Short-lived positron emitters in beam-on PET imaging during proton therapy


Short-lived positron emitters in beam-on PET imaging during proton therapy

Dendooven, P.; Buitenhuis, H. J. T.; Diblen, F.; Heeres, P. N.; Biegun, A. K.; Fiedler, F.; van Goethem, M.-J.; van der Graaf, E. R.; Brandenburg, S.

The only method for in-vivo dose delivery verification in proton beam radiotherapy in clinical use today is positron emission tomography (PET) of the positron emitters produced in the patient during irradiation. PET imaging during irradiation maximizes the number of detected counts and minimizes biological washout. In such a scenario, also short-lived positron emitters will be observed. We determined which short-lived positron emitters are relevant by measuring their production in the stopping of 55 MeV protons in water, carbon, phosphorus and calcium. The most copiously produced short-lived nuclides and their production rates relative to the relevant long-lived nuclides are: 12N (T1/2 = 11 ms) on carbon (9% of 11C), 29P (T1/2 = 4.1 s) on phosphorus (20% of 30P) and 38mK (T1/2 = 0.92 s) on calcium (113% of 38gK). No short-lived nuclides are produced on water. The production on PMMA and 4 tissue materials is calculated from the experimental results. The number of decays, integrated over an irradiation, is calculated as function of the duration of the irradiation. For an irradiation in (carbon-rich) adipose tissue, 12N dominates the PET image up to an irradiation duration of 70 s. On bone tissue, 12N dominates over 15O during the first 8-15 s (depending on the carbon-to-oxygen ratio). The short-lived nuclides created on phosphorus and calcium provide 2.5 times more decays than the long-lived ones during a 70 s irradiation. Bone tissue will thus be better visible in in-beam PET compared to PET imaging after an irradiation. 12N needs to be considered in PET imaging during proton beam irradiations as its large positron range blurring may noticeably degrade image quality. Investigations into the energy-dependent production of 12N, 29P and 38mK and their effect on the quality of in-vivo treatment verification in proton therapy with PET imaging are urgently needed.

Keywords: PET; dose monitoring; short lived isotopes

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Publ.-Id: 21888