Carcinoid: Definition

Carcinoid is an older term for a type of slow-growing cancer that arises from neuroendocrine cells — specialized cells found throughout the body that release hormones and chemical signals. In modern pathology, this tumor type is more commonly called a well-differentiated neuroendocrine tumor (NET). The two terms refer to the same general category of tumor, and you may see either or both used in your pathology report, depending on the organ involved and which naming convention your pathologist follows. The term “carcinoid” is still widely used for lung tumors, where it has a specific diagnostic meaning, but is used less often in other locations.

Where do carcinoid tumors develop?

Carcinoid tumors can develop anywhere neuroendocrine cells are found. The most common sites are the gastrointestinal tract — particularly the small intestine, appendix, rectum, and stomach — and the lungs. Less commonly, they arise in the pancreas, thymus, and other organs. Despite the older perception that carcinoids are primarily lung tumors, the majority occur in the gastrointestinal tract.

What does a carcinoid tumor look like under the microscope?

When a pathologist examines a carcinoid tumor under the microscope, the cells typically appear very uniform — similar in size and shape. They are arranged in organized clusters, nests, ribbons, or gland-like structures. One of the most characteristic features is the appearance of the nucleus: the genetic material (chromatin) is evenly dispersed in a fine, granular pattern that pathologists describe as “salt and pepper” — resembling tiny dark dots scattered on a pale background. This nuclear pattern is a hallmark of neuroendocrine differentiation and is seen across all well-differentiated neuroendocrine tumors regardless of location.

Additional tests using immunohistochemistry (IHC) are usually performed to confirm the neuroendocrine nature of the cells. Markers such as chromogranin and synaptophysin are characteristically positive in carcinoid tumors.

Are carcinoids cancerous?

Yes — all carcinoid tumors are considered malignant (cancerous) because they have the potential to spread to other parts of the body, including lymph nodes and the liver. However, most carcinoids are slow-growing, and many patients live for years or even decades after diagnosis, particularly when the tumor is small and low-grade. The risk of spread depends on the location, size, and grade of the tumor.

How are carcinoid tumors graded?

Grading describes how quickly tumor cells divide and how aggressively the tumor is likely to behave. Pathologists assess two key features:

Mitotic count — the number of cells actively dividing per defined area of tissue.
Ki-67 index — a laboratory test that measures the percentage of tumor cells actively growing. A higher Ki-67 percentage indicates a faster-growing tumor.

Based on these features, well-differentiated neuroendocrine tumors are classified as grade 1, grade 2, or grade 3. In the lung specifically, the terminology differs: low-grade tumors are called typical carcinoid and intermediate-grade tumors are called atypical carcinoid, based on the mitotic count and the presence or absence of necrosis.

What is carcinoid syndrome?

Some carcinoid tumors — particularly those in the gastrointestinal tract — produce hormones such as serotonin that can cause a distinct set of symptoms known as carcinoid syndrome. Symptoms include facial flushing, diarrhea, abdominal cramping, and wheezing. Importantly, carcinoid syndrome almost always occurs only when the tumor has spread to the liver, because the liver normally breaks down the hormones before they can circulate in the blood. If the tumor is confined to the gastrointestinal tract without liver metastases, carcinoid syndrome is very uncommon.

Questions to ask your doctor

Is my tumor referred to as a carcinoid or a neuroendocrine tumor — and what is the difference in my case?
What grade is my tumor, and what does this mean for its likely behavior?
Has the tumor spread, and what is the recommended treatment?