Effects of fractionated irradiation on mice bearing subcutaneous tumors

Subcutaneous tumors (ST) were grown in the sacral parts of male ICR strain mice. The part of the animal that bore ST with 10 +- 2 mm in diameter was locally irradiated with a Cesium-137 source. Six graded doses between 3000 to 8000 rads were given in 10 fractions of one fraction per day. Tumor size, mean survival time (MST), tumor control rate (TCR), residual or recurrent tumor fraction (REF) and mortality rate (MR) were followed for 120 days for each group of animals. The best therapeutic effect for mice bearing ST was obtained with 4000 rads of γ-radiation given in 10 fractions. Under such treatment, no visible tumor was detected after 40 days, MST was greater than 93 days, TCR greater than 87%, REF and MR were less than 13% and 33% respectively. Irradiation by a larger dose (6000 rads) did produce both lower REF and higher TCR. Mortality rate, however, increased to 83% and MST dropped to 40 days. The changes in tumor size were the same as those of 4000 rads. The result with a smaller dose (3000 rads) showed incomplete tumor regression, which caused a MR of 56%, TCR 56%, REF 44% and MST 82 days. Similar results were obtained if the animals were irradiated under anesthesia