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Honeycombing is a term used to describe multiple air-filled cystic spaces usually 3-10 mm in diameter that are stacked in rows or clusters.
Honeycombing occurs in a subpleural distribution and the cysts have well-defined walls. Honeycombing can be microcystic (<4 mm cysts), macrocystic honeycombing (>4 mm cysts), or mixed microcystic and macrocystic. Multiplanar reformats can be helpful in differentiating honeycombing from traction bronchiectasis and paraseptal emphysema.
Honeycombing is the most specific sign associated with Usual interstitial pneumonia (UIP) pattern of interstitial lung disease (ILD). In a study done by Lynch et al., honeycombing was present in up to 90% of the patients with a diagnosis of Idiopathic pulmonary fibrosis (IPF). Another study conducted by Sumikawa et al. demonstrated that the HRCT feature that distinguished IPF from cellular nonspecific interstitial pneumonia (NSIP) and fibrotic NSIP were the extent of honeycombing (odds ratio, 5.16 and 2.10, respectively).
Honeycombing can sometimes constitute more than 70% of the fibrotic portions of the lungs in ILDs and this can be referred to as “exuberant honeycomb sign”. It is associated more commonly with connective tissue diseases (CTD) UIP rather than IPF UIP.
Honeycombing can be present in other conditions such as NSIP and chronic hypersensitivity pneumonitis (HP). However, ground glass opacities dominate in NSIP and honeycombing, if present, is usually minimal. Similarly, HP can also demonstrate honeycombing, but other features such as centrilobular nodules, mosaic attenuation, and upper lobe predominant fibrosis are often present, differentiating it from UIP.
https://doi.org/10.1053/j.ro.2018.12.009
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Honeycombing appearance on HRCT images. The cystic spaces reproduced on HRCT images (A and C); a real honeycomb (B, removed from apartment window of one of the authors). The honeycombing appearance consists of a cluster of cystic air spaces, usually with a diameter of 3–10 mm, although they may reach 2.5 cm in size
Palmucci, S., Roccasalva, F., Puglisi, S. et al. Clinical and radiological features of idiopathic interstitial pneumonias (IIPs): a pictorial review. Insights Imaging 5, 347–364 (2014). https://doi.org/10.1007/s13244-014-0335-3
(a) Honeycombing sign in a 75-year-old woman with the diagnosis of usual interstitial pneumonia. Axial HRCT of the chest shows peripheral and lower predominant multilayers of cysts (arrows). (b and c) Exuberant honeycombing in a 65-year-old man with history of rheumatoid arthritis and underlying diagnosis of usual interstitial pneumonia. Axial HRCT of the chest shows peripheral- and lower lobe predominant florid honeycombing compatible with “exuberant honeycomb sign.”
Radiographic Signs and Patterns in Interstitial Lung Disease. Seminars in Roentgenology. 54(1):66-72. doi:10.1053/j.ro.2018.12.009