A Monte Carlo study of the cellular dosimetry for ³³S neutron capture therapy

An interesting isotope, sulfur-33 (a stable isotope), has been proposed as an additional neutron-capture agent and a cooperative target for improving boron neutron capture therapy. The ³³S (n, α)³⁰Si reaction emits the α particles useful for neutron capture therapy. In this study, the dosimetric characteristics of sulfur-33 neutron capture therapy (SNCT) were investigated at the cellular level using the Monte Carlo method. The Monte Carlo N-Particle (MCNP) code was employed to simulate the transport of α particles emitted from different source regions of a single-cell model, including the cell surface (CS), cytoplasm (Cy), and nucleus (N). The model cells were defined as two concentric spheres consisting of the nucleus and cytoplasm. The radius of the cell model was Rc = 6 μm Rn = 3 μm. The S-values (S_C←C, S_C←CS, S_N←N, S_N←Cy, S_N←CS) were compared to the results with the Medical Internal Radiation Dosimetry (MIRD) method. Finally, S-values of the cellular model of several geometries were calculated.The relative differences of MCNP versus MIRD method S-value for SNCT ranged from 0.28% to 4.29% for C to C (S_C←C), -2.82% to 0.05% for Cs to C (S_C←Cs), 0.54% to 4.42% for N to N (S_N←N), -0.72% to 3.14% for Cy to N (S_{N ←Cy}), and -1.76% to 0.36% for CS to N (S_N←CS). The ratios of S_N←N/S_C←C, S_N←CS/S_C←C, S_N←Cy/S_C←C decreased with increasing the cell and nucleus size. The S-value simulated by MCNP is in agreement with the MIRD method, which is proved to be reliable for the cellular dosimetry of SNCT by the Monte Carlo method. The Monte Carlo method can be used instead of the MIRD method to accurately calculate cellular S-value, which solves the problem of cell shape limitation in MIRD method (which is suitable for regular spherical cellular model).