Table 1.

Comparisons of double-strand breaks (DSBs) induced by proton beam therapy (PBT) versus photon irradiation.

Cell Line	Method(s)	Proton Energy	Photon Energy	Observation (Proton vs. Photon)	Ref
ONS76 medulloblastoma; MOLT4 leukemia cells	γH2AX foci by immunofluorescence	200 MeV	10 MV X-rays	~1.2–1.6-fold increase in DSB foci and ~1.2–1.5-fold larger in size 30–180 min post-irradiation	[32]
HeLa; SQ20B HNSCC cells	Pulse-field gel electrophoresis	76 MeV, 201 MeV	622 keV 137Cs γ-rays	~1.2-fold increase in DSBs. No differences between PBT energies, nor along the SOBP	[9]
IN528 and T4213 glioblastoma stem-like cells	Alkaline and neutral comet assay	N.S.	320 kV X-rays	~1.2–1.6-fold higher numbers of DSBs at 20–48 h post-irradiation	[33]
TrC1 prostate cancer cells; murine embryonic fibroblasts	Histone γH2AX and 53BP1 foci by immunofluorescence	187 MeV entrance dose	320 kV X-rays	Similar numbers of DSBs at 0.5–24 h post-irradiation	[34]
AG01522 skin fibroblasts	53BP1 foci by immunofluorescence	60 MeV entrance dose	225 kV X-rays	Similar numbers of DSBs at 0.5–24 h post-irradiation	[35]
Wild-type, HR-, and NHEJ-deficient Chinese hamster ovary cell lines	Histone γH2AX foci by immunofluorescence	138 MeV	200 kV X-rays	Similar initial induction of DSBs	[36]
HeLa; UMSCC74A and UMSCC6 HNSCC cells	Neutral comet assay	58 MeV entrance dose; 11 MeV distal edge	100 kV X-rays	No difference in DSB repair kinetics	[37]

N.S. refers to not specified. HR—homologous recombination; NHEJ—non-homologous end-joining; SOBP—spread-out Bragg peak.