Figure 5.

Illustrative sketch of the PGT method. The therapeutic proton p (the projectile) slows down as it penetrates the target and interacts with a nucleus, which emits a prompt gamma ray. The time between the entrance of the proton to the target (the start flag) and the arrival of the gamma ray to the detector (the stop flag at the yellow rectangle) encodes the proton transit time and gamma time of flight, which can be correlated to the depth of interaction of the proton (gamma emission point).