Changes in lung morphology and cell number in radiation pneumonitis and fibrosis: a quantitative ultrastructural study.

We used stereologic-morphometric techniques to obtain a detailed quantitative picture of the changes in lung ultrastructure of rats at 12 and 26 weeks after unilateral thoracic irradiation with 3000 cGy. At 12 weeks post-radiation, the total number type 1 epithelial cells, type 2 epithelial cells and capillary endothelial cells were decreased 50-70%, total type 1 epithelial and capillary surface areas were decreased 55-60%, and the total volume of intracapillary blood was decreased 75%. The interstitial cells and matrix together accounted for more than 9% of the peripheral lung tissue volume including air, compared to 3% in controls. The numerical density of interstitial cells was increased to 3-fold the control value. The numerical density of interstitial cells was increased to 3-fold the control value. Although fibroblasts still comprised the largest interstitial cell subgroup, the numerical density of mast cells was increased over 150-fold and other inflammatory and immune cells were increased to a lesser extent. At 26 weeks post-radiation, the number, volume, and surface area of the type 1 epithelium and capillary endothelium had further decreased to only 5-10% of control values. The total number of type 2 epithelial cells was reduced by 75% but the volume density was actually increased because of a 4-fold increase in the mean cell volume. The interstitial cells and matrix now comprised over 77% of total peripheral lung tissue volume including air as compared to 6% in controls. Mast cells and plasma cells comprised 11% and 19% of all interstitial cells respectively and the densities of these cells were 540 and 180-fold the control value respectively. The relation of these morphometric findings to the results of previous morphologic studies is discussed.