## 3 Results
### Increase in Cancer Cell Death Following Proton Irradiation in the Presence of BSH  
Exposure of the two cancer cell lines DU145 and PANC-1 at the newly designed beamline with 685-keV monoenergetic protons (calculated incident LET \(\approx\) 30 keV/um) \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**CR-39**} & **Irradiation** & \multirow{2}{*}{**Counts**} & **Fluence** & **Fluence\({}_{\text{expected}}\)** & **Dose** & **Dose\({}_{\text{expected}}\)** \\  & **Time (s)** & & **(cm\({}^{-2}\))** & **(cm\({}^{-2}\))** & **(mGy)** & **(mGy)** \\ \hline \#1 & \(15\pm 1\) & \(36\pm 6\) & (4.20 \(\pm\) 0.11)\(\cdot 10^{4}\) & (4.33 \(\pm\) 0.09)\(\cdot 10^{4}\) & 1.77 \(\pm\) 0.05 & 1.82 \(\pm\) 0.04 \\ \#2 & \(30\pm 1\) & \(71\pm 8\) & (8.4 \(\pm\) 0.2)\(\cdot 10^{4}\) & (8.6 \(\pm\) 0.2)\(\cdot 10^{4}\) & 3.53 \(\pm\) 0.08 & 3.62 \(\pm\) 0.08 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Values of measured and expected counts, fluence and doses for the irradiated SSTDs CR-39; all values are reported as MEAN \(\pm\) SEM. Figure 5: Counts obtained for two SSTDs CR39 (**a**) #1 and (**b**) #2 moving along the two main diagonals; the mean value with standard deviation is reported as solid and dashed blue line. in the presence of the \({}^{11}\)B carrier BSH resulted in a significant reduction in clonogenic survival compared to cultures irradiated in the absence of BSH. Survival dose-response curves for the two cell lines are shown in Figures 6 and 7; fitting parameters and values for Dose-Modifying Factor at 10% survival level (DMF\({}_{10}\)) are reported in Tables 2 and 3. Figure 6: Clonogenic cell death induced in DU145 prostate cancer cells by the monoenergetic proton beam at the newly implemented beamline: Dose-responses obtained after irradiation in the presence (black circles) or the absence (white circles) of the \({}^{11}\)B carrier BSH are shown. Data are averaged from three separate experiments. Error bars represent the SEM. Figure 7: Clonogenic cell death induced in PANC-1 pancreatic cancer cells by the monoenergetic proton beam at the newly implemented beamline: Dose-responses obtained for irradiation in the presence (black circles) or the absence (white circles) of the \({}^{11}\)B carrier BSH are shown. Data represent the averaged mean from two independent experiments. Error bars represent the SEM. In line with previously obtained results at other facilities [16,17], in our hands BSH did not exert any cytotoxic effect per se: the measured PE, which expresses the proliferative ability of unirradiated cells, was the same for samples treated or untreated with BSH, yielding values of around 0.64 and 0.46 for PANC-1 and DU145 cell lines, respectively. All dose-response curves obtained from proton-irradiated cell cultures exhibit loss of cancer cell proliferative competence that is linearly dependent on the dose, as shown by the absence of a significant \(\beta\) value (Tables 2 and 3) when experimental SFs data were fitted to the linear-quadratic Equation (4). For non-BSH treated cells, lack of a shouldered dose response, and hence of an appreciable portion of radiation-induced sublethal damage at low dose, reflects the relatively high LET of the incident proton beam. The presence of the BSH, on the other hand, results in a significant increase in the ability of proton irradiation to cause cancer cell death as clearly shown by even steeper clonogenic dose-response curves obtained for the two examined cell lines compared to those for cells exposed to protons in the absence of BSH (Figures 6 and 7). Specifically, the DMF\({}_{10}\), which quantifies BSH-associated exacerbation of proton-induced cell death, was found to be about 1.85 for the DU145 cell line and 1.56 for PANC-1 cells (Tables 2 and 3), indicating that the pB reaction leads to an enhancement of proton biological effectiveness by over 80% and 50%, respectively.