Can Iodine Be Used As A Contrast Agent For Protontherapy Range Verification? Measurement Of The I-127(P,N)Xe-127m (Reaction) Cross Section In The 4.5-10 Mev Energy Range

Full exploitation of the therapeutic potential of protontherapy is still prevented by uncertainties in the determination of the proton range within the patient's body. Several methods have been proposed to verify the proton range in-vivo by measuring the secondary activity (either PET or prompt-gamma) induced by proton fields. However, none of the presented solutions has yet reached clinical maturity, due to low activity counts and different spatial distributions of dose and proton-induced activity profiles.The quality of verification images can be largely improved by the use of contrast agents that provide an increased induced radioactivity near the Bragg peak region. Iodine is a contrast agent already approved for a number of medical applications. It presents a promising proton reaction channel producing the metastable isomer Xe-127m, with a half-life of 69.2(9) s, which could be used for online range verification. However, cross section production for this reaction has not been measured down to small proton energies present near the Bragg peak.To overcome this lack of data, we put in place a specific set-up consisting of two pairs of collinear opposed LaBr3(Ce) detectors and a fully digital acquisition system and performed irradiations in the low energy (4-10 MeV) range in the microbeam line at the Center for Micro Analysis of Materials (Madrid, Spain).We report on a new measurement of the I-127(p,n)Xe-127m reaction cross section in the 4.5-10 MeV energy region using the activation technique. The accuracy of the measured cross sections is better than 30%, and the observed values are in agreement with theoretical calculations using the default parameters of the TALYS code. Significant activity was observed for proton energies above 4.5 MeV, corresponding to a range in water of less than 0.3 mm. Our results support the viability of I-127 as a contrast agent for proton radiotherapy.