Alveolar Sarcoid

Alveolar sarcoid refers to an atypical presentation of pulmonary sarcoidosis. There are large opacities ranging in diameter from 1 cm to 4 cm with a rounded or elongated shape, irregular edges and blurred margins with or without air bronchograms. They are typically found in the parahilar or peripheral regions.

The opacities don't represent a true alveolar process, but result from the confluence of a large number of interstitial granulomas. Indeed, on CT, small nodules are often visible around these large opacities, an appearance that has been called the galaxy sign.

Another pattern of alveolar sarcoid is the so-called "fairy ring." This refers to circumferentially organized opacities. On mediastinal windows, this can mimic central necrosis, but lung windows reveal apparently normal lung centrally. It is hypothesized that the central area represents spontaneous resolution of granulomatous inflammation with new granulomatous inflammation developing peripherally.

References

• Marlow TJ, Krapiva PI, Schabel SI, Judson MA. The "fairy ring": a new radiographic finding in sarcoidosis. Chest. 1999 Jan;115(1):275-6.
• Park HJ, Jung JI, Chung MH, Song SW, Kim HL, Baik JH, Han DH, Kim KJ, Lee KY. Typical and atypical manifestations of intrathoracic sarcoidosis. Korean J Radiol. 2009 Nov-Dec;10(6):623-31.