[Impact of localized microwave hyperthermia on the pH-distribution in malignant tumors (author's transl)].

The impact of localized microwave hyperthermia (2.45 GHz) on the tissue pH-distribution was investigated in C3H mouse mammary tumors. The measurements were performed in microareas of the tissue using pH-microelectrodes with tip diameters approximately 1 micrometer. pH-readings during localized heating revealed that tissue temperatures exceeding 42 degrees C were followed by a pronounced pH-drop in tumors with a negligible amount of necrosis. Tissue pH-values measured immediately after heating (43 degrees C for 60 min) were lowered by an average of 0.54 +/- 0.19 pH-units as compared with preheating data (2p less than 0.005). In large tumors with extensive necrosis the mean tissue pH-value was distinctly higher (pH = 7.21) than in smaller tumors (pH = 6.75). Significant changes of the mean pH-values during hyperthermia were not obvious in these larger mammary carcinomas. The pH-drops measured in the extracellular space of smaller tumors during hyperthermia are most likely paralleled by an intracellular pH-decline. Both effects may enhance the therapeutical efficiency of elevated tissue temperatures.