Carcinoid Syndrome (cont.)
How common are carcinoid tumors and carcinoid syndrome?
The prevalence of carcinoid tumors is difficult to
determine since many carcinoid tumors are not detected because they produce no
symptoms. In one autopsy series, carcinoid tumors were estimated to have a
prevalence of 8/100,000 people annually, but 90% of the carcinoids found at
autopsy were not the cause of death.
The carcinoid syndrome is very rare. This is because
many carcinoid tumors do not produce hormones. Another reason that the syndrome
is rare is that hormones released by carcinoid tumors, particularly from tumors
within the abdomen, often are destroyed by the liver before they reach the
general circulation to cause
symptoms. For example, hormones produced by small intestinal carcinoid tumors
are released into the portal vein. The portal
vein blood passes through the
liver before reaching the heart and the general circulation. As the portal vein
blood passes through the liver, the hormones are destroyed by the liver.
Only those carcinoid tumors that can release hormones
directly into the general circulation (and not into the portal veins which pass
through the liver) can produce the carcinoid syndrome. Thus, the most common
cause of carcinoid syndrome is small intestinal carcinoid tumors that have
metastasized to the liver. The metastases in the liver can release hormones
directly into the circulation. Another rare example is carcinoid tumors of the
bronchial airways. Carcinoid tumors in the bronchial airways can release
hormones directly into the general circulation via the pulmonary veins without passing through the liver.
What is the prognosis and natural history of carcinoid tumors?
Carcinoid tumors can be benign (non-cancerous) or
malignant (cancerous). Benign carcinoid tumors are typically small (less than 1
cm). They usually can be removed completely and, in most cases, they do not come
back. Cells from benign carcinoid tumors do not spread to other parts of the
body. Benign carcinoid tumors typically produce no symptoms and are commonly
found by chance during flexible sigmoidoscopy or upper gastrointestinal
endoscopy.
Cancerous carcinoid tumors are typically large (larger
than 2 cm) at the time of diagnosis. Cells from these malignant tumors can invade and damage tissues
and organs near the tumor. Moreover, malignant cells can break away and enter
the bloodstream or lymphatic system and
spread to form new tumors in other parts of the body. (The distant tumors are
called metastases.) Common sites for carcinoid metastases include the lymph
nodes, liver, lung, bone, and skin.
Carcinoid tumors between 1.0 and 2.0 cm in size have approximately a 10%
chance of being cancerous at the time of diagnosis.
Carcinoid tumors are typically slow growing. They grow
much more slowly than other cancers such as colon, pancreas, liver, and lung
cancer. Many small carcinoid tumors produce no symptoms and are not fatal; they
are found incidentally at autopsy. Even patients with larger, malignant
carcinoid tumors (with or without metastasis) can survive years or decades with
a good quality of life. This is especially true with modern treatments to
control the carcinoid syndrome and to control the growth of the carcinoid
tumors. However, there is a rare form of carcinoid tumor called adenocarcinoids
that are more aggressive
than the typical malignant carcinoid tumor, and they have a poor prognosis. An
experienced pathologist can
identify adenocarcinoid tumors by examining the tissue from the tumor under a
microscope.
Next: Where do carcinoid tumors occur? »
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