Neuroendocrine Tumors (NETs): A Plain-Language Overview

Neuroendocrine tumors, or NETs, start in an unusual kind of cell. These cells act like a hybrid of nerve cells and hormone-producing cells. These cells sit throughout the body. They are most dense in the lungs, the pancreas, and the digestive tract — especially the stomach, small intestine, and appendix. That is why NETs can turn up in so many different organs. Older medical writing often calls these tumors carcinoid tumors. That name is still common. Doctors increasingly prefer the term neuroendocrine tumor instead, since it is more accurate.

Most cancers are described mainly by stage. Stage means how far a cancer has spread. NETs are described just as much by grade. Grade means how fast the cells are dividing. This split matters. NETs can behave very differently, even at similar stages.

• Grade 1 cells look close to normal and divide slowly.
• Grade 2 falls in between grade 1 and grade 3.
• Grade 3 cells look highly abnormal and divide quickly.

Grades 1 and 2 are called well-differentiated tumors. The cells still resemble their tissue of origin. These tumors tend to grow slowly, sometimes over many years without causing problems. Grade 3 splits into two different things. One is a well-differentiated NET that has simply reached a high grade. The other is a separate, more aggressive category called neuroendocrine carcinoma. It behaves much more like a fast-growing cancer. This distinction changes treatment significantly. It is worth asking specifically which one applies to you.

Some NETs release hormones directly into the bloodstream, including a hormone called serotonin. When that happens, it can cause a cluster of symptoms called carcinoid syndrome:

• Redness or a feeling of warmth in the face and neck, often called flushing.
• Diarrhea, abdominal pain, or a bloated feeling.
• Wheezing or other trouble breathing.
• A fast heartbeat.
• Over time, damage to the heart valves, called carcinoid heart disease.

Carcinoid syndrome usually shows up only after a tumor has spread to the liver. The liver normally breaks down these hormones before they reach the rest of the body. Once the tumor sits in the liver itself, hormones can bypass that filter. They reach the bloodstream directly instead. That is why this specific pattern of symptoms signals spread, not just a random set of complaints.

Treatment depends heavily on grade, location, and whether the tumor is producing hormones. Surgery to remove the tumor is often possible. It can be curative for tumors found early. Some tumors are slow-growing, well-differentiated, and have already spread. For these, treatment can include drugs called somatostatin analogs. They block hormone release, which controls both tumor growth and carcinoid syndrome symptoms at once. Faster-growing tumors and neuroendocrine carcinomas are treated more like other aggressive cancers. Chemotherapy is often part of that plan. Because so many NETs grow slowly, monitoring with regular scans is sometimes the right choice, rather than immediate aggressive treatment. That is a very different mindset than most other cancers call for.

• What grade is my tumor, and is it well-differentiated or a neuroendocrine carcinoma?
• Is my tumor producing hormones, and do I have or am I at risk for carcinoid syndrome?
• Is surgery an option, or is monitoring the better approach right now?
• If I have carcinoid syndrome symptoms, what should prompt me to call you?

• American Cancer Society — What Is a Gastrointestinal Neuroendocrine Tumor?
• American Cancer Society — Neuroendocrine Tumors and Carcinoid Tumors
• National Cancer Institute — Gastrointestinal Neuroendocrine Tumors Treatment (PDQ), Patient Version