Studies on single or hybrid nanoparticle (NP) formulations.

NP type	Model	Beam sources	Important outcomes	Ref.
Single metal and metal oxide nanoparticles
TiO2 (un-/doped)	PRESAGE phantom	kV Photons	• X-ray energy differential therapeutic effects observed in PRESAGE	94 and 182–184
	In vitro	MV photons	• NP radioenhancement effect during MV X-ray irradiation explained by increased ROS generation
	In vivo		• Rare earth dopants (e.g. Sm, Gd, Nd, Eu, Er, Tb) increased X-ray induced ROS, and apoptosis markers
(PAA-) TiO2/H2O2	In vitro	kV Photons	• NPs increased intracellular H2O2 concentrations by gradual release thereof leading to increased radiotherapy efficiency	163, 185 and 186
	In vivo		• Increased ˙OH radical and H2O2 concentration, DNA damage and apoptosis during X-ray irradiation
MnFe2O4	In vitro	kV Photons	• Oxygen delivery via catalytic H2O2 decomposition	144 and 187
	In vivo (normoxia and hypoxia)	MV photons	• Hypoxic conditions were alleviated in vitro and in vivo
			• Decreased HIF-1a levels, and increased apoptosis and DNA damage under irradiation during hypoxic conditions
			• Immune-modulating effect: suppression of PD-L1 expression and increased infiltration of T cells even after irradiation
			• PEGylated NPs showed good cytocompatibility, passive tumor accumulation, O2 generation via H2O2 decomposition, GSH consumption via glutathione-peroxidase-like activity, enhanced ROS and double-strand break levels in hypoxic conditions, and hypoxia attenuation and good radioenhancement efficiency in vivo.
SPIONS (γFe2O3, Fe3O4)	In vitro	kV Photons	• Excellent biocompatibility	81
			• Increased ROS production via Fenton and Haber–Weiss reactions from released iron ions and catalytically active nanoparticle surface
			• X-ray irradiation led to additional oxidative stress from increased catalytically active nanoparticle surfaces
CuO	In vitro	MV photons	• Increased ROS levels with X-ray treatment and CuO NPs	83
	In vivo		• Increased radiosensitivity by the NP-induced modulation of the cell cycle distribution towards increased G2/M phase
			• Increased level of self-destructive autophagy observed with the combination of CuO NPs and X-rays
ZnO	In vitro	kV Photons	• Radioenhancing effects explained by increased apoptosis and DNA damage; oxidative stress as possible driver identified	188–191
	In vivo	MV photons	• Discussed cytotoxic effects of ZnO: dissolution of Zn++ in acidic conditions; e−h+ pair production even in the dark leading to surface ROS production
			• Discussed genotoxic effects of ZnO: oxidative stress
			• Gd-doped ZnO NPs increased cells in G1 phase and decreased DNA repair efficiency
			• ZnO-CaffeicAcid NPs showed radioenhancement via ROS/oxidative stress generation, DNA damage, DNA repair, and mitochondrial dysfunction, suppression of cell cycle checkpoint machinery and cell death promotion (via apoptosis, necrosis, and disregulations of gene and protein expressions)
Y2O3	In vitro	kV Photons	• Increased ROS and DNA double-strand breaks with NPs alone or in combination with X-rays	103
			• NPs affected irradiation induced DNA damage and repair response
			• Synergistic effects of NP treatment and irradiation proposed by clonogenic assay
Ag	In vitro	MV photons	• Increased apoptosis levels after irradiation with Ag NPs	192–197
	In vivo		• Antiproliferative activity in combination with radiotherapy
			• Decrease of mitochondria membrane potential, promotion of apoptosis and enhanced destructive autophagy under irradiation in hypoxic conditions with Ag NPs
			• Increased ROS and protective autophagy during irradiation with Ag NPs
			• Radiosensitization might be Ag+ cation release dependent
Gd chelate (AGuIX)	In vitro	kV Photons,	• Good safety profile	2, 178 and 198–202
	In vivo	In human	• Rapid and safe renal elimination
