Purpose:If rectal balloon were not used in rectum cancer treatment,there is a lack of reproducibility of rectum shape on each treatment and irregularly inserted air inside the rectum can cause inhomogeneity of density with surrounding tissue,which could be resulted in uncertainty of dose delivery.In this research,we estimated the accuracy of TPS(Treatment Planning System)dose calculation and actual dose,and compared dose distribution when homogeneity of rectum and surrounding tissue increases as inner materials of rectal balloon changes.Materials and Methods:Cylindrical PMMA RPLGD phantom was used to measure dose at rectal wall and Air,water and PMMA were used as inner balloon materials.Total 12 plans(prostate and rectum cancer 3D-CRT(3 Dimensional Conformal Radiation Therapy)and IMRT(Intensity Modulated Radiation Therapy)plans with air,water,and PMMA balloons)were made using Varian Eclipse ver.8.9 to estimate influence of rectal balloon material on dose distribution.Each 16 glass dosimeters were located at rectum wall so that the point doses were compared with TPS.Result:As Homogeneity increase,Dose distribution was improved.As CT HU value of inner balloon material increase from 0(air)to 1000(water)and 1120(PMMA),Homogeneity Index(HI)and Conformity Index(CI)were increased about 20%and 5%,respectively.And considerable difference,between TPS and RPLGD reading,was measured with air balloon when water and PMMA balloons had less variation.Conclusion:It was shown that the dose distribution and the accuracy of TPS were better when the density of the balloon material was similar to the density of surrounding tissue.Especially when air is inserted into rectum,there is a possibility of difference between actual dose and TPS calculation.Thus,it is needed to look forward to find a method to increase treatment accuracy using tissue-equivalent inner balloon materials.