Breast cyst

A breast cyst is a small sac filled with fluid inside the breast. It is not cancer: it belongs to the group called benign breast disease, meaning the conditions that are not cancerous.

To understand where it comes from, you have to look at how the breast is built on the inside. Milk is made in tiny clusters of glands, and each cluster drains into a small duct; that unit, the glands plus their duct, is called the terminal duct lobular unit. When those end ducts widen, fluid stays trapped inside and a cyst forms. It is like a small water balloon hanging at the tip of a branch.

When you feel it, it usually moves around in the tissue instead of sitting anchored in place, and it feels rubbery, like a soft eraser. And it rarely comes alone: most patients have several.

What your fingers cannot tell you is whether that lump is full of fluid or solid. Two imaging tests together settle that: in a woman over 35 with a lump you can feel, mammography with ultrasound is what separates a cyst from a solid lesion. And there is an in-between case, the complex cyst, which has fluid and also a solid part inside: that one does get worked up with a biopsy, meaning taking a small piece to look at under the microscope.