Comparative effects of water-bath- and microwave-induced hyperthermia on survival of Chinese hamster ovary (CHO) cells

Suspensions of asychronous CHO cells in the log phase of growth were subjected to a water-bath or to 2450-MHz-microwave hyperthermia at a temperature of 44 °C for a period of 60 minutes. Immediately following hyperthermic treatment, the cells were equilibrated at 37 °C, counted, and plated out for colony assays, to quantitate survival of cells. Microwave heating was performed by inserting the long dimension of a plastic culture tube, which contained 8 × 10
⁵
cells in 3 ml of media, into a waveguide parallel to the E-field vector; CW energy was intermittently applied to maintain the suspension at 44 ± 0.1 °C. Hyperthermia was induced in a similar sample of cells from the same culture by immersing the culture tubes into a water bath at 44 ± 0.05 °C. Temperature-rise profiles of the two hyperthermic treatments were matched by monitoring suspensions of cells with the Liquid Crystal Fiberoptic temperature probe. Cells subjected to both sources of hyperthermia under comparable conditions showed no significant difference in survival. The D
o
(slope of the exponential region) of the survival curve was 8.3 minutes for the water-bath treatment compared with 8.1 minutes for the microwave treatment. Similarly, the D
q
, (a measure of the survival curve that indexes cell repair) was 23.4 minutes for the water-bath treatment and 23.7 minutes for the microwave treatment. Intrasample uniformity of heating and accurate thermometry were found to be highly critical to the demonstration of parity.