		MV photons	• Preferential accumulation in tumor due to EPR effect after intravenous injection
		MeV hadrons	• Possible emission of low-energy photoelectrons and Auger electrons leading to higher ROS
			• Dose enhancement higher for kV than MV photon, and C6+ than He2+ ion irradiation
			• Photon radiation-induced ROS and DNA double-strand break increase and DNA repair reduction observed
			• Ion irradiation increased DNA damage mediated by ROS
Gd2O3	In vitro	kV Photons	• Compared to a solution of separate Gd-atom species, Gd2O3 NPs showed higher ROS generation, suggesting a Gd–Gd interatomic de-excitation-driven nanoradiator effect	203 and 204
		MeV protons	• Increased hydroxyl radical and ROS production, increased DNA double-strand breaks and cell cycle arrest at G2/M phase
		C6+ ions	• Increased apoptosis and cytostatic autophagy identified as radiosensitization mechanism
HfO2 (NBTXR3)	In vitro	kV Photons	• NBTXR3 has a good safety profile and improves radiotherapy efficiency via physical mechanisms	4, 24, 156–159 and 205–208
	In vivo	In human	• Increased necrosis, DNA double-strand breaks, micronuclei formation and an activation of the cGAS-STING pathway detected after irradiation
		MV photons	• In combination with radiotherapy: enhancement of early apoptosis, early necrosis and late apoptosis/necrosis; abscopal effects driven by an increased CD8+ cell infiltration
			• Modulation of the immunopeptidome observed in vivo leading to an anti-tumor immune response
WO3−x	In vitro	kV Photons	• Increased DNA double-strand breaks and apoptosis after irradiation with NPs	209 and 210
	In vivo		• Remarkable synergistic effect of radiotherapy and phototherapy (PTT/PDT)
Pt	In vitro	kV Photons	• ROS scavenging capabilities of Pt NPs during irradiation detected, which might counteract nanoparticle radioenhancement	70 and 211–213
		MV photons	• Pt NPs amplified radiation therapy by confined production of ROS in nano-volumes around nanoparticles
		C6+ ions	• Amplified DNA damage detected during hadron therapy
Au	In vitro	kV Photons,	• Physical, chemical and biological mechanisms described	1, 45, 104, 109, 121, 124, 176 and 214–220
	In vivo	MV photons,	• Less radioenhancement observed at MV compared to kV irradiations
		Protons	• Observed enhancement effects at MV energies cannot be explained by physical effects
			• Enhancement effects are cell-line specific
			• Cell cycle arrest in G2/M phase has been observed for some cell lines
			• Cytoplasmic damage can drive DNA damage; mitochondrial function identified as possible driver
			• Increased DNA dmage and/or mitochondrial and/or ER stress with and/or without irradiation lead to increased apoptosis and/or necrosis
			• Au NPs have radiosensitization effect via various biological mechanisms, such as weakening the detoxification system
Bi2O3	In vitro	kV Photons	• In vitro enhancement ratio for kV irradiation higher compared to MV irradiation	62, 66 and 221
		MV photons	• Monte Carlo studies predicted dose enhancement for kV irradiation but not for MV irradiation
			• Radiocatalytic surface of NPs hypothesized to lead to water splitting
BiFeO3	Gel dosimetry	kV Photons	• Promising results in radiotherapy amplification, magnetic hyperthermia and imaging	222
	In vitro
BiGdO3	MAGIC gel dosimeter	kV Photons	• Multifunctional pegylated nanoparticles showed radiation enhancement in gel dosimetry, in vitro and in vivo.	223
	In vitro
	In vivo
Bi2WO6	In vitro	kV Photons	• Bi2WO6 generated radiocatalytic ROS in an acellular system, with ˙OH as main oxidative species identified	224
	In vivo	MV photons	• Radiosensitization in vitro was found in a clonogenic assay together with increased ROS and DNA double strand break levels
			• Efficient radiotherapy enhancement shown in vivo without toxicity effects in the time span of 30 days.
Hybrid nanoparticle systems
Au–TiO2 hybrid	In vitro	kV Photons	• Production of large amount of ROS during irradiation	225
	In vivo		• Synergistic X-ray therapeutic effect of Au and TiO2 due to their interfacial contact
Au@MnO2-PEG	In vitro	kV Photons	• O2 generation from H2O2 decomposition via MnO2 shell and relieve of cellular hypoxia	226
	In vivo		• H2O2 decomposition led to Mn2+ release and enhanced T1-weighted MR imaging
			• Compared to the individual components, Au@MnO2 core@shell hybrid NPs led to synergistic radioenhancement effects in vitro and in vivo and to more increased double-strand breaks and apoptosis levels
Au@Pt nano-dendrites	In vitro	Photons (energy N/A)	• Integration of CT imaging, PTT & RT with synergistic therapeutic effects	227
Au:Pt-PEG	Plasmid DNA	kV Photons	• NPs enhanced the MV X-ray induced double-strand breaks in plasmid DNA mainly by influencing the chemical stage that takes place after the physical interaction	228 and 229
	In vitro	MV photons	• NP radioenhancing effect with kV X-rays observed in vitro and in vivo by enhancing double-strand breaks, apoptosis and relieving of hypoxia via H2O2 decomposition
	In vivo
WS2:Gd3+-PEG 2D-nanoflakes	In vitro	Photons (energy N/A)	• Triple-modal imaging: CT/MR/photo acoustic	230
	In vivo		• Synergistic PTT/RT therapeutic effects observed
			• X-ray enhanced DNA double-strand breaks observed
Gd2(WO4)3:10%Tb@PEG/MC540	In vitro	kV Photons	• X-ray induced generation of 1O2 observed from photo excitation of surface coupled merocyanine (MC) 540	231
	In vivo		• Synergistic radioenhancement and PDT effects observed in vitro and in vivo
			• Dual-modal imaging properties (CT/MR)
Fe3O4@Ag	In vitro	MV photons	• Synergistic radioenhancement effects of the Fe3O4 core and Ag shell as compared to the individual components alone	232
			• Radiosensitivity enhancement through decrease of the cytoprotective autophagy at an early stage, followed by an increase of calcium-dependent apoptosis at a later stage
Gd2O3/BSA@MoS2-HA	In vitro	kV Photons	• Enhanced DNA double-strand breaks in combination with X-rays in vitro	233
	In vivo		• In vivo tumor growth inhibition in combination with X-rays
			• Best therapeutic effects in combination with X-ray and PTT treatments
			• In vivo multimodal imaging properties (MSOT/CT/MR)
BiNPs@SiO2@BamCS/PCM	In vitro	kV Photons	• Acellular detection of elevated ROS (˙OH, 1O2 and ˙O2−) under X-ray irradiation	234
	In vivo		• Elevated cell death, ROS and DNA double-strand breaks detected in combination with X-rays in vitro
			• Depleted GSH levels due to hyperthermia-released and H2O2-activated proalkylation agent BamCS
			• Excessive ROS and irreversible depletion of endocellular GSH led to cell death via mitochondria-mediated apoptosis pathway
			• Tumor growth inhibition in combination with X-rays
			• Synergistic PTT/RT effects observed
Nanoscale metal–organic frameworks (nMOFs)
MnTCPP–Hf–FA	In vitro	kV Photons	• O2 generation via catalytic H2O2 decomposition	235
	In vivo		• Decreased HIF-1a expression suggested hypoxia alleviation due to intracellular H2O2 decomposition and O2 generation
			• Increased ROS levels and DNA double-strand breaks after irradiation even in hypoxic conditions
UiO-66-NH2(Hf)	In vitro	Photons (energy N/A)	• Radioenhancement due to increased ROS levels, DNA double-strand breaks and cell apoptosis	236
	In vivo
Hf6-DBB-Ru	In vitro	kV Photons	• MOFs accumulated in mitochondria through cationic nature of Ru-based photosensitizer	237
	In vivo		• Irradiation induced ˙OH radical generation from Hf6 units and 1O2 generation from the DBB-Ru photosensitizer
			• Clonogenic radioenhancement effects observed, along with increased double-strand break, 1O2, lipid peroxidation (COX-2 upregulation) and apoptosis/necrosis levels after irradiation
			• Irradiation induced depolarization of the mitochondrial membrane leading to cytochrome c release and caspase-3 activation (apoptosis pathway)
			• Effective in vivo tumor regression after intratumoral or intravenous nMOF administration and low dose (6 or 8 Gy) X-ray therapy, with increased levels of necrosis and apoptosis.
W18@Hf12-DBB-Ir	In vitro	kV Photons	• Hierarchical assembly of Hf12, DBB-Ir, and W18 generated 3 distinct ROSs: ˙OH, 1O2 and ˙O2−, respectively	238
	In vivo		• Irradiation in vitro lead to enhanced ROS, DNA double-strand breaks and apoptosis/necrosis
			• Superb anticancer efficacy (>99% tumor growth inhibition) on two in vivo tumor models with irradiation