Cyst

What it means

A fluid-filled pouch, usually round or oval, with a thin wall around it. These pouches can form almost anywhere — in kidneys, liver, ovaries, breasts, thyroid, spine, skin, the brain — and most adults carry a few without ever knowing. They form for many reasons: a blocked duct, a leftover from development, a follicle that didn't reabsorb, or simple age-related changes.

Why it appears on a CT or MRI report

Radiologists describe each one in detail: the organ it sits in, its size in millimetres, whether the contents look like clear water on the images, how thick the wall is, whether there are any solid bits or strands inside, and whether it lights up with contrast dye. The word "simple" is the key term — a simple finding has clear watery contents, a paper-thin wall, no internal divisions, and no enhancement. That picture is overwhelmingly benign and usually needs nothing more.

What it usually means

The vast majority are simple and benign, and they are found by accident when imaging is done for an unrelated reason. Common examples include simple kidney cysts (almost universal after age 50), liver cysts, ovarian follicular cysts, breast cysts, and small spinal cysts. These usually need no treatment and no follow-up. A small share are described as "complex" — meaning the wall is thicker than expected, the contents are not pure fluid, there are internal divisions called septations, or part of the wall takes up contrast. Complex ones occupy a wider range, from old bleeding inside a previously simple sac, to infection, to a small share that are actually solid tumours masquerading as fluid collections. Standardised grading systems (Bosniak for kidneys, for example) help radiologists sort the complex ones by how worrying they look. Symptomatic ones — large enough to press on a nerve, an organ, or a duct — sometimes need to be drained or removed.

When to follow up

If the report calls it simple and recommends no follow-up, you can usually file it away as background information and mention it next time you see your doctor. If the description includes words like complex, septated, thick-walled, hyperdense, or enhancing, ask your doctor what the imaging plan is — that often means a follow-up scan or a referral. Pain in the area, a lump you can feel, or pressure symptoms are worth raising regardless of the imaging description.

A plain-language way to picture it

Picture a tiny water balloon tucked inside the tissue. A simple one is a clear balloon with smooth, thin rubber and pure water inside — easy to ignore. A complex one is the same balloon, but with cloudy water, a thicker wall, maybe a partition splitting it in half. The cloudier the contents and the thicker the wall, the more carefully the radiologist looks.