As stated above, there is increasing evidence that cells that are not directly exposed to radiation, but are the progeny of cells that were irradiated many cell divisions previously, may express a high level of gene mutations, cell lethality and chromosome aberration. Collectively, this phenomenon has become known as genomic instability [53
]. Genomic instability and the bystander effect have one thing in common, namely that both involve non-targeted effects in non-irradiated cells; in one case, being in the progeny of irradiated cells; and in the other, being in the close neighbors of irradiated cells. The observations that 1) several cell cycle checkpoint genes such as cyclin B1 and RAD51 have been shown to be over-expressed in radiation induced bystander cells [54
]; and 2) that DNA repair deficient cells have a higher bystander chromosomal aberration and mutagenic response [55
] provide a possible link between genomic instability and bystander response. This link is further strengthened by a variety of in vivo
and in vitro
]. Since genomic instability is considered a predisposition factor for carcinogenesis, it has been postulated that radiation induced non-targeted/bystander effects may promote secondary cancer induction in radiotherapy patients [57
From animal studies with X-rays, there is evidence that irradiation of partial of the lung in mice can induce a non-targeted response in the non-irradiated part of the lung through the induction of inflammatory cytokines [58
]. With irradiation of the lower region of the lung, the frequency of micronuclei increased in the out-of-field upper lung relative to the sham-irradiated group. The induction of micronuclei in the non-targeted lung tissues was inhibited by superoxide dismutase (SOD) and L-NG
-Nitroarginine methyl ester (L-NAME), a non-specific inhibitor of nitric oxide synthase, which suggested that production of reactive oxygen species and nitric oxide resulted in indirect DNA damage and induced a bystander effect in the neighboring tissue [59
]. Furthermore, there is recent evidence that irradiation of the lower abdomen of mice with X-rays results in the induction of inflammatory response [61
] as well as mutations and COX-2 induction [62
] in out of field lung tissues.
Using the radiosensitive Patched-1+/−
) mouse model system that has a defect in radiation-induced activation of the ATR-Chk1 checkpoint signaling pathway [63
], Mancuso et al
reported induction of medulloblastoma in the non-irradiated brain tissues after partial irradiation of the lower half of the animal with a 3 Gy dose of X-rays [64
]. A significant increase in medulloblastoma (39%) occurred in the partial body irradiated heterozygous mice compared to the sham-treated group. The study also showed the induction of γH2AX, a marker of DSBs and apoptosis in bystander cerebellum. Although these short-term bystander responses can be detected in different mouse strains after similar treatment, the carcinogenesis in cerebellum was specific for the heterozygous animals and suggested that the endpoints are dependent on the genotype of animals.
Based on human serum analyses, there is clear evidence that plasma clastogenic factors are present many years after radiation exposure from the Japanese atomic bomb survivors, Chernobyl liquidators and from radiotherapy patients [65
]. To provide a better estimate of the frequency distribution of second primary tumor sites in relation to previous irradiation volumes, a cohort of 115 pediatric patients who developed such cancers were studies [69
]. shows the frequency of second tumors as a function of distance from the irradiation site. It can be estimated that ~22% of these secondarily derived tumors arise from a distance of at least 5 cm from the irradiated site and ~6% arise from a distance that is > 10 cm away. A peak second primary tumor frequency of ~31% was identified in volumes receiving less than 2.5 Gy and a total of 10~15 % of these tumors are estimated to arise in tissues receiving less than 0.5 Gy [69
]. Although these findings are suggestive, nonetheless, the data highlight the potential of second tumor development outside the treatment field and at much lower dose level.
Fig. 4 Frequency of second primary tumor among 115 pediatric patients among a cohort of 4581 treated with radiation for various primary solid tumors (reproduced with permission from ).