SU‐C‐204‐07: The Production of Short‐Lived Positron Emitters in Proton Therapy

Purpose: To investigate the production and effect of short‐lived positron emitters when using PET for in‐vivo range verification during a proton therapy irradiation. Methods: The integrated production of short‐lived positron emitters in the stopping of 55 MeV protons was measured in water, carbon, phosphorus and calcium targets. The experimental production rates are used to calculate the production on PMMA and a representative set of 4 tissue materials. The number of decays integrated over an irradiation in these materials is calculated as function of the duration of the irradiation, considering irradiations with the same total number of protons. Results: The most copiously produced short‐lived nuclides and their production rates relative to the relevant long‐lived nuclides are: 12‐N (T1/2 = 11 ms) on carbon (9.5% of the 11‐C production), 29‐P (T1/2 = 4.1 s) on phosphorus (20% of the 30‐P production) and 38m‐K (T1/2 = 0.92 s) on calcium (113% of the 38g‐K production). No short‐lived nuclides are produced on water. The most noticeable Result is that for an irradiation in (carbon‐rich) adipose tissue, 12‐N will dominate the PET image up to an irradiation duration of 70 s. On bone tissue, 15‐O dominates over 12‐N after 7–15 s (depending on the carbon‐to‐oxygen ratio). Conclusions: The presence of 12‐N needs to be considered in PET imaging during proton beam irradiations as, depending on tissue composition and PET scanning protocol, it may noticeably deteriorate image quality due to the large positron range blurring. The results presented warrant investigations into the energy‐dependent production of 12‐N, 29‐P and 38m‐K and their effect on PET imaging during proton irradiations.