R

radiation therapy The use of ionizing electro-
magnetic energy particles or waves to destroy can-
cer cells. Radiation is the emission of energy in a
pattern of rays, such as visible light. Ionizing radi-
ation is a form of energy of sufficient intensity to
alter the electronic charge of atoms and the struc-
ture of molecules, such as ultraviolet light. At high
exposure, such alteration changes the structures
of cells. The effects of ionizing radiation on cellu-
lar structure can cause as well as treat cancer.
The substances that contain the ionizing energy
particles are radioactive isotopes, also called
radionuclides or radioisotopes, most of which
occur in the natural environment though scien-
tists can cultivate them in the laboratory for con-
sistency and ready availability. As the radioisotope
disintegrates it releases radiation at a known rate,
which allows the radiation oncologist to deter-
mine the appropriate exposure times and frequen-
cies.

RADIOISOTOPES USED IN RADIATION THERAPY
cesium-137 (^137 Cs) cobalt-60 (^60 Co)
gold-198 (^198 Au) iodine-125 (^125 I)
iodine-131 (^131 I) iridium-192 (^192 Ir)
palladium-103 (^103 Pd) phosphorus-32 (^32 P)
radium-226 (^226 Ra) yttrium-90 (^90 Y)

The radiation oncologist determines which
radioisotope or type of radiation to use based on
the type of cancer, its location in the body, and
how extensively the tumor has spread. Some
types of radiation are more effective for penetra-
tion to tumors deep within the body and others
are more effective for treating tumors close
beneath the surface of the SKIN. How rapidly the
radioisotope dissipates is important for internal
radiation therapy in which the radiation oncolo-

gist implants radioactive pellets into the body to

directly expose the cancer tumor to the radiation.

How Radiation Therapy Works to Treat Cancer

A key characteristic of cancer cells is that they

divide rapidly and without much organization.

Radiation therapy works by damaging the DNA

within the cells, which prevents them from divid-

ing. Though all cells in the body are vulnerable to

such damage, radiation therapy selectively targets

the tumor, limiting the exposure of healthy cells to

the radiation. Because normal cells do not divide as

rapidly as cancer cells and they divide in an organ-

ized process, they are able to recover from the radi-

ation exposure. The exceptions are the cells of

fast-growing tissues such as skin and HAIR, which

may experience some damage as a result of radia-

tion therapy. The radiation oncologist structures

shields and blocks to protect healthy tissue from the

radiation as much as possible.

A fundamental premise of radiation therapy is

fractionation—dividing a lethal DOSEof radiation

into numerous sublethal doses administered over

a period time. The standard protocol for radiation

therapy in the United States delivers the fraction-

ated dosage of radiation daily five days a week for

two to nine weeks. Depending on the cancer and

the capabilities of the treatment facility, some radi-

tion therapy protocols use radioisotopes and deliv-

ery methods that allow fewer doses administered

over a shorter time and that vary the intensity of

radiation according to the type and location of the

cancer.

Types of Radiation Therapy

There are two basic types of radiation therapy,

external beam and internal radiation. Some peo-

ple receive both external beam and internal radia-

tion therapy, depending on the type and location

388