Table 2.

DNA double strand break (DSB) repair pathway choice following proton beam therapy (PBT) versus photon irradiation.

Cell line	Irradiations	Outcome	Ref
Wild-type, HR- and NHEJ-deficient Chinese hamster ovary cell lines	200 MeV protons and 137Cs γ-rays	NHEJ is the major pathway for both photons and low-LET protons	[43]
Wild-type and NHEJ-deficient Chinese hamster ovary cell lines	14.4 MeV plateau protons and 667 keV 137Cs γ-rays	NHEJ is the major pathway for both photons and low-LET protons	[44]
Wild-type, HR-, and NHEJ-deficient Chinese hamster ovary cell lines	138 MeV protons and 200-kV X-rays	Dependence on HR following protons	[36]
A549 lung cancer; glioblastoma cells	138 MeV protons and 200 kV X-rays	Dependence on HR following protons	[45]
HeLa	21 MeV protons	Higher proportion of cells undergoing HR following protons	[46]
Non-small-cell lung cancer cells	235 MeV protons and 250 kV X-rays	HR only partly required following protons	[47]

LET—linear energy transfer; HR—homologous recombination; NHEJ—non-homologous end-joining.