Radiation therapy was recognized as a potential cause of cancer many years ago.Patients undergoing radiation therapy may have an increased risk of second malignancies as a result of radiation away from the treatment field.The effect of radiation on the risk of developing a second cancer after radiation therapy depends on different factors such as prescribed dose,treated area and the patient's age.In this study the radiation induced secondary cancer of the lungs,heart and contralateral breast after conventional radiation therapy of breast was assessed.In order to estimate the radiation induced secondary risk of organ at risks in radiation therapy of breast cancer,radiotherapy treatment plans of 200 women with breast cancer which treated using Elekta compact 6 MV linear accelerator was studied.For each case high precision contour of organ at risks was redrawn and the total dose received by each organ during radiotherapy process was calculated by CorePLAN treatment planning system.Nominal cancer risk coefficients from ICRU 103(2007)and NCRP 116(1993)were used to estimate the secondary cancer risks of organ at risks.The average age of women in our study when breast cancer was diagnosed was 49.6 years.All of diagnoses were made after 2013.This study indicated an induced secondary cancer risk from conventional radiation therapy of breast following radiotherapy in the most important organ at risks involved in radiation therapy treatment field.As a result,the risk of secondary cancer in lungs,heart and contralateral breast was estimated 5.74±0.67%,1.83±0.47%and 0.69±0.21%respectively.Radiation induced secondary cancer risk of organ at risks following radiation therapy of breast cancer is associated with the delivered dose to these organs.Although the absolute risk is relative low,the growing number of long-time survivors after breast cancer treatment highlights the need for advances in normal tissue sparing radiation techniques.