# Investigating the Secondary Thermal Neutron Intensity of Neutron Capture-Enhanced Proton Therapy
## 4 Discussion  
For the clinical application of NCEPT, it is essential to develop drugs that increase intracellular boron concentration and understand the behavior of secondary neutrons generated in the body. In this study, we focused on the neutrons generated within the body during scanning PBT and investigated their impact on the thermal neutron fluence distribution using MCSs. The accuracy of the MCS beam model was verified by comparing the calculated thermal neutron fluence with measurements obtained through the activation \begin{table}
\begin{tabular}{c c c} \hline \hline
**Target Center Depth [cm]** & \
\begin{tabular}{c} **Number of Proton-Beam** \\ **Particles [n]** \\ \end{tabular} &
\begin{tabular}{c} **Average Thermal Neutron** \\ **Fluence Within the Target** \\ **[1/cm\({}^{2}\)/Gy]** \\ \end{tabular} \\ \hline
5.5 & 3.39 \(\times\) 10\({}^{11}\) & 2.98 \(\times\) 10\({}^{7}\) \\
10.5 & 3.45 \(\times\) 10\({}^{11}\) & 5.23 \(\times\) 10\({}^{7}\) \\
15.5 & 3.86 \(\times\) 10\({}^{11}\) & 7.14 \(\times\) 10\({}^{7}\) \\
20.5 & 4.35 \(\times\) 10\({}^{11}\) & 8.70 \(\times\) 10\({}^{7}\) \\
25.5 & 4.91 \(\times\) 10\({}^{11}\) & 1.01 \(\times\) 10\({}^{8}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of protons and average thermal neutron fluence at varying depths under the same dose. Figure 5: MCS results for different target depths. Target center depths of 5.5, 10.5, 15.5, 20.5, and 25.5 cm are shown in green, orange, yellow, cyan, and red, respectively. Solid lines are thermal neutron fluences, and dotted lines are SOBPs produced by proton beams. analysis of a gold wire. The results showed good agreement, confirming that our beam model was generally accurate. Based on the MCS results, the internal thermal neutron distribution generated by PBT was found to be isotropic around the proton-beam axis, and it was evident that the fluence was highly dependent on the proton-beam energy and the number of irradiated protons. Notably, the thermal neutron fluence varied in intensity with depth, peaking not within the treatment target volume but at a location proximal to the source, outside of the target volume. This result is the divergence between the proton-beam Bragg peak and the peak of the thermal neutron fluence increasing as the proton-beam energy level rises. This effect was particularly pronounced in the low-energy neutron region below 1.0 eV; these findings highlight the importance of considering unexpected BNCRs due to thermal neutron peaks occurring outside the target volume. The 3.54% discrepancy observed between the thermal neutron fluence measured using gold wire activation analysis and the fluence obtained from MCS results may be attributed to the simplifications in modeling the materials within the beam nozzle. Other possible causes include the nuclear reaction cross-sections used in MCSs, the suppression of secondary particle transport other than protons, and the definition of the thermal neutron energy range. However, since this discrepancy was sufficiently small compared to the typical uncertainty range in neutron measurements, we concluded that our MCS results were within the acceptable range. Previous studies have investigated thermal neutron generation and distribution in PBT. [PERSON] et al. emphasized that most secondary neutrons produced during PBT were thermal neutrons, consistent with our PHITS simulation analysis [9]. These results validate our assumption that the measured neutron fluence primarily comprises thermal neutrons. [PERSON] et al. observed that heavy ions produced forward-directed and knockout neutrons with significant angular dependence, whereas the neutron dose from proton radiation therapy showed minimal angular dependence [24]. These findings suggest that the neutrons generated by PBT are produced isotopically, and our study confirmed these findings.