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Purpose:Radiation induced bystander effect(RIBE)depends on several parameters including total dose,dose rate,radiation quality,tissue type and volume of the irradiated tissue in radiotherapy.The present study was designed to assess uncertainties in bystander effects induced by high doses of ionizing radiation.Methods:In this study,QU-DB and MRC5 cell lines were studied.One hour following to irradiation,the medium transfer technique was applied to induce bystander effect in unirradiated cells.The three end-points examined in this study were MTT assay(to evaluate percentage of survival),colony assay(to evaluate survival fraction)and MN assay(to assess chromosome aberrations).Results:RIBE reduced number of MNs as well as survival fractions in doses higher than 4 Gy in lung tumoral cell line.Evaluation of chromosome aberrations by MNs counting and survival fraction by colony assay,we were assured that bystander effect had actually decreased at doses higher than 4 Gy.In other words high doses of ionizing radiation could produce a "negative feedback".Conclusions:Medium dilution technique was used to find out the unexplained cause of RIBE reduction when high doses of ionizing radiation are delivered.Interestingly,this observation indicated that the quantity of bystander signals produced in the 8%diluted medium(extracted from 6 Gy irradiated cells)did not show significant difference compared with the corresponding signals in the 6%diluted medium(extracted from 8 Gy irradiated cells).This observation is in favour of negative feedback theory.Based on our results the dose range by which tissue samples were irradiated in this work are approximately alike to dose range normally used in Radiosurgery,brachytherapy,etc.Therefore it may be concluded that in such conditions normal tissues are lesser affected by RIBE signals generated by highly irradiated tumoral cells.