in proton therapy. However, the use of a PET/CT at a remote site results in significant signal loss due to decay of ¹⁵O signal. We assessed the feasibility of using a mobile NeuroPET scanner (PhotoDetection Systems) for proton field verification in the treatment room and compared its performance with that of off-site PET/CT in parallel phantom studies.

Methods A PMMA phantom (9x9x20 cm³) was irradiated with a Φ5cm spread-out Bragg-peak field, 16cm water equivalent (WE) range, 10cm WE modulation, and 8 Gy dose. After a delay of ~7 minutes, the activated phantom was imaged for 30 minutes in listmode and reconstructed with OSEM. The 50% distal fall-off depths along the activity depth profiles at 5 positions (range verification) and 50% fall-off beam widths at 2 depths (lateral field verification) were calculated for 30 min and last 2 min acquisitions.

Results The measured distal fall-off depths/lateral field widths (at 5, 10 cm depths) were 125.7±1.1/ (51.6±0.6, 52.8±0.8) mm for NeuroPET and 124.2±0.8/ (52.0±0.1, 53.0±0.4) mm for PET/CT for overall acquisitions, and 120.3±2.2/ (48.9±1.2, 49.9±0.9) mm for NeuroPET and 120.4±7.1/ (50.6±1.0, 51.9±1.2) mm for PET/CT for the last 2 minutes.

Conclusions In-room PET imaging is feasible and yields comparable accuracy to delayed PET/CT. However better precision can be achieved with NeuroPET at low statistics, which is the case with proton induced imaging. Further improvement in precision is expected when imaging immediately after treatment.