## 4 Discussion  
Despite not being yet routinely implemented in clinics worldwide, BNCT has been long advocated as a strategic radiotherapy (RT) modality, particularly against radiote-sistant tumors, due to its much greater relative biological effectiveness (RBE) compared to conventional RT (CRT) based on low-Linear Energy Transfer (LET) photons and/or electrons [10; 17; 46; 47; 48]. For this reason, we did not perform cellular irradiations in the  
Figure 6: Comparison of the degree of complexity measured in the complex exchanges scored between BNCT- and X-ray-irradiated samples as expressed in terms of the number of chromosomes involved per complex CA. Figure 7: The number of breaks giving rise to the complex exchanges detected by m-FISH in BNCT- and X-ray-irradiated samples confirms the much higher degree of DNA damage complexity elicited by the BNCT-associated \({}^{10}\)B(n,\(\alpha\))\({}^{7}\)Li reaction. absence of the \({}^{10}\)B carrier. Indeed, a boost in BNCT-related research and clinical application is occurring nowadays as the development of compact accelerator-based neutron sources paves the way for its in-hospital use, and hence for more advanced clinical trials [12; 49]. The potential superiority of BNCT over CRT stems from the radiobiological properties of the high-LET products of the \({}^{10}\)B(n,\(\alpha\))\({}^{7}\)Li reaction [13; 46]. In fact, while several studies have clearly demonstrated how effectively BNCT impairs the reproductive integrity of cancer cells, as reviewed in [7; 16], for example, scant data exist on the BNCT-specific biomolecular mechanisms presiding over such elevated RBE [24; 25; 50]. It is a long-standing notion that high-LET particles are likely to cause damage sites containing multiple lesions in close spatio-temporal vicinity [28; 51]. Such highly clustered lesions, on a nanometer scale, may encompass base damages, Single- and Double-Strand Breaks (SSBs and DSBs, respectively), and challenge the repair capacity of the cells, impacting the major known pathways for restitution of DSBs (the most cytotoxic lesions), i.e., homologous recombination (HR), the classical nonhomologous end-joining (c-NHEJ) and the backup nonhomologous end-joining (b-NHEJ). Specifically, clustered damage repair remains an open question, but recent findings seem to point to alternative NHEJ pathways to participate in the case of highly complex DSBs [26; 52] leading to a lower rate of faithful rejoining. This, in turn, is likely to lead to the formation of chromosome aberrations (CAs), with a greater occurrence of complex rearrangements compared to low-LET radiation reflecting the denser ionization pattern of energy deposition events along the tracks [32; 53] such as those of the particles emitted in the \({}^{10}\)B(n,\(\alpha\))\({}^{7}\)Li reaction. Hence, a causal link between the superiority of BNCT at cell killing and an increased occurrence of lethal aberrations compared to lower LET radiations is likely to exist [29], although this has never been convincingly demonstrated in radiobiological studies thus far. We therefore investigated the frequency of CAs in BPA-treated cells exposed to a thermal neutron beam by two well-established chromosome labeling techniques, i.e., WCP and m-FISH, and compared the observed frequencies of overall CAs and of complex-type aberrations with those measured following low-LET (X-rays) irradiation. Human mammary epithelial MCF-10A cells were used to avoid the confoundingly high baseline CA frequency from genomically unstable cancer cells. As expected, the overall aberration occurrence was found to increase with dose for both types of irradiations (X-rays and BNCT) as revealed by both techniques (Figure 2). However, BPA-treated samples exposed to BNCT exposure did consistently show a significantly higher frequency of all types of CAs scored by both techniques.