The Dynamics of Tumor Cords in an Irradiated Mouse Mammary Carcinoma with a Large Hypoxic Cell Component

The tumor cord model represents a histologically based framework for interpretation of radiobiological phenomena, particularly the resistance to radiation conferred by absence of oxygen. For the mammary carcinoma T50/80 grown in B6D2F1 male mice, average oxygenation was poor, based on tumor growth delay after irradiation. There was no improvement in radiobiological oxygenation for several days after a high dose of radiation. This was consistent with events in the cords of the tumor, where although up to 20% of all cells became pyknotic by 8 hr, the cords did not shrink for at least 2 days. The cellular kinetics of populations of intact and dead cells, adjacent to and remote from the capillaries of the cords, were examined for up to 60 hr after irradiation and it was found that: (i) before treatment, average LI (adjacent) was 13% and LI (remote) was 2%, (ii) after irradiation, cells expressed pyknosis after passing through the S phase of the cell cycle, so that (iii) at early intervals there was a larger proportional rise in pyknotic cells in the adjacent than the remote zone. However, (iv) at later intervals there was always a higher proportion of dead cells in the remote zone.