Radiation-induced changes in pulmonary collagen metabolism were studied in Syrian hamsters given multiple thoracic doses of60 Co radiation to achieve cumulative exposures of 6000, 4000, and 2000 R. At 13-14 wk after initial exposure, 6000- and 4000-R exposures had increased incorporation of injected [14 C]proline into pulmonary collagenous protein which suggested an increased collagen synthesis. By 21-22 wk after exposure, increased pulmonary soluble collagen was noted. Increased pulmonary scarring was indicated by a variable increase in native collagen at 13-36 wk. A collection of alveolar macrophages at 7-8 wk, followed by inflammation at 13-14 wk and a beginning of pulmonary fibrosis at 13-19 wk were noted. At 21-22 wk after exposure a somewhat more marked pulmonary fibrosis and some epithelialization were observed. Hemosiderin deposits were also observed at 35-36 wk after exposure, but pathologic processes were lessened by this time. The early activation of collagen synthesis presumably caused the radiation-induced fibrosis. Later, when collagen tended to accumulate, the synthetic rate was normal. The activation of collagen synthesis caused by external thoracic irradiation resembles that caused by thoracic irradiation from the internal emitter, <tex-math>${}^{144}{\rm Ce}$</tex-math>. Moreover, it demonstrates the usefulness of monitoring collagen biosynthesis by [14 C]proline incorporation into the lung.

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