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Changes in Intratumor pH by Two Heatings¹

Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota, Minneapolis, Minnesota 55455

It is a known fact that pH in rodent tumors decline significantly upon heating most likely due to breakdown of the tumor blood circulation. We recently observed that tumor blood vessels become thermotolerant after being heated with a sublethal thermal dose. The purpose of the present study was to reveal whether heating can reduce intratumor pH when the tumor vessels are thermotolerant. When the SCK tumors of A/J mice were heated at 42.5°C for 1 h, the tumor vessels became most thermotolerant at 18 h postheating, as measured with the ⁸⁶Rb uptake method. The intratumor pH in the control SCK tumors was 7.05 ± 0.14 (SD), and it significantly decreased to 6.70 ± 0.08 (P < 0.001) after heating at 44.5°C for 1 h. However, when the tumor vessels were thermotolerant, i.e., 18 h after heating at 42.5°C for 1 h, reheating at 44.5°C for 1 h could not reduce the intratumor pH. We concluded that such a failure to increase tumor acidity by a second heating at temperatures as high as 44.5°C was due to vascular thermotolerance developed by the first heating.

¹ This work supported by National Cancer Institute Grants CA13353 and CA44114.

² To whom requests for reprints should be addressed, at Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota, Harvard Street at East River Road, Box 494 UMHC, Minneapolis, MN 55455.

Received 4/ 9/90. Accepted 7/30/90.