By Mayo Clinic Staff

Tumors and cysts are two distinct types of growths.

• 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, also called benign, but sometimes cancer can cause a cyst.
• Tumor. A tumor is any 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, also called malignant.

Cysts that appear uniform after an ultrasound exam or a CT scan are almost always benign and should simply be watched.

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. A biopsy is a procedure to remove a sample of tissue for testing in a lab to see if it is cancer.