Feasibility and therapeutic effect of neutron spectra with different characteristics based on BNCT for head and lung cancers

BACKGROUND: Boron neutron capture therapy (BNCT) is widely recognized as an important treatment for malignant brain tumors and melanomas due to its targeting ability and the production of secondary particles with high linear energy transfer (LET). However, clinical trials for deep-seated tumors are less common than those for superficial tumors. Considering the substantial differences in therapeutic effects that neutron energy spectra produced by different devices may have on tumors, there is a need to explore how these variations impact treatment efficacy across tumor depths. PURPOSE: This study investigates the advantages and disadvantages of neutron spectra with different characteristics for treating tumors at varying depths. The objective is to evaluate how different neutron sources influence therapeutic outcomes, providing insights into their suitability for clinical applications in BNCT. METHODS: Using the Monte Carlo Particle and Heavy Ion Transport code System (PHITS), we studied the neutron spectra generated by three neutron sources: from a nuclear reactor, from a low-energy proton accelerator with a lithium target, and from a high-energy proton accelerator with a beryllium target. Specifically, we compared their therapeutic effects on head tumors at depths of 3 and 6 cm, and a lung tumor at a depth of 9 cm. The simulations assessed dose delivery, critical organ exposure, and treatment parameters to determine the effectiveness of each neutron source. RESULTS: The results reveal that for the treatment of a head tumor at a depth of 3 cm, all three neutron sources delivered the prescribed dose to the tumor while maintaining doses to critical organs within acceptable limits. Notably, the accelerator-based neutron sources offered shorter treatment times compared to the reactor-based source. For a head tumor at a depth of 6 cm, only the neutron source from the low-energy proton accelerator with a lithium target met the treatment requirements without exceeding critical organ doses, although the homogeneity index (HI) value decreased. When treating a lung tumor at a depth of 9 cm, all three neutron sources achieved therapeutic doses to normal lung tissue, but resulted in excessive skin doses. Additionally, analysis of dose-volume histogram (DVH) curves, HI values, and two-dimensional dose distribution maps was conducted, alongside discussions of the required tumor-to-normal tissue (T/N) ratio necessary for effective BNCT of deep-seated tumors. CONCLUSIONS: This study provides a theoretical basis for the optimization and selection of neutron sources for BNCT treatment of tumors at various depths from a dosimetric perspective. The findings offer a reference for the clinical application of BNCT, particularly in addressing the challenges of treating deep-seated tumors, and highlight the need for tailored neutron spectra to enhance therapeutic outcomes.