Effect of hyperthermia on the hypoxic fraction in an experimental mammary carcinoma in vivo.

The influence of hyperthermia on the fraction of clonogenic hypoxic cells in a CaH mammary carcinoma was investigated using the TCD50 values for radiation treatment under normal or clamped conditions. A one-hour heat treatment at 41.5 and 42.5 degree C given four hours after radiation reduced the hypoxic fraction to 2.3 X 10(-1), 7.4 X 10(-3) and 6.5 X 10(-3) respectively, when compared with the content of hypoxic cells in non-heated tumours. The results indicate that moderate hyperthermia is able to destroy selectively a large proportion of the fraction of radioresistant hypoxic cells in a solid tumour. This effect is probably due to the increased heat sensitivity of cells in a chronically hypoxic and nutritionally deprived acidic environment.