Radiation-induced apoptosis in vivo: therapeutic significance of apoptosis in radiation therapy

Apoptotic cell death is frequently found in certain tumor cells after irradiation; however, the incidence is not always high in vivo. Seven tumors were transplanted to nude mice, and their organs were histologically examined after irradiation to study the therapeutic significance of apoptosis in radiation therapy. A high incidence of apoptosis was found only in radiosensitive tumors or normal cells with wild-type p53, but the peak incidence in most cells was only a few percent or less. However, the calculated total incidence of apoptotic cell death was much higher than the actual peak incidence, because the half-life of apoptosis is very short. Even in radioresistant tumors, total radiation-induced apoptosis was estimated to be about 10 percent. These results suggest that apoptotic cell death in radiotherapy may be more important in vivo than previously estimated.