Radioprotection by Pentobarbitone Sodium of a Murine Tumourin Vivo

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Abstract
The effect of 2 methods of immobilization on the local control of mouse implanted MTI anaplastic tumors at 80 days was studied. The anesthetic was pentobarbitone sodium (probably the most widely used for immobilization during irradiation). Also investigated were the effect of rendering the tumors fully hypoxic (by occlusion of their blood supply), and the effect of immobilization procedures on the tumor temperatures. The probability of local tumor control as a function of X-ray dose was shown. For the mice that were unanesthetized, and received X-rays only, the dose of X-rays required to control 50% of the tumors (i.e., TCD 50) was 63.6 .+-. 1.3 gray [X-ray dose] (.+-. 1 SEM [standard error of the mean]). The effect in unanesthetized mice of clamping of the tumors to render them fully hypoxic was to increase the radioresistance of the tumors to 69.5 .+-. 1.4 gray, an increase of 5.9 gray. Assuming a hypoxic Do of 3.6 gray, this would indicate a hypoxic proportion under air-breathing conditions of 19% (SEM range 9-14%). The protective effect of anesthetizing with pentobarbitone sodium on the TCD 50 was even greater than that of clamping off the blood supply to the tumor. The TCD 50 was increased to 75.4 .+-. 1.6 gray, an increase of 11.8 gray. The radioprotective effect of pentobarbitone sodium on the TCD 50 was large: an extra 11.8 gray were required. The progressive drop in the tumor temperature from 4.degree. to 9.degree. C during the course of irradiation would suggest that the blood flow to the tumor was also reduced.