In this section you will learn:
• Cancer is not one disease, but likely more than 200 different types
of the disease.
• Changes in the genetic material in a cell underpin cancer
initiation and development in most cases.
• A cancer cell’s surroundings influence the development and
progression of disease.
• The development of cancer is a process that occurs over a period
of many years.
• The most advanced stage of cancer, metastatic disease, accounts for
more than 90 percent of cancer deaths.
imatinib, dasatinib, nilotinib,
bosutinib, and ponatinib —
block the abnormal protein
that causes most cases
of chronic myelogenous
What is Cancer?
At its simplest, cancer can be considered a disease in which normal cells start “behaving badly”,
multiplying uncontrollably, ignoring signals to stop, and accumulating to form a mass that is generally
termed a tumor (see Developing Cancer, p. 17).
Unfortunately, research has taught us that cancer is anything but simple.
First and foremost, there are perhaps as many as 200 different types of cancer, each named for the organ
or type of cell from which it originates. Moreover, cancer is complex at every level, from populations, to
individuals, to specific cancers, to the molecular and genetic defects that drive these cancers.
Despite cancer’s complexity, we are beginning to exploit our growing knowledge of the molecular changes
that generally drive cancer initiation and development for the benefit of patients, providing new ways to
reduce the burden of cancer (see sidebar on The Virtuous Cycle of Biomedical Research, p. 9).
The Origins of Cancer
An in-depth understanding of what happens when normal cells become cancerous is essential if we are to
answer the question: What is cancer?
We know that to keep our bodies healthy, most cells multiply or divide in a tightly controlled process to
replace old and damaged cells. Sometimes, this well-regulated process goes awry, and cells do not die
when they should or new cells form when they should not. These extra cells can accumulate, forming a
tumor. What upsets this delicately balanced system and causes cancer?