Response of viable tumor cords to a single dose of radiation

A C3H mouse mammary tumor contained cords of viable tissue separating vascular and connective stroma from regions of necrosis. The mean distance from capillary to necrotic tissue was about 90 ,um, a value close to the calculated diffusion distance for oxygen. Doses of 600 and 2000 rads y-radiation to the tumor caused increasing delays in tumor growth but no significant reduction in volume; 3000 rads given under conditions of acute local hypoxia caused slight tumor regression followed by slow regrowth. After each of these doses the mean thickness of tumor cords shrank rapidly to about 50% of the control value at about 12 hr after irradiation, and then started to increase. The mean concentration of cells within the tumor cords also decreased after irradiation. Thus, changes in tumor volume were not representative of microscopic changes in cell number. The rate of recovery of cord thickness was dose dependent, and after 2000 rads or 3000 rads hypoxic cords regrew to a mean thickness less than that in unirradiated tumors. Changes in tumor oxygenation with time after irradiation are dependent on these changes in microarchitecture and are compared with rates of reoxygenation that have been estimated by other methods.