Use of short-lived positron emitters for in-beam and real-time β range monitoring in proton therapy.

Aim: The purpose of this work is to evaluate the precision with which the GEANT4 toolkit simulates the production of β emitters relevant for in-beam and real-time PET in proton therapy.

Background: An important evolution in proton therapy is the implementation of in-beam and real-time verification of the range of protons by measuring the correlation between the activity of β and dose deposition. For that purpose, it is important that the simulation of the various β emitters be sufficiently realistic, in particular for the N short-lived emitter that is required for efficient in-beam and real-time monitoring.

Methods: The GEANT4 toolkit was used to simulate positron emitter production for a proton beam of 55 MeV in a cubic PMMA target and results are compared to experimental data.

Results: The three β emitters with the highest production rates in the experimental data (C, O and N) are also those with the highest production rate in the simulation. Production rates differ by 8% to 174%. For the N isotope, the β spatial distribution in the simulation shows major deviations from the data. The effect of the long range (of the order of 20 mm) of the β originating from N is also shown and discussed.

Conclusions: At first order, the GEANT4 simulation of the β activity presents significant deviations from the data. The need for precise cross-section measurements versus energy below 30 MeV is of first priority in order to evaluate the feasibility of in-beam and real-time PET.