Fractionation of Combined Heat and Radiation in Asynchronous CHO Cells: I. Effects on Radiation Sensitivity

Hyperthermia (10 min at 45°C) immediately prior to graded radiation doses (hX) reduced the D0 by a factor of 1.4 from that of the radiation-only survival curve (X). However, fractionated treatment consisting of 10 min at 45°C + 4 Gy (hx) followed at various times by hX (hx + time + hX) resulted in a D0 which increased with longer fractionation intervals and for an interval of 24 hr was 1.4 times that of the control heat-radiation survival curve (hX) and thus became similar to the D0 of the control radiation survival curve (X). Furthermore, a radiation survival curve 24 hr after 10 min at 45°C + 4 Gy (hx + 24 + X) showed a D0 which was larger than that of the control radiation survival curve (X), but the increase was less (factor of 1.2) than that for the fractionation of hx + 24 + hX relative to hX. The fractionation of radiation alone (x + 24 + X), or a pretreatment by heat alone (h + 24 + X) did not increase significantly the D0 of the radiation survival curve over that of the control (X). However, a pretreatment by radiation alone, 24 hr before combined heat and radiation (x + 24 + hX), increased the D0 also by a factor of 1.4 compared to that of the hX control. However, in contrast to the control radiation survival curve (X) and the fractionation survival curve hx + 24 + hX, the fractionation survival curve x + 24 + hX had no shoulder (n = 1). These results indicate that cell killing by combined heat and radiation in fractionated protocols cannot be predicted from the inactivation kinetics of combined heat and radiation based on single treatments when the fractionation interval is more than one cell-cycle time.