Cysts (and cyst-like structures) on HRCT

Because I like to simplify things, I consider three types of "cysts" -- thin-walled cysts, honeycombing and emphysema. I realize the "holes" from emphysema aren't really cysts, but when you are looking at the scan, often the major challenge is to decide if the black circles you are seeing are thin walled cysts or holes from emphysema, so I like to include that differential here.

Thin-Walled Cysts

If you decide that the structures you are looking at are thin-walled cysts, then you are home free (assuming you are correct). The two major considerations are generally easy to differentiate clinically.

• Langerhans Cell Histiocytosis -- In general, the cysts will have strange shapes and show a prediliction for the upper lung fields. Also, this disease is associated with smoking.
• Lymphangioleiomyomatosis -- These cysts tend to be circular and evenly distributed througout the lungs. The disease is commonly found in premenopausal women. Associated findings may include spontaneous pneumothorax and chylothorax.

Honeycombing

Honeycombing is a sign of end-stage lung and is not specific for any particular etiology. It can be recognized as thick-walled, air-filed cystic structures which are often clustered, in multiple layers and often subpleural in location. If the underlying etiology is unknown, try looking in less-involved regions of the lungs and try to identify the predominant pattern there.

Centrilobular Emphysema

As stated previously, emphysema is not a "cystic" disease, but the appearance can be similar. In emphysema, the air-filled hole wil have no perceptible wall. Other types of emphysema (including panacinar, paraseptal, cicatricial, and bullous emphysema have different appearances and are not confused with thin-walled cysts, though occasionally, paraseptal emphysema may be similar in appearance to honeycombing).