Table 1. A comparison between photon-based techniques and proton-based techniques.

	Photon-based techniques	Particle-based techniques
Technique	Involves firing beams multiple times from different angles	Uses particle accelerators to form a single beam of high-energy protons [44]
Mechanism	Radiation delivered from an external source; dose decreases for deeper tissues	Distribution follows a Bragg-peak: Low doses delivered on entering target tissues with a steep maximum at a specific energy-dependent depth [44]
Delivery methods	2DCRT, 3DCRT, IMRT	Via heavy particles; Involves Passive scattering, uniform scanning, active scanning [46]
Comparison	Poorer OS	Better OS [54]
	Less localized radiation exposure® lower doses delivered, more collateral damage	More localized particle exposure® higher doses delivered, less collateral damage [45]
	Poorer dose distribution	Better dose distribution due to narrow Bragg-peak range [44]
	Poorer energy distribution	Better energy distribution (via higher linear-energy transfer)
	The exponential decrease in radiation as depth increase	Uniform coverage at all depths
	DNA damage may be reparable	Induce irreparable damage to DNA
	More dependent on oxygen availability ® hypoxic tumors show poorer response	Less dependent on the oxygen availability of tumor tissue® hypoxic tumors show better response [45]

2DCRT: Two-dimensional conventional radiotherapy; 3DCRT: Three-dimensional conformal radiotherapy; IMRT: Intensity-modulated radiotherapy; OS: Overall survival; DNA: Deoxyribonucleic acid