Response of 9L Tumor Cells to Hyperthermia and X Irradiation

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Abstract
Asynchronous gliosarcoma (9L) cells [rat] were exposed in vitro to temperatures from 39.0-45.0.degree. C. Except for 42.0.degree. C, dose-response curves showed both shoulder and exponential survival regions. The parameters To, Tq, and n were used to describe the survival curves for different temperature treatments. An Arrhenius plot of 1/To vs. 1/K showed a break at 43.0.degree. C with an activation energy of 509 kcal/mol for temperatures below and 109 kcal/mol for temperatures above 43.0.degree., respectively. Shoulder width was maximal at 42.5.degree. C with Tq 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-h hyperthermic treatments at 41.5-43.0.degree. C generally resulted in small changes in the Do [mean lethal dose] of the radiation survival curve, the Dq [quasi threshold dose] varied significantly with temperature. A synergism factor was defined as the quotient of the percentage survival expected in the event the 2 modalities interacted additively divided by the percentage survival actually observed from combined treatment. The synergism factor exhibited a maximum at 42.5.degree. C indicating that this may be a critical biological temperature.