Background. Radioimmunotherapy and other forms of biologically targeted radiopharmaceutic treatment appear to show unexpected efficacy in many patients with lymphoma, neuroblastoma, and several other types of nonepithelial malignancies. The radiobiologic mechanisms responsible for this high clinical radioresponsiveness are unclear, but must involve some sort of cytotoxic enhancement or sensitization to protracted courses of low-dose-rate radiation exposure. Materials a n d Methods. A series of in vitro experiments was performed with malignant lymphoma cell lines exposed under various conditions to high-dose-rate external beam radiotherapy or low-dose-rate 9OY radiation. Data were collected on cell cycle effects, DNA fragmentation, and modulation of cytotoxicity by caffeine and treatment sequence alterations.
Results. The data showed that some malignant lymphoma lines are highly sensitive to low-dose-rate radiation and that a portion of the cytotoxicity appears to be mediated by the induction of radiation-associated apoptosis (programmed cell death). Cell cycle effects of lowdose-rate radiation (such as G2M block) appear to be relatively minor in this experimental system. Agents that modulate apoptosis (such as the calcium-releasing agent caffeine) significantly enhance cell kill and DNA fragmentation after 9oY treatment.
Conclusions. These results suggest that radiation-associated apoptosis may be important in the radiobiology of targeted radiopharmaceutical therapy. Cancer 1994;