Medical Application of Nuclear
Energy
By Paik Nam-sun, M.D., Ph.D.
Korea Cancer Center Hospital

Nuclear energy is very useful and advantageous for medical application, for example, cancer diagnosis, treatment and confirmation of cancer recurrences. Of course, nuclear energy can induce carcinogenesis. During World War II nuclear bombs (fatman and little boy in Hiroshima and Nagasaki in Japan) initiated many thyroid cancer and leukemia.
The first use of radiation for medical purposes started with an accident. In 1898 Marie and Pierre Curie discovered an element that gave off radiation naturally and called it radium. Henri Becquerel, another pioneer in the new field of radiation, was carrying a piece of radium in his vest pocket, and it caused a severe burn on his chest. It was this burn that led to exploration of how this radiant energy interacts with human cells, and how it could help treat diseases. Soon scientists were using both X rays and radium to treat skin cancer. The first report of a patient being cured by radiation was presented in 1899.

How Radiation Is Produced
Most modern radiation therapy departments today use a high-energy form of X rays produced by a special machine called a linear accelerator. This machine accelerates electrons, which then bounce off a metal target and produce X rays. Faster electrons have higher energy and produce higher-energy X rays.
Linear accelerators also produce electron beams for radiation therapy. (Electrons are not photons and cannot penetrate the body as deeply as X rays can.) The electron beam is produced by re-moving the metal target from the path of the accelerated electrons. This kind of radiation therapy is often used for skin cancer. Elec-tron beam "boosts" are sometimes useful for delivering additional radiation to the "tumor bed" (the area where the tumor was growing) following a course of penetrating photon radiation.
Radiation therapy can also use other parts of the atom. "Heavy particle" radiation uses the neutrons or protons (which are found in the center of the atom) and are much larger than electrons. These heavy particles are produced by a machine called a cyclotron. "Proton beam" therapy uses a more precise beam than an X ray beam. "Neutron beam" therapy uses a beam with different characteristics than X rays and may have advantages for tumors resistant to standard X rays. Neutrons have been used to treat prostate and salivary gland tumors.
There are other kinds of photons that are also used in radiation therapy. Gamma rays behave like X rays but are produced by radioactive elements such as radium and cesium.

Dividing the Dose-Fractionation
The goal of radiation treatment is to deliver the radiation beam at a dose that will prevent tumor cells from growing, but will allow normal cells to recover. The total dose of radiation will not be given all at once. A fraction of the total dose will be given with each treatment. This is called fractionation. A course of treat-ment usