Cyst. A cyst is a sac that may be filled with air, fluid or other material. A cyst can form in any part of the body, including bones, organs and soft tissues. Most cysts are noncancerous (benign). Although cancers can form cysts.
Some common examples of cysts include sebaceous (epidermoid) cysts — small bumps that form just beneath the skin — cysts that occur in the liver (hepatic), cysts that occur in the kidneys (renal), and breast and ovarian cysts. It’s important to note, however, that nearly all cancers are capable of producing cysts.
Cysts that appear uniform after examination by ultrasound or a computerized tomography (CT) scan are almost always benign and should simply be observed. If the cyst has solid components, it may be benign or malignant and should have further evaluation. The best test to determine whether a cyst or tumor is benign or malignant is to take a sample of the affected tissue — or, in some cases, the entire suspicious area — by removing it and studying it under a microscope. This is known as a biopsy.