Document Type: Review Article
Ph.D Student in Medical Physics Research Center, Bu-Ali Research Institute, Mashhad University of Medical Sciences, Mashhad, Iran
Professor, Medical Physics Research Center, Bu-Ali Research Institute, Mashhad University of Medical Sciences, Mashhad, Iran
Introduction: Radiation effects observed in cells that are not irradiated are known as non-targeted effects.
Radiation induced bystander effect (RIBE) as a kind of non-targeted effect has been introduced in recent
years. RIBE occurs in unexposed cells which are related to adjacent or distant irradiated cells. RIBE
contradict with "target theory" which necessitates radiation traversal through the nucleus for affecting cells.
Methods: To understand this phenomenon, some investigations and review articles which deal with this
subject were studied. Most investigators have applied in vitro experimental methods to investigate RIBE in
animal or human cells, but in vivo experiments are also increased in recent years.
Results: Both low and high LET radiation cause RIBE. Cell responses to RIBE are including: cell death,
chromosomal damage, mutagenesis, neoplastic transformation, genomic instability, cell cycle delay,
modification of gene expression and radiosensitivity alteration. At low doses that only some cells are
irradiated this phenomenon is significant. However RIBE exists at high doses, due to predomination of direct
radiation effects it is less considered. There is link between RIBE and other non-targeted effects such as
adaption and genomic instability.
Conclusion: At low doses, there is no linear relationship between outcome and the number of cells hit by
radiation. Thus this will affect the basic principle of defining dose limits which have been initiated by linear
no threshold curve. There is a scope to exploit RIBE in radiotherapy, and also it is important to avoid
undesirable bystander effects in normal tissues.