Influence of momentum acceptance on range monitoring of ¹¹C and ¹⁵O ion beams using in-beam PET

Abstract

In heavy-ion therapy, the stopping position of primary ions in tumours needs to be monitored for effective treatment and to prevent overdose exposure to normal tissues. Positron-emitting ion beams, such as 11C and 15O, have been suggested for range verification in heavy-ion therapy using in-beam positron emission tomography (PET) imaging, which offers the capability of visualizing the ion stopping position with a high signal-to-noise ratio. We have previously demonstrated the feasibility of in-beam PET imaging for the range verification of 11C and 15O ion beams and observed a slight shift between the beam stopping position and the dose peak position in simulations, depending on the initial beam energy spread. In this study, we focused on the experimental confirmation of the shift between the Bragg peak position and the position of the maximum detected positron-emitting fragments via a PET system for positron-emitting ion beams of 11C (210 MeV u−1) and 15O (312 MeV u−1) with momentum acceptances of 5% and 0.5%. For this purpose, we measured the depth doses and performed in-beam PET imaging using a polymethyl methacrylate (PMMA) phantom for both beams with different momentum acceptances. The shifts between the Bragg peak position and the PET peak position in an irradiated PMMA phantom for the 15O ion beams were 1.8 mm and 0.3 mm for momentum acceptances of 5% and 0.5%, respectively. The shifts between the positions of two peaks for the 11C ion beam were 2.1 mm and 0.1 mm for momentum acceptances of 5% and 0.5%, respectively. We observed larger shifts between the Bragg peak and the PET peak positions for a momentum acceptance of 5% for both beams, which is consistent with the simulation results reported in our previous study. The biological doses were also estimated from the calculated relative biological effectiveness (RBE) values using a modified microdosimetric kinetic model (mMKM) and Monte Carlo simulation. Beams with a momentum acceptance of 5% should be used with caution for therapeutic applications to avoid extra dose to normal tissues beyond the tumour when the dose distal fall-off is located beyond the treatment volume.

Radioactive Beams in Particle Therapy: Past, Present, and Future

“…Even if this shift is smaller than the one observed using stable ions for treatment and projectile fragments for PET imaging (Figure 2), it increases with the momentum acceptance. Recent measurements at HIMAC shows that for 11 C, the shift is around 2 mm at 5% acceptance and is reduced to 0.1 mm at 0.5% momentum acceptance [84]. Momentum spreads can therefore translate in significant range spreads at the site of stopping ( Table 2).…”

Radioactive Beams for Image-Guided Particle Therapy: The BARB Experiment at GSI

“…The atomic interaction (energy loss, energy-loss straggling and angular straggling) of the ion-beam with the tissue is the dominant physical process involved in the ion-beam therapy, and the accurate understanding of corresponding properties like range and range straggling are of very high practical importance. At HIMAC (Japan), the ranges of various PET isotopes ( 10,11 C, 14,15 O) have been investigated extensively (27,44,45). The range distribution of the selected fragments is primarily determined by the initial energy distribution.…”

“…Most of these problems are automatically overcome if β + -radioactive ion beams (RIB) are directly used for both treatment and imaging. Such radioactive ion beams would improve the count rate by an order of magnitude ( 26 ), reduce the shift between measured activity and dose ( 27 ), and mitigate the washout blur of the image with short-lived isotopes and in-beam acquisition, eventually leading to sub-mm resolution. Attempts to use RIB in therapy started almost half a century ago during the heavy ion therapy pilot project at the Lawrence Berkeley Laboratory (CA, USA) ( 28 ), but they were always hampered by the low intensity of the secondary beams produced by fragmentation of the primary ion used for therapy ( 29 ).…”

Depth dose measurements in water for 11C and 10C beams with therapy relevant energies

“…This, together with the short half-lives of the radioisotopes (20.33 min for 11 C and 19.3 s for 10 C), allows to reduce the time required to collect the PET signal, opening the possibility of an online range monitoring and also reducing the biological washout of the PET signal [ 15 , 16 ]. Additionally, differently than in the case of stable beams, with RIB the activity peak matches the mean ranges of the primaries and is correlated with the 80% distal fall-off after the Bragg peak (and almost coincides with the peak position in case of negligible beam momentum spread) [ 17 ].…”