Carcinoid

Carcinoid is the most common primary small bowel neoplasm, occurring most often in the distal ileum. Pathologically, it arises from enterochromaffin cells in the crypts of Lieberkühn. It is a hormonally active tumor, secreting, most notably, serotonin, in addition to a host of other hormones. Serotonin is broken down to 5-HIAA by the liver, which is secreted in the urine. This allows for urine 5-HIAA to act as a tumor marker.

All carcinoid tumors are POTENTIALLY malignant (but not all are malignant). Histology cannot predict malignant potential (i.e. “benign” and “malignant” tumors can appear identical histologically). Thus, malignancy is based on invasion and metastasis. Tumors less than 1cm in diameter are usually asymptomatic and rarely metastasize. Tumors between 1-2cm metastasize about 50% of the time. Tumors larger than 2cm metastasize about 90%. Regional metastasis is usually to the mesentery or regional lymph nodes.

Metastatic carcinoid possesses three very interesting characteristics, not seen in other neoplasms. First, metastases are usually LARGER than the primary tumor. Second (as discussed below), carcinoid frequently metastasizes to the mesentery, where often calcifies and causes a local desmoplastic response, leading to the pathognamonic appearance of a “stellate, calcified mesenteric mass.”  Third, because the tumor is hormonally active, metastasis to the liver leads to carcinoid syndrome.

Carcinoid syndrome is a costellation of clinical signs, incuding flushing, diarrhea, and bronchospasm. It should be emphasized that carcinoid sydrome can only occur after intestinal carcinoid has metastasized to the liver. This is because the hormones that cause the signs of carcinoid syndrome are broken down in the liver. The venous drainage of the bowel is via the portal vein, which drains to the liver.  Metastases within the liver bypass this portal drainage and empty into the systemic circulation, where they are not broken down, leading to symptomatology.

The “Rule of 1/3’s” is a useful way to remember the properties of carcinoid tumors:

•           1/3 of primary small bowel tumors

•           1/3 occur in small bowel (> 90% in ileum)

•           1/3 are multiple

•           1/3 have other primary malignancies

The clinical presentation of carcinoid is nonspecific (unless liver metastases have occurred, causing carcinoid syndrome).  Enlarging tumor may lead to ulceration and intussusception, with intermittent obstruction and bleeding.