## 4 Radiation-Induced Bystander and Abscopal Effects
### Studies of RIBE and RIAE and Their Mediators
These \"out-of-field\", or abscopal effects have been described as \"an action at a distance from the irradiated volume but within the same organism\" [76]. The discovery of the RIBE has prompted the description of radiation-induced abscopal effects (RIAE) as distant in vivo bystander effects, and their further investigation. Several studies reported in vivo RIAE in animal models. Using strategies that involve partial head or body 1 Gy X-ray irradiation, profound genetic and epigenetic changes were identified in shielded organs, such as skin and spleen [77; 78]. Others reportedDSBs, apoptosis, and tumor induction in shielded cerebellum of 3 Gy X-ray-irradiated radiosensitive Ptch1 heterozygous mice [79]. Strikingly, the results of the in vivo non-targeted effects can be transmitted to future generations. While genomic instability was known to be elevated in offspring of parents whose germ cells were directly irradiated prior to conception as part of a medical procedure [80], male rats given cranial irradiation (20 Gy applied as two doses of 10 Gy on two consequent days) had an accumulation of unrepaired DNA damage in their sperm cells [81]. This effect was manifested as epigenetic dysregulation in the unexposed progeny conceived after paternal exposure. RIBE and RIAE are potential contributors to the well-documented clinical phenomenon of secondary cancers, a major concern in cancer RT, affecting more than 1% of patients [40; 82]. Clinically, patients often suffer from systemic side-effects, such as fatigue, diarrhea, and weight loss during local RT. Additionally, RIAE can cause damage to a distant, non-irradiated tumor [83]. The reported instances of spontaneous anti-tumor abscopal effects following local RT as a sole therapy are rare. In a review by [PERSON] et al. [84], 10 cases of abscopal effects in RT patients with a range of solid tumor types were reported, and in another review, 35 cases over 45 years were reported, out of millions of patients treated during this period [85]. The clinical reports of anti-tumor RIAE have covered a wide range of doses given in either single or multiple fractions; however, they are more prominent following larger doses, supporting the use of hypofractionation [86]. Most published studies of RIAE in animals and patients describe biological effects observed in distant, non-irradiated normal or tumor tissues (Figure 4a,b). However, published data also describes the transmission of bystander signals from one irradiated animal to another non-irradiated animal (Figure 4c). The first observation of such inter-animal communication was made when non-irradiated rats and mice were placed in the same cage with irradiated animals for one to two weeks [87]. The researchers observed that the peripheral blood of the non-irradiated animals showed a marked decrease in leukocytes and a lower immune reactivity. A similar communication was also shown between fish [88]. In these experiments, unirradiated rainbow trout showed bystander effects after swimming with trout that had been exposed to radiation. This was later confirmed in zebrafish [89] and with proteomic analysis of non-irradiated, bystander rainbow trout [90]. Bystander responses have also been found to persist throughout the lifespan of fish after they have been irradiated in the early life stages [91]. Below, we review the studies conducted at the AS and ESRF facilities to explore various types of non-targeted effects of ionizing radiation and summarize the biological mechanisms discovered.