The production of positron emitters with millisecond half-life during helium beam radiotherapy

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Abstract
Therapy with helium ions is currently receiving significantly increasing interest because helium ions have a sharper penumbra than protons and undergo less fragmentation than carbon ions and thus require less complicated dose calculations. For any ion of interest in hadron therapy, the accuracy of dose delivery is limited by range uncertainties. This has led to efforts by several groups to develop in vivo verification techniques, including positron emission tomography (PET), for monitoring of the dose delivery. Beam-on PET monitoring during proton therapy through the detection of short-lived positron emitters such as 12 N ( T 1/2 = 11 ms), an emerging PET technique, provides an attractive option given the achievable range accuracy, minimal susceptibility to biological washout and provision of near prompt feedback. Extension of this approach to helium ions requires information on the production yield of relevant short-lived positron emitters. This study presents the first measurements of the production of short-lived positron emitters in water, graphite, calcium and phosphorus targets irradiated with 59 MeV/u 3 He and 50 MeV/u 4 He beams. For these targets, the most produced short-lived nuclides are 13 O/ 12 N ( T 1/2 = 8.6/11 ms) on water, 13 O/ 12 N on graphite, 43 Ti/ 41 Sc/ 42 Sc ( T 1/2 = 509–680 ms) on calcium, 28 P ( T 1/2 = 268 ms) on phosphorus. A translation of the results from elemental targets to PMMA and representative tissues such as adipose tissue, muscle, compact and cortical bone, shows the dominance of 13 O/ 12 N in at least the first 20 s of an irradiation with 4 He and somewhat longer with 3 He. As the production of 13 O/ 12 N in a 3 He irradiation is 3–4 times higher than in a 4 He irradiation, from a statistical point of view, range verification using 13 O/ 12 N PET imaging will be about 2 times more precise for a 3 He irradiation compared to a 4 He irradiation.

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