Mapping 15O Production Rate for Proton Therapy Verification

Purpose: This work was a proof-of-principle study for the evaluation of oxygen-15 (O-15) production as an imaging target through the use of positron emission tomography (PET), to improve verification of proton treatment plans and to study the effects of perfusion. Methods and Materials: Dynamic PET measurements of irradiation-produced isotopes were made for a phantom and rabbit thigh muscles. The rabbit muscle was irradiated and imaged under both live and dead conditions. A differential equation was fitted to phantom and in vivo data, yielding estimates of O-15 production and clearance rates, which were compared to live versus dead rates for the rabbit and to Monte Carlo predictions. Results: PET clearance rates agreed with decay constants of the dominant radionuclide species in 3 different phantom materials. In 2 oxygen-rich materials, the ratio of O-15 production rates agreed with the expected ratio. In the dead rabbit thighs, the dynamic PET concentration histories were accurately described using O-15 decay constant, whereas the live thigh activity decayed faster. Most importantly, the O-15 production rates agreed within 2% (P>.5) between conditions. Conclusions: We developed a new method for quantitative measurement of O-15 production and clearance rates in the period immediately following proton therapy. Measurements in the phantom and rabbits were well described in terms of O-15 production and clearance rates, plus a correction for other isotopes. These proof-of-principle results support the feasibility of detailed verification of proton therapy treatment delivery. In addition, O-15 clearance rates may be useful in monitoring permeability changes due to therapy. (C) 2015 Elsevier Inc. All rights reserved.