In the acute phase (4 to 12 weeks after completion of radiation therapy), the histological reaction is that of diffuse alveolar damage and consists of hyaline membranes in the alveolar ducts and respiratory bronchioles while the alveolar spaces fill with an exudate of proteinaceous material. This corresponds to the ground-glass opacities typically manifesting on CT. The reticular pattern that can be seen in this phase is due to congestion of capillaries and oedema of the interstitium (Fig. 6c) [20] .