Tissue Repair and Repopulation in the Tumor Bed Effect

These experiments were designed to study the kinetics and magnitude of cell repair and repopulation in tissues whose damage results in the tumor bed effect. The right hind thighs of mice were irradiated with single doses or two equal γ-ray fractions. Interfraction intervals ranging from 30 min to 24 h (to measure the kinetics of repair from sublethal damage) and 6 and 12 weeks (to determine the extent of repopulation) were used. One day after the second radiation dose 5× 10⁵ FSA tumor cells were inoculated into the center of the irradiated field. Radiation dose-response curves were obtained by calculating the time required for tumors to reach 12 mm diameter. No recovery occurred within 6 h of the radiation delivery as measured by this assay. Some recovery, 3.2-4.6 Gy above a single radiation dose, occurred when the interval between two fractions was 24 h. With increasing interfraction intervals of 6 and 12 weeks further dose sparing occurred in the amount of 5.0-6.9 and 7.5-8.3 Gy, respectively. The data suggest that repopulation is the major contributor to the radiation dose-sparing recovery of stromal tissue and that some proliferative response may occur as early as 1 day after the first irradiation.