Feasibility of using distal endpoints for In-room PET Range Verification of Proton Therapy

In an effort to verify the dose delivery in proton therapy, Positron Emission Tomography (PET) scans have been employed to measure the distribution of β+ radioactivity produced from nuclear reactions of the protons with native nuclei. Because the dose and PET distributions are not directly comparable, the range verification is currently carried out by comparing measured and Monte Carlo (MC) simulation predicted PET distributions. In order to reduce the reliance on MC, MC-PET and dose distal endpoints were compared to explore the feasibility of using distal endpoints for in-room PET range verification. MC simulations were generated for six head and neck patients with corrections for radiological decay, biological washout, and PET resolution. One-dimensional profiles of the dose and MC-PET were examined along the direction of the beam and covering the cross section of the beam. The chosen endpoints of the MC-PET (y-intercept of the linear fit to the distal falloff) and MC dose (20-50% of maximum dose) correspond to where most of the protons are below the threshold energy for the nuclear reactions. The difference in endpoint range between the distal surfaces of the dose and MC-PET are compared and the spread of range differences are assessed. Among the six patients, the mean difference between MC-PET and dose depth was found to be -1.5 mm to +3.7 mm between patients, with a standard deviation of 1.3 to 6 mm across the individual beams.