Table 1.

Summary of the radiobiology of proton radiation compared to photon radiation.

Biological Process	Photon Irradiation	Proton Irradiation
Relative biological effectiveness		General RBE of 1.1 is assumed in planning Biological data reveal varying RBE
DNA damage response	NHEJ is main repair pathway	Induction of clustered DNA damage leading to larger γH2AX foci and slower repair kinetics HR becomes more important for repair
Cell cycle distribution	G2/M arrest	G2/M arrest can be more extensive and prolonged
ROS		Induction of more ROS
Cell death	Activation of prosurvival response	Induction of more cell death Activation of proapoptotic response
Radioresistance	Hypoxia: increased expression of HIF-1α CSCs are less sensitive Activation of prosurvival pathways	Hypoxia: decreased expression of HIF-1α CSCs are more sensitive Decreased activation of prosurvival pathways