Radiation-induced lung disease (RILD) due to radiation therapy is common. Radiologic manifestations are usually confined to the lung tissue within the radiation port and are dependent on the interval after completion of treatment. In the acute phase, RILD typically manifests as ground-glass opacity or attenuation or as consolidation; in the late phase, it typically manifests as traction bronchiectasis, volume loss, and scarring. However, the use of oblique beam angles and the development of newer irradiation techniques such as three-dimensional conformal radiation therapy can result in an unusual distribution of these findings. Awareness of the atypical manifestations of RILD can be useful in preventing confusion with infection, recurrent malignancy, lymphangitic carcinomatosis, and radiation-induced tumors. In addition, knowledge of radiologic findings that are outside the expected pattern for RILD can be useful in diagnosis of infection or recurrent malignancy. Such findings include the late appearance or enlargement of a pleural effusion; development of consolidation, a mass, or cavitation; and occlusion of bronchi within an area of radiation-induced fibrosis. A comprehensive understanding of the full spectrum of these manifestations is important to facilitate diagnosis and management in cancer patients treated with radiation therapy.

The CT halo sign indicates ground glass attenuation surrounding a pulmonary nodule on CT. Although it was initially proposed as an early, specific finding of invasive pulmonary aspergillosis, it can be caused by many other pathological conditions such as infection, neoplastic and inflammatory diseases. The halo of ground glass attenuation pathologically represents pulmonary haemorrhage, tumour infiltration, or non-haemorrhagic inflammatory processes. Although non-specific, this sign is important because the clinical setting and associated radiological features may give a clue to the differential diagnosis. In this review, we demonstrate the spectrum of pulmonary diseases showing the "CT halo sign" on thin-section CT and discuss their radiological and clinical features.

Large nodules arising from the coalescence of small nodules may be seen in active tuberculosis and in sarcoidosis. The CT finding was termed "clusters of small nodules" instead of the "sarcoid galaxy sign" in this article. A single cluster of small nodules, clusters of small nodules in the superior segment of the lower lobe, or clusters of small nodules not associated with lymphadenopathy or associated with tree-in-bud lesions would favor the diagnosis of active pulmonary tuberculosis rather than pulmonary sarcoidosis.

New lung lesions that develop during medication for tuberculous pleural effusion should be considered a transient worsening that ultimately improves with continuation of medication.

Multilocular thymic cysts typically manifest on CT as unilocular or multilocular cystic thymic masses, often with soft-tissue attenuation components. CT cannot be used to distinguish neoplastic from nonneoplastic soft-tissue components.