A potential method for in vivo range verification in proton therapy treatment.

Proton therapy potentially offers excellent dose conformality and reduction in integral dose. The superior dose distribution is, however, much more sensitive to the radiological depth along the beam path than for photon fields. Variations in this depth due to inaccurate planning calculation, setup uncertainty, respiration-induced organ motion, etc, could result in either an 'undershooting', missing the distal portion of the target volume entirely, or an 'overshooting', delivering the full prescription dose to the normal tissue beyond the target volume. In vivo dose verification plays an important role in the quality assurance of the treatments. The point dose measurements widely used for photon treatments are, however, insufficient for protons due to the particular characteristics of the proton depth-dose distribution. In this work, we explore a method for in vivo range verifications in proton treatments using range-modulated passive scattering fields. The method utilizes the time dependence of the dose distribution delivered by these fields. By measuring the time dependence of the dose rate at any point in the target volume, the residual range of the beam at this point can be obtained with millimeter accuracy.