Activation measurements of an iodinated contrast media for online range verification in proton therapy

The use of contrast agents has previously been proposed as a novel method to increase the activation close to the Bragg peak, aiming to improve the quality of proton range monitoring in vivo. In a recent work, we demonstrated the feasibility of $^{127}$I for online verification, thanks to its high cross-section (200 mbarn at 10 MeV) and low energy production threshold for $^{127m}$Xe. The spectroscopy-based method relies on identifying two specific delayed $\gamma$ lines (at 124.6 and 172.5 keV), which can be analyzed via single-photon emission computed tomography (SPECT). In this work, we present a proof-of-principle study to investigate proton activation in a commercial iodinated contrast media (ICM) for radiology. Five measurements were conducted at different proton energies (6-10 MeV), equivalent to the last millimeters of clinical proton range. Activation in the ICM was measured with four LaBr$_3$(Ce) scintillators. The contribution from iodine was separated from the activation of the solvent, yielding excellent agreement with previously reported data. These results demonstrate the potential of this technique and pave the way for further testing in clinically relevant scenarios.