RESPONSE OF 9L TUMOR-CELLS TO HYPERTHERMIA AND X-IRRADIATION

Asynchronous gliosarcoma (9L) cells were exposed in vitro to temperatures from 39.0 to 45.0°C. Except for 42.0°C, dose-response curves showed both shoulder and exponetial survival regions. The parameters T0, T(q) and n were used to describe the survival curves for different temperature treatments. An Arrhenius plot of 1/T0 vs 1/K showed a break at 43.0°C with an activation energy of 509 kcal/mole for temperatures below and 109 kcal/mole for temperatures above 43.0°, respectively. Shoulder width was maximal at 42.5°C with T(q) equal to 126 min and n equal to about 90. Cells remained attached to the growing surface of the tissue culture flask throughout the time necessary to establish survival curves based on colony-forming ability. However, for exposure times resulting in less than 1% survival, cells detached from the flask and floated into the supernatant medium. The rate of cell detachment corresponded to the cells' loss of ability to exclude the trypan blue exclusion dye. Combining graded doses of ionizing radiation with 1-hr hyperthermic treatments at 41.5-43.0°C generally resulted in small changes in the D0 of the radiation survival curve, but the D(q) varied significantly with temperature. A synergism factor was defined as the quotient of the percentage survival expected in the event the two modalities interacted additively divided by the percentage survival actually observed from combined treatment. The synergism factor exhibited a maximum at 42.5°C indicating that this may be a critical biological temperature.