Reduced thermal sensitivity of the vasculature in a slowly growing tumour.

The thermal sensitivity of a slowly growing murine mammary carcinoma has been studied, and correlated with several vascular parameters. This tumour, CaRH, had previously been shown to be particularly resistant to hyperthermia when applied as 1 h immersion in a 42.8 degrees C water bath, with or without added X-rays. In the present study water bath temperatures of 43, 44 and 45 degrees C were used as well as a more complex system of water bath plus radiofrequency currents which produced a uniform intratumour temperature of 43 degrees C. Following treatment the tumour blood volume and relative capillary perfusion were estimated using radioactive tracer techniques. Only the treatments which gave at least 60 per cent reduction in blood perfusion yielded significant growth delays or thermal radiosensitization. These data have been compared with values for six other murine tumours. If, instead of comparing the exposure temperature, we compare the blood flow reduction, it is seen that all of the tumours are similar in their thermosensitivity. A higher temperature may be needed to cause vascular shutdown in more slowly growing tumours, but it is achieved with an intratumour temperature of 43 degrees C for 1 h. This may correlate with endothelial cell proliferation rates, which are similar in CaRH to the values measured in human tumours. The more rapid vascular expansion of the fast-growing tumours may result in a more fragile and thermosensitive capillary network. The hypoxic fraction, which is a measure of vascular inadequacy, also correlates with thermal sensitivity.