Stereotactic radiosurgery: Relative biological effect of continuous vs. intermittent radiation exposures

Abstract

AbstractLinear accelerator‐based stereotactic radiosurgery is increasingly used in place of gamma knife equipment. Conforming radiation isodoses to complex volumes requires multiple isocenters and arcs per isocenter and may result in a protracted and intermittent radiation exposure. The experiments in this study were designed to quantitate the difference between continuous and intermittent radiation schedules which are typical for gamma knife and linear accelerator‐based radiosurgery, respectively. Two radiobiologically well‐characterized cell lines, V79 Chinese hamster and U‐87MG human malignant glioma cells, were irradiated in vitro to 12 Gy delivered as a continuous 5 min exposure and an intermittent exposure lasting maximally 112 min. The effect of continuous vs. intermittent irradiation to a dose of 31.5 Gy was also measured in vivo using FSaII fibrosarcomas, implanted into C3Hf/Sed mice, in a tumor growth assay. The in vitro experiments demonstrated a significant reduction in biological radiation effect reflected in 2‐ to 3‐fold increases in cell survival with intermittent vs. continuous irradiation schemes. Similar effects were seen with the FSaII tumors. The reduced effect of the intermittent irradiation schemes is most likely due to repair processes after radiation exposure, such as Potentially Lethal Damage Repair (PLDR) and Sublethal Damage Repair (SLDR), which can occur during 5–15 min intervals between radiation increments of intermittent stereotactic radiosurgery treatments. © 1995 Wiley‐Liss, Inc.