The role of selective COX-2 inhibitors in reactive oxygen species formation in osteosarcoma cells after X-ray irradiation. Selective inhibition of COX-2 is thought to prevent carcinogenesis in some malignant tumors. In this study, in an effort to enhance the effectiveness of osteosarcoma treatment, we investigated the effect of a selective inhibitor of COX-2, with or without irradiation. We also asked whether selective COX-2 inhibitors increase the effect of X-ray irradiation, with regard to reactive oxygen species (ROS) formation in an osteosarcoma cell line. Our results showed that the presence of COX-2 inhibitor without irradiation results in faint spots of ROS formation that do not appear in the absence of COX-2 inhibitor. However, COX-2 inhibitor did not induce ROS formation when combined with irradiation. Thus, radiotherapy with selective COX-2 inhibitions has limitations in the treatment of radioresistant osteosarcoma to obtain the effective achievement, it is indispensable to combine another agent in future studies.