in This secTion You WiLL Learn:
• cancer is noT one disease; i T is
a coLLec Tion of man Y diseases
characTerized bY The uncon TroLLed
groWTh of ceLLs.
• changes in The gene Tic ma TeriaL in a
normaL ceLL underPin cancer ini TiaTion
and deveLoPmenT in mos T cases.
• a cancer ceLL’s surroundings
infLuence The deveLoPmen T and
Progression of disease.
• The mos T advanced s Tage of cancer,
me Tas Ta Tic disease, accoun Ts for more
Than 90 Percen T of cancer dea Ths.
• The more We KnoW abouT The bioLogY
of cancer, The more PreciseLY We can
Preven T, de Tec T, diagnose, and Trea T i T.
Cancer is not one disease; it is a collection of many diseases
that arise when the processes that control the multiplication
and life span of normal cells go awry.
As humans develop, we grow, through extensive cell
multiplication, from a single cell to an estimated 37. 2
trillion cells in an adult body ( 20). When a person matures,
the pace of cell multiplication slows. In adults, normal cells
primarily multiply only to replace cells that die either due
to exposure to a variety of external factors or naturally as a
result of normal cellular wear and tear, which is related to
the number of times the cell has multiplied.
When the processes that control the multiplication and
life span of normal cells go awry, the cells start multiplying
uncontrollably, fail to die when they should, and begin to
accumulate. In body organs and tissues, these cancerous
cells form a tumor mass, and in the blood or bone marrow,
they crowd out the normal cells.
Without medical intervention, over time, some cancerous
cells gain the ability to invade local tissues, and some
spread, or metastasize, to distant sites. The progression of
a cancer to metastatic disease is the cause of most cancer-related deaths.
Changes, or mutations, in the genetic material of cells are
the primary cause of cancer initiation and development.
Not all mutations contribute to cancer development, but the
greater the chance that a cell will acquire a mutation, the
greater the chance that the cell will acquire a mutation that
will cause cancer. The identity, order, and speed at which
a cell acquires genetic mutations determine the length of
time it takes for a cancer to develop and are influenced by
numerous interrelated factors (see sidebar on Why Me? Why
This Cancer? p. 19).
of cancer deaths are a result
of metastatic disease.
(Tcga) is an international
program started by the
national cancer institute and
national human genome
research institute in 2006 to
catalog the genetic mutations
associated with over 20
different cancer types.