Extreme variation in cancer incidence across
different tissues is well known; for example,
the lifetime risk of being diagnosed
with cancer is 6.9% for lung, 1.08% for
thyroid, 0.6% for brain and the rest of the
nervous system, 0.003% for pelvic bone and
0.00072% for laryngeal cartilage (1–3). Some of
these differences are associated with well-known
risk factors such as smoking, alcohol use, ultraviolet
light, or human papilloma virus (HPV)
(4, 5), but this applies only to specific populations
exposed to potent mutagens or viruses. And such
exposures cannot explain why cancer risk in
tissues within the alimentary tract can differ by
as much as a factor of 24 [esophagus (0.51%),
large intestine (4.82%), small intestine (0.20%),
and stomach (0.86%)] (3). Moreover, cancers of
the small intestinal epithelium are three times
less common than brain tumors (3), even though
small intestinal epithelial cells are exposed to
much higher levels of environmental mutagens
than are cells within the brain, which are protected
by the blood-brain barrier.