Perilymphatic nodules are interstitial nodules (usually well-defined) which are characterised by distribution. Nodules are subpleural, along fissures, along interlobular septa, and adjacent to the bronchovascular bundles.
The differential diagnosis of nodules in a perilymphatic distribution is limited. The most common cause is sarcoidosis (typically symmetrical and upper lobes). Also common is lymphangitis carcinomatosis (often asymmetrical and lower lobe). Silicosis and coal-worker’s pneumoconiosis uncommonly give this appearance. Rare causes include amyloidosis and lymphoid interstitial pneumonitis.
In this case the diagnosis was sarcoidosis.
Reference: Webb WR, Higgins CB. Thoracic Imaging: Pulmonary and Cardiovascular Radiology. Lippincott, Williams & Wilkins 2005
Credit: Dr Laughlin Dawes