  For the in vivo range verification in proton therapy, it has been tried to
measure the spatial distribution of the prompt gammas generated by the
proton-induced interactions with the close relationship with the proton dose
distribution. However, the high energy of the prompt gammas and background
gammas are still problematic in measuring the distribution. In this study, we
suggested a new method determining the in vivo range by utilizing the time
structure of the prompt gammas formed with the rotation of a range modulation
wheel (RMW) in the passive scattering proton therapy. To validate the Monte
Carlo code simulating the proton beam nozzle, axial percent depth doses (PDDs)
were compared with the measured PDDs with the varying beam range of 4.73-24.01
cm. And the relationship between the proton dose rate and the time structure of
the prompt gammas was assessed and compared in the water phantom. The results
of the PDD showed accurate agreement within the relative errors of 1.1% in the
distal range and 2.9% in the modulation width. Average dose difference in the
modulation was assessed as less than 1.3% by comparing with the measurement.
The time structure of prompt gammas was well-matched within 0.39 ms with the
proton dose rate, and this could enable the accurate prediction of the in vivo
range.
