A Monte Carlo study of the Cellular Dosimetry for SNCT Comparison with MIRD S Values

Background An interesting isotope, sulfur-33 (a stable isotope), has been proposed as an additional neutron capture or a cooperating target for enhancing boron neutron capture therapy. Recently, 33S was considered as the substitute for boron-10 in neutron capture radiotherapy. The 33S (n, α)30Si reaction emits the α particles useful for neutron capture therapy. Based on the Monte Carlo method, we investigated the dosimetric characteristics of the cellular model of several geometries for the sulfur-33 neutron capture therapy (SNCT). Methods Monte Carlo N-Particle (MCNP) was used for simulating the transport of α particles emitted by SNCT from the cellular model surface (CS), cytoplasm (Cy), or nucleus (N) of a single cell model. 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 (SC←C, SC←CS, SN←N, SN←Cy, SN←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. Results The relative differences of MCNP versus MIRD method S values for SNCT ranged from 0.28% to 4.29% for C to C (SC←C), -2.82% to 0.05% for Cs to C (SC←Cs), 0.54% to 4.42% for N to N (SN←N), 0.72% to 3.14% for Cy to N (SN ←Cy), and -1.76% to 0.36% for CS to N (SN←CS). The ratios of SN←N/ SC←C, SN←CS/SC←C, SN←Cy/ SC←C decreased with increasing the cell and nucleus size. Conclusion 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 Model Carlo method can be used instead of the MIRD method to accurately calculate cellular S values, which solves the problem of cell shape limitation in MIRD method (which is suitable for regular spherical cellular model).