Tumors in the breast are basically of two types, based on their internal structure: cystic tumors and solid tumors. We call cystic tumors simply "cysts." The main characteristic of cystic tumors, or cysts, is that their internal material is fluid. In short, cysts are benign tumors filled with fluid. Cysts in the breast are not malignant and do not become malignant, or cancerous.

Due to various hormonal changes, these cysts can grow, shrink, increase in number, or decrease in number over time. The growth and increase in number of these cysts do not necessarily indicate malignancy, while shrinkage and decrease in number do not necessarily indicate good. These changes are similar to weight gain and weight loss.

If cancer develops in the breast, it doesn't happen because these cysts become cancerous. Cancers arise from normal mammary glands.