Monte Carlo Simulation Study of Annihilation Gamma Yield from C-11 and C-12 Irradiation in Water and PMMA Phantoms

With technical advances in radioactive isotope generation, isotopes with positron decay (PDI) become a viable alternative to their stable counterparts for hadron therapy. Due to substantial signal boosting for positron emission tomography (PET), PDI is promising for accurate and precise online beam range (R
_B
) verification. In this study, we quantified the online beam range verification ability of C-11 ion beams for PET imaging. Geant4 was used to simulate the dose and gamma photon distributions inside the phantom generated from C-11 and C-12 ion beams impinging water and PMMA phantoms. Quark Gluon String Pre-compound (QGSP), Binary Ion Cascade (BIC) and High Precision (HP) Neutron model physics-list was used to govern the relevant physics processes for four therapeutic energies (E
_T
) (to reach the same penetration depth at each energy level for two carbon species). The annihilation gammas (AGs) were determined by a PET system in GATE and the retrace-back method. AG yields with different acquisition times from start of ion history were compared between C-11 and C-12 beams. Exactly after the beam was turned on, AG with time tag ≤ 5 minutes were collected and analyzed. For the water phantom at the lowest E
_T
, the measured AG from C-11 was 8 folds higher than that from C-12 beam, while dropped to 1.09 folds for the highest E
_T
. In general, with the increasing E
_T
, AG yield rates increased for both species. Full width at half maximum (FWHM) of AG distribution peaks were 0.61 mm and 3.3 mm for C-11 at the lowest and highest E
_T
respectively. The results in PMMA were similar to those in water. The simulation results show that C-11 is an excellent candidate for accurate and precise online R
_B
verification using PET.