About
one-third of all patients admitted to U.S. hospitals are diagnosed or treated
using radioisotopes. Most major hospitals have specific departments dedicated
to radiation medicine. In all, about 112 million nuclear medicine or radiation
therapy procedures are performed annually, with the vast majority used in
diagnoses. Radioactive materials used as a diagnostic tool can identify the
status of a disease and minimize the need for surgery, reducing the risks from
postoperative infection.
Radioisotopes
give doctors the ability to "look" inside the body and observe soft
tissues and organs, in a manner similar to the way x-rays provide images of
bones. Radioisotopes carried in the blood also allow doctors to detect clogged
arteries or check the functioning of the circulatory system. Some chemical
compounds concentrate naturally in specific organs or tissues in the body. For
example, iodine collects in the thyroid while various compounds of
technetium-99m* (Tc-99m) collect in the bones, heart, and other organs. Taking
advantage of this proclivity, doctors can use radioisotopes of these elements
as tracers. A radioactive tracer is chemically attached to a compound that will
concentrate naturally in an organ or tissue so that a picture can be taken. The
process of attaching a radioisotope to a chemical compound is called labeling.
To
detect problems within a body organ, doctors use radio-pharmaceuticals or
radioactive drugs. Radioisotopes that have short half-lives are preferred for
use in these drugs to minimize the radiation dose to the patient. In most
cases, these short-lived radioisotopes decay to stable elements within minutes,
hours, or days, allowing patients to be released from the hospital in a
relatively short time.
The
radioisotope used in about 80 percent of nuclear diagnostic procedures is
Tc-99m. The penetrating properties of its gamma rays and its short (6-hour)
half-life help reduce risk to the patient from more prolonged radiation
exposure.
Because
of their short half-lives, certain radio-pharmaceuticals must be produced,
shipped to the hospital, and then used within a couple of weeks. Short-lived
radionuclides such as Tc-99m, gallium-67, and thallium-201 are often used to
diagnose the functioning of the heart, brain, lung, kidney, or liver. For
example, Tc-99m is used to diagnose osteoporosis, a condition caused by calcium
deficiency in older people, especially women.
To
evaluate the presence of heart disease, a radioisotope is injected into a
patient's bloodstream while he or she is exercising on a treadmill. The
radioisotope travels toward the heart, allowing doctors to follow the blood
flow on a screen. While looking at the image, doctors can check for reduced
blood flow through the arteries, a possible signal of heart disease.
Nuclear imaging is also used to evaluate brain function. Organic
radio-chemicals are labeled with F-18 and then injected into the bloodstream. A
device called a gamma camera detects radiation emitted from
the organ, displaying an image that can enable the physician to detect
blockages or other dysfunctional activity.
For
some diagnostic tests, the patient need not come into contact with
radioactivity at all. The tests are performed on blood or other fluids taken
from the patient, using a procedure called radio-immunoassay. These tests can
detect some diseases by identifying and measuring the amounts of hormones,
vitamins, enzymes, or drugs in the body.
The
same property that makes radiation hazardous can also make it useful in helping
the body heal. When living tissue is exposed to high levels of radiation, cells
can be destroyed or damaged so they can neither reproduce nor continue their
normal functions. For this reason radioisotopes are used in the treatment of
cancer (which amounts to uncontrolled cell division). Although some healthy
tissue surrounding a tumor may be damaged during the treatment, mostly
cancerous tissue can be targeted for destruction.
A
device called a teletherapy unit destroys malignant tumors
with gamma radiation from a radioisotope such as cobalt-60 (Co-60). Teletherapy
units use a high-energy beam of gamma rays to reduce or eradicate tumors deep
within the body. These units are licensed by the NRC because they use byproduct
material that is produced only by a nuclear reactor.
Another
treatment, called brachytherapy, destroys cells by-placing the radioisotope (in
the form of a seated source) directly into the tumor. Generally, two techniques
are used for this type of treatment: (1) direct, manual implantation of a
radiation source by a physician or (2) automated implantation using a device
called a remote afterloader. The NRC as well as Agreement
States license these brachytherapy devices. Using these devices, a small, thin
wire or sealed needle containing radioactive material, such as iridium-192
(Ir-192) or iodine-125 (1-125), is inserted directly into the cancerous tissue.
The radiation from the isotope attacks the tumor as long as the device is in
place. When the treatment is complete, long-lived material (Ir- 1 92) is
removed, but short-lived radioisotopes (1-125) may be left permanently. This
technique is used frequently to treat mouth, breast, lung, and uterine cancer.
Brachytherapy
and teletherapy procedures are performed only in hospitals or clinics by
trained medical personnel. Strict controls and safety requirements set by the
NRC or the Agreement States must be followed. For example, treatment rooms must
have adequate shielding to prevent scattered radiation from penetrating into an
adjacent room. Radiation monitors must be used and patients carefully observed
at all times during treatment.
Many
types of cancer, such as Hodgkin's disease (cancer of the lymph glands) and
cancers of the cervix, larynx, and skin, can be treated by radiation alone.
Boron capture neutron therapy has been used on a trial basis recently to treat
potentially fatal brain cancer. In this procedure, the diseased brain tissue incorporates
a neutron-absorbing isotope and then is exposed to neutron radiation
originating from a nuclear research reactor. The energy and radiation emitted
as a result of the neutron activation slow down the growth of cancer cells and,
in some cases, completely kill them.
The
overall objectives of NRC's safety rules for radiation medicine are to ensure
that patients receive only the exposure medically prescribed and that the
radiation is delivered in accordance with the physician’s instructions.
NRC
regulations require that physicians and physicists have special training and
experience to practice radiation medicine. The training emphasizes safe
operation of' nuclear-related equipment and accurate record-keeping.
When
using radiation as a medical treatment, the physician weighs the potential
benefits against the risk of side effects. Intense radiation exposure often
destroys tumors that would prove fatal, but side effects such as hair loss,
reduced white blood cell count, and nausea can be sever and must be monitored
carefully.