Hypothermia-enhanced human tumor cell radiosensitivity

Ablation of neoplastic disease by freezing has found increasing utility as a potential therapeutic modality. To assess the effect of cooling temperatures on cellular radiation response, an established human cervical carcinoma cell line (HTB35) was subjected to holding temperatures of 0, 5, or 15 degrees C for up to 24 h before irradiation. Survival was measured by in vitro clonogenic assay of colonies containing at least 50 cells. Cooling for up to 12 h did not significantly decrease survival, but after 24 h survival fell to 75% of control cultures grown at 37 degrees C. X-irradiation immediately after cooling for 24 h resulted in 1.6-fold enhanced radiosensitivity. However, the radiosensitizing effect decayed rapidly if the cooled cells were returned to normal growth temperature for 6 h or longer before irradiation and subculture. Both temperature and cooling duration influenced the radiation response. With 0, 5, or 15 degrees C, radiosensitivity increased after 3, 6, or 12 h, respectively, and progressively rose with up to 24 h of cooling. By flow cytometric analysis, no statistically significant difference was observed in the S-phase fraction between control cells and those cooled to 0 degree C for 24 h. These data demonstrate cooling-enhanced in vitro radiation sensitivity which is dependent upon cooling temperature, duration, and rewarming interval before irradiation. While cell cycle redistribution does not appear to be a factor in the increased radiosensitivity, differences in the radiation survival curves between cooled versus normothermic cells suggest that diminished capacity for sublethal damage repair may be a significant influence on the changes which were observed.