Embryonic Gut Derivation
Foregut Carcinoid Tumors
Located in the lungs, thymus, stomach, first part of the duodenum (small intestine), or the pancreas.
Gastric Carcinoids (carcinoids of the stomach) are further classified as:
• Type 1: Associated with chronic atrophic gastritis (inflammation of the stomach lining) caused by hypergastrinemia (high levels of the hormone gastrin).
• Type 2: Implicated with Zollinger Ellison Syndrome and MultipleEndocrine Neoplasia Type 1 (MEN-1).
• Type 3: Sporadic, not associated with hypergastrinemia, can cause Atypical Carcinoid S yndrome and are frequently malignant.
Pulmonary Carcinoids (carcinoids in the lungs) are further classified as:
• Typical Pulmonary Carcinoids (benign or low-grade malignant): Considered to be well-differentiated, commonly located in the center of the lungs, and rarely metastasize.
• Atypical Pulmonary Carcinoids (low-grade malignant): Poorly differentiated, commonly located in the periphery of lungs, characterized by frequent mitoses (cellular division), and frequently metastasize.
Midgut Carcinoid Tumors
Located in the small intestine, appendix, or right colon (large intestine).
Hindgut Carcinoid Tumors
Located in the transverse colon, sigmoid colon, or rectum.
Presence of Clinical Syndrome
A functioning carcinoid tumor secretes biochemically active substances such as hormones, which cause specific clinical syndromes such as Carcinoid Syndrome or Zollinger-Ellison syndrome.
A non-functioning carcinoid tumor secretes specific substances but these substances are either inactive and/or do not cause any clinical syndrome.
Inherited Versus Sporadic
Cancer causing mutations arise randomly
Cancer causing mutations are inherited due to MEN-1 or other familial factors
Carcinoid tumors can be familial or sporadic. Inherited carcinoid cancer refers to carcinoid cancer which is genetically inherited whereas sporadic carcinoid cancer has no hereditary basis. Carcinoid tumors are generally thought to be sporadic, except for a small proportion of which occur as a part of MEN (multiple endocrine neoplasia) syndromes. Other familial factors contribute to a small proportion of carcinoid tumors; these are less understood than MEN-1 causes.
Carcinoid tumors are graded based on how fast they are growing. The World Health Organization in 2017 updated the classification system based upon their clinical pathological criteria, regardless of origin, size, or anatomical extent of the tumor.
- Grade 1 (Low grade) well-differentiated
- Grade 2 (Intermediate grade) well-differentiated
- Grade 3 (High grade) poorly differentiated, occasionally well-differentiated
Differentiation is based on one measure called a Ki-67 index. The Ki-67 index calculates the number of cells that are dividing as a way to see if it is a slow-growing (well differentiated) or fast-growing tumor (poorly differentiated). Well-differentiated tumors have a Ki-67 index below 20% and poorly differentiated tumors have a Ki-67 index between 20-100%.