Shokohzaman Soleymanifard - Ph.D Student in Medical Physics Research Center, Bu-Ali Research Institute, Mashhad University of Medical Sciences, Mashhad, Iran
Mohammad Taghi Bahreyni Toossi - 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.

Radiation Induced Bystander Effect, Non-Targeted Effects of Radiation, Linear No Threshold Relationship (LNT), Radiotherapy