## 4 Radiation-Induced Bystander and Abscopal Effects  
### Studies of RIBE and RIAE and Their Mediators  
Cancer RT is based on the dogma that radiation kills targeted cells. This dogma is challenged by the radiation-induced bystander effect (RIBE), in which unirradiated (naive) neighboring cells can also be damaged after receiving signals from irradiated populations, whether through direct cell-to-cell contact or exposure to conditioned medium (Figure 3) [39; 40; 41; 42]. RIBE is now a well-established consequence of ionizing radiation and manifests as increased genomic abnormalities and loss of viability of 'naive' cells. [PERSON] and [PERSON] [43] published the first modern report on RIBE. Their data showed that alpha particles directed at 1% of a given cell group induced sister chromatid exchanges in 30% of that population. The authors supported their findings with statistical data showing that the proportion of cells damaged by a single alpha particle was less than the number of cells that exhibited sister chromatid exchanges. Later it was discovered that the targeted cells release damage-inducing substances into the medium, and medium conditioned by irradiated cultures was shown to induce various types of genotoxic events in unirradiated cultures [40]. Researchers in this field agree that the induction of RIBE is mediated by signals transmitted from irradiated cells to non-irradiated cells [44; 45]. Bystander effects are known to be communicated in two ways, both discovered around the same time by different researchers. The first is by gap-junction intercellular communication [42], and the second is extracellular soluble factors [41]. These mechanisms were the only known communication pathways for several years until it was recently discovered that UV photons from irradiated cells can also trigger bystander responses [46; 47]. Affected bystander cells exhibit increased levels of micronucle, apoptosis, mutations, altered DNA damage and repair, senescence arrest [40; 48], and increased levels of phosphorylated histone H2 AX (\(\gamma\)-H2 AX) [49], a marker of DNA double-strand breaks (DSBs) [50; 51; 52]. In contrast to DSB dynamics in irradiated cells, where maximal \(\gamma\)-H2 AX signal is observed 30 min after irradiation, foci formation is delayed in bystander cells. In 3D tissue models irradiated with microbeams, the maximal number of \(\gamma\)-H2 AX foci was observed 12-48 h after irradiation and gradually decreased over 7 days [51]. Mediators of RIBE include several inflammatory cytokines that were found at elevated levels in the medium conditioned by irradiated cells [53; 54]. The cytokine TGF-\(\beta\), when added to cell cultures, induced increased DSB levels similar to those induced by the conditioned medium [55]. IL-8 was found to be associated with an increase in ROS [56], which itself plays a role in RIBE [57; 58; 59]. In addition to ROS and NAD (P)H oxidase, TNF-\(\alpha\) was also found to be involved in the bystander signaling process [60]. Nitric oxide was found to play a role in the bystander signaling process when cell proliferation was induced in neoplastic human salivary gland cells [61; 62]. Increased radioresistance was observed in p53 wild-type glioblastoma cells [63] after exposure to nitric oxide-containing conditioned media. COX2 [64] and serotonin [65] have been implicated in the process of bystander signaling, with the latter confirmed by several publications [66; 67; 68; 69]. Recent research has shown that long-noncoding RNAs contained in exosomes are bystander candidates [70; 71; 72], while UV photons are the only physical factor shown to induce bystander effects [46; 47]. Although RIBE are usually considered detrimental to the cell, there are also reports of protective bystander effects in vitro, such as (i) bystander signaling leading to a reduction in neoplastic transformation [73] and (ii) evidence that bystander effects trigger adaptive responses [74; 75]. In fact, for decades, radiation oncologists have reported reactions in normal, unirradiated tissues after RT.