  Protontherapy is hadrontherapy fastest-growing modality and a pillar in the
battle against cancer. Hadrontherapy superiority lies in its inverted
depth-dose profile, hence tumour-confined irradiation. Protons, however, lack
distinct radiobiological advantages over photons or electrons. Higher LET
(Linear Energy Transfer) $^{12}$C ions can overcome cancer radioresistance: DNA
lesion complexity increases with LET, resulting in efficient cell killing, i.e.
higher Relative Biological Effectiveness (RBE). However, economic and
radiobiological issues hamper $^{12}$C-ion clinical amenability. Thus,
enhancing proton RBE is desirable. To this end, we exploited the p + $^{11}$B
$\rightarrow$3$\alpha$ reaction to generate high-LET alpha particles with a
clinical proton beam. To maximize the reaction rate, we used sodium borocaptate
(BSH) with natural boron content. Boron-Neutron Capture Therapy (BNCT) uses
$^{10}$B-enriched BSH for neutron irradiation-triggered alpha-particles. We
recorded significantly increased cellular lethality and chromosome aberration
complexity. A strategy combining protontherapy ballistic precision with the
higher RBE promised by BNCT and $^{12}$C-ion therapy is thus demonstrated.
