Ganglion cyst

What it means

A ganglion cyst is a smooth, round or oval sac filled with thick, jelly-like fluid that forms just under the skin, connected to a nearby joint capsule or tendon sheath by a thin stalk. They develop when fluid that normally lubricates a joint or tendon leaks out and pools into a pocket, forming a balloon-like swelling. They are by far the most common soft-tissue lump found around the wrist and hand, and also occur frequently around the knee, ankle, and foot.

Why it appears on a CT or MRI report

Ganglion cysts are usually diagnosed clinically, simply by feeling a smooth, mobile lump, but they are often confirmed or further characterized with ultrasound or MRI, especially when the lump is deep, painful, or in an unusual location. On MRI, they appear as well-defined, fluid-filled structures with a very characteristic bright signal, and the report will typically describe the size, exact location, and — when visible — the stalk connecting the cyst to the adjacent joint or tendon. This appearance is distinctive enough that radiologists can usually identify a ganglion cyst with a high degree of confidence.

What it usually means

Ganglion cysts are benign, meaning they are not cancer and do not spread or invade surrounding tissue. Many cause no symptoms at all and are noticed only as a painless lump, sometimes changing size over time or even disappearing on their own. Others can cause a dull ache, especially with repetitive wrist or hand motion, or press on a nearby nerve to cause tingling or weakness, though this is less common. Ganglion cysts are more frequent in people who do repetitive hand or wrist movements and are seen across a wide age range, though most often in younger and middle-aged adults.

When to follow up

A ganglion cyst that isn't bothering you generally doesn't need any treatment beyond simple observation. It's worth seeing a doctor if the lump is painful, growing quickly, restricting movement, or associated with numbness, tingling, or weakness, which can suggest pressure on a nearby nerve. Treatment options when needed range from simple aspiration (draining the fluid with a needle) to surgical removal, though cysts can sometimes come back after either approach. A lump that feels hard, fixed in place, or is growing rapidly should be checked promptly to rule out other causes.

A plain-language way to picture it

Think of a small water balloon that has ballooned out from a tiny leak in a garden hose fitting, filled not with water but with a thick, gel-like fluid, and still connected back to that fitting by a narrow channel. That's a ganglion cyst — a self-contained pocket that has bulged out from a joint or tendon sheath. It can sit there quietly for years, shrink away entirely, or occasionally refill after being drained, because the small leak it grew from is often still there.