The survival response of HeLa cells to pulsed${\rm CO}_{2}\text{-laser}$ irradiation was evaluated and compared to that obtained after short heat treatment in a water-bath. The survival curve of exponentially growing cells subjected to single pulses (70 nsec) of varying infrared energy densities is composed of an initial shoulder up to$750\ {\rm mJ}/{\rm cm}^{2}$ followed by an exponential decline ($D_{0}=90\ {\rm mJ}/{\rm cm}^{2}$). A quite similar curve was obtained when the cells were heated for 2 min at temperatures ranging from 37 to 55°C. In this case, the response was characterized by a shoulder up to 47.5°C and a$D_{0}=1{{}^\circ}{\rm C}$. In both cases split-dose experiments yielded survival response curves which indicated that for short exposure times the recovery is complete within 30 min. The cell responses to treatment at 44°C for a period of time varying from 30 to 180 min or to series of 300, 500, 600, or$800\ {\rm mJ}/{\rm cm}^{2}$ laser pulses separated by 1-min intervals yielded survival curves that were all characterized by an exponential decay with no apparent shoulder. But as the time between pulses became shorter than 1 min, a progressive bending of the curves resulted, indicating changes in cell sensitivity. Theoretical considerations on laser radiation interaction in cells show that an elevation of temperature is the only factor influencing the survival. All the results suggest a temperature- and exposure-time-dependent mode of cell response to hyperthermia.

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