Observations of thermal gradients in perfused tissues during water bath heating.

Actual thermal gradients in perfused tissues are difficult to observe using thermocouples because of thermal conduction along the probes. We have used fine type-K (chromel-alumel) probes, which have a much lower thermal conductivity than equivalent-sized type-T (copper-constantan) thermocouples, to examine thermal gradients in two mouse tumour systems during water bath heating. The results indicate substantial heterogeneity in temperature distribution even in tumours transplanted in the foot and immersed to a depth of 2 cm in a 44 degrees C water bath for 20 min, i.e. thermal gradients greater than 1 degree C/mm were observed in KHT fibrosarcomas. The temperature heterogeneity for water bath heating is primarily a result of blood flow and appears to be tumour-specific. Temperature measurements using an excised perfused canine kidney demonstrate that increased perfusate volume flow increases the range of tissue temperatures. Consistent with theory, an artifactual improvement in temperature homogeneity resulted when temperature was measured using type-T thermocouples instead of type-K probes. These results emphasize the difficulties in obtaining accurate temperature measurements during experimental and clinical hyperthermia. Even extensive measurements of temperature in tissues may underestimate the true range of heterogeneity unless factors such as thermal smearing are controlled.