## 2 Materials and Methods
_ROS, superoxide and hydroxyl radical assays._ The MCF-7, MCF-10A and A459 cells were seeded in 96-well plates at a cell per well density of 2-3 \(\times\) 10\({}^{4}\) and were incubated overnight. The cell medium was substituted by one containing the NPs at a concentration of 10 ug/mL. After 24 h incubation, the NP-enriched medium was aspirated off. According to the different assay protocols the respective -dye solution was filled in. Subsequently, half of the 96-well plates were treated with X-rays at a single dose of 1 Gy. _ROS assay._ 2',7'-dichlor fluorescein diacetate (DCFH-DA) was dissolved in DMSO to obtain a 0.01 M stock solution which was diluted with DMEM to 100 uM. The cells were incubated in this solution for 30 min. After cellular uptake, the acetate groups of DCFH-DA were cleaved off so that the DCFH molecules remained inside the cells. Afterwards, the wells were washed with PBS and 100 uL PBS were added in each well. The cells were exposed to X-radiation or remained non-irradiated. ROS oxidize intracellular DCFH to fluorescent DCF. Upon excitation at 480 nm the fluorescence emission of the DCF dye was detected at 528 nm. _Superoxide assay._ The relative intracellular concentration of superoxide radicals (O\({}_{2}\)\({}^{\bullet}\)) was determined by means of the MitoSOX\({}^{\text{TM}}\) Red mitochondrial superoxide indicator, following the manufacturer's protocol. The dye was dissolved in DMSO at a concentration of 2.5 mM and diluted to 5 uM in [PERSON]'s balanced salt solution (HBSS). After 10 min incubation followed by X-ray irradiation, the cells were washed and 100 uL of HBSS were added. The fluorescence of MitoSOX\({}^{\text{TM}}\) Red was excited at 530 nm, and the fluorescence intensity was detected at 590 nm. _Hydroxyl radical assay._ 3'-(p-hydroxyphenyl)-fluorescein (HPF) selectively reacts with HO\({}^{\bullet}\). The 5 mM HPF solution was diluted to 5 uM in Krebs-Ringer buffer. After removal of the cell culture medium, 100 uL of HPF solution is added, and the cells were incubated therein for 30 min. Then the cells were washed and exposed to X-radiation. The fluorescence emission of intracellular HPF was excited at 490 nm and detected at 515 nm. _Clonogenic cell survival assay._ The MCF-7, MCF-10A and A459 cells were grown in 6-well plates and were incubated in culture medium enriched with the respective kind of NPs (10 ug/mL) overnight. After X-ray irradiation at single dosages of 1, 2 or 3 Gy, the cells were detached, seeded and grown in 6-well plates for 2 weeks to form colonies. The cell colonies were fixed and stained with a mixture of 0.5% (\(w/v\)) crystal violet in 50/50 methanol/water for 30 min. Colonies containing more than 50 cells were counted. The colony number was used to calculate the surviving fraction (SF). The survival curves were fitted to a linear quadratic function (ln SF = \(-\)(\(\alpha\)D + \(\beta\)D\({}^{2}\))). The X-ray enhancing effect was assessed by determining the dose modifying factor (DMF) from the X-radiation survival curves upon calculating the ratio of radiation doses at the 50% survival level (NP-treated radiation dose divided by the control radiation dose). _Statistical Analysis._ Data are presented as arithmetic mean values \(\pm\) standard error (SE). Statistical analysis was performed using the analysis of variance (ANOVA) with post hoc Bonferroni correction for multiple comparisons. A value of \(p\) < 0.05 was considered to be statistically significant.