Quantitative assessment of radionuclide production yields in in-beam1 and offline PET measurements at different proton irradiation facilities

Objective: Reliable radionuclide production-yield data are a prerequisite for positron-emission-1
tomography (PET) based in-vivo proton treatment verification. In this context, activation data acquired2
at two different treatment facilities with different imaging systems were analyzed to provide3
experimentally determined radionuclide yields in thick targets and were compared with each other to4
investigate the impact of the respective imaging technique.5
Approach: Homogeneous thick targets (PMMA, gelatine, and graphite) were irradiated with mono-6
energetic proton pencil-beams at two distinct energies. Material activation was measured (i) in-beam7
during and after beam delivery with a double-head prototype PET camera and (ii) offline shortly after8
beam delivery with a commercial full-ring PET/CT scanner. Integral as well as depth-resolved +-9
emitter yields were determined for the dominant positron-emitting radionuclides 11C, 15O, 13N and (in-10
beam only) 10C. In-beam data were used to investigate the qualitative impact of different monitoring11
time schemes on activity depth profiles and their quantitative impact on count rates and total activity.12
Main results: Production yields measured with the in-beam camera were comparable to or higher13
compared to respective offline results. Depth profiles of radionuclide-specific yields obtained from the14
double-head camera showed qualitative differences to data acquired with the full-ring camera with a15
more convex profile shape. Considerable impact of the imaging timing scheme on the activity profile16
was observed for gelatine only with a range variation of up to 3.5 mm. Evaluation of the coincidence17
rate and the total number of observed events in the considered workflows confirmed a strongly18
decreasing rate in targets with a large oxygen fraction.19
Significance: The observed quantitative and qualitative differences between the datasets underline20
the importance of a thorough system commissioning. Due to the lack of reliable cross-section data, in-21
house phantom measurements are still considered a gold standard for careful characterization of the22
system response and to ensure a reliable beam range verification