Enhancement of lung colony-forming efficiency by local thoracic irradiation: interpretation of labeled cell studies.

The dynamics of 125IUdR-labeled tumor cell retention in the lungs have been compared with the efficiency of lung colony formation over a wide range of inoculum sizes in normal mice and in mice pretreated with 1000 rads local thoracic irradiation (LTI). These studies indicate that the increase in lung colony yield after LTI cannot be ascribed to improved intravascular trapping or interstitial lodgement of tumor cells. Hypotheses purporting that active specific or nonspecific host defense might be impaired by LTI were reassessed but rejected in favor of the idea that radiation-induced alterations in the physiological mocroenvironment of the lungs improve an intrinsically poor survival capability of intravenously disseminated tumor cells.