Tumors and cysts are two distinct entities.

• 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), but sometimes cancer can cause a cyst.
• Tumor. A tumor is any abnormal mass of tissue or swelling. Like a cyst, a tumor can form in any part of the body. A tumor can be benign or cancerous (malignant).

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. Often this is done with repeat imaging to see if the cyst grows over time.

The best test to determine whether a cyst or tumor is benign or malignant is a biopsy. This procedure involves removing a sample of the affected tissue — or, in some cases, the entire suspicious area — and studying it under a microscope.