The effect of vascular occlusion on the thermal sensitization of a mouse tumour.

The effect of occluding the blood supply to a mouse tumour (with a metal clamp) has been studied for both irradiation and heating. Local heat was applied by immersion in a water bath for one hour at 42.8 degrees C or for 15 minutes at 44.8 degrees C. Occlusion of the blood supply during heating has a profound cytotoxic effect on the tumour, even in the absence of irradiation. Most tumours treated with 42.8 degrees C for one hour under clamped conditions were locally controlled whether they were irradiated or not. Tumours heated with their blood supply unobstructed showed a lesser sensitivity to heat, seen as an increased sensitivity to X rays with a thermal enhancement ratio of 1.8--2.6. With the shorter period of more intense heat (44.8 degrees C for 15 min), the effect of increasing the clamping time before heating was studied. The proportion of tumours locally controlled increased from 33% if the clamp was applied immediately before heating to 83% if the clamp was present for 60 minutes before heating commenced. No cures were observed for heat applied immediately before clamping, or immediately after release of the clamp. Accumulation of metabolic products or pH changes are implicated as the factors which alter the thermal sensitivity of these tumour cells.