What’s the Difference Between a Cyst and a Tumor?

A cyst is a sac-like pocket of tissue, filled with fluid, air, tissue, or other material that can form anywhere in the body. Cysts can be tiny or very large, and most cysts are benign (not cancerous). There are hundreds of different types of cysts that form for many different reasons, such as infections or blockages in ducts.

Tumors, also known as neoplasms, are generally solid masses of tissue that form from abnormal new growth of cells. Tumors and neoplasms can be described as benign, premalignant, or malignant (cancerous).

Types of benign tumors include adenomas, lipomas, hemangiomas, myomas, and fibroids. A benign tumor is not cancer, and usually does not spread to other parts of the body. However, some benign tumors can become malignant, and others, while remaining benign, can grow to a size that encroaches on important structures in the body and causes serious symptoms.

Pancreatic cancer cells. Most pancreatic cysts are benign, but some can be associated with pancreatic cancer.
Pancreatic cancer cells. Most pancreatic cysts are benign, but some can be associated with pancreatic cancer.

Most tumors are not cysts, and most cysts are not necessarily tumors, though in certain situations the terms may be used interchangeably.

People may notice cysts or tumors that they can feel on or near the surface of the body. Increasingly, cysts or tumors are discovered incidentally when someone has an MRI, CT scan, or ultrasound exam for another reason. For example, cysts or tumors may be detected in the liver, kidneys, or pancreas during an MRI scan of the abdomen.

Cysts can often be diagnosed by their appearance in an imaging scan, but further tests may be recommended. An example of a test involves draining the cyst’s fluid by means of a needle or catheter so that it can be analyzed by a pathologist. Many cysts don’t need to be removed; in some cases, if there is a suspicion of cancer, or if the cyst is very large and causing symptoms, surgical removal may be recommended.

Fluid-filled cysts in the pancreas are more frequently detected these days because the increasing use of abdominal imaging for other reasons. Most pancreatic cysts are benign, but some can be associated with pancreatic cancer. Because the procedure to remove precancerous cysts of the pancreas is a major, complex operation, many factors go into the decision about whether to recommend surgery for an individual’s cyst or whether to monitor it at regular intervals to detect “worrisome” changes that suggest the cyst is suspicious for a cancer. Experts in the Interdisciplinary Pancreatic Cystic Tumor (IMPACT) Clinic at Dana-Farber have extensive experience in evaluating patients with pancreatic cystic tumors.

In general, a person who notices a lump should have it examined if it is growing quickly, is associated with bleeding, pain, or tenderness, or is interfering with daily activities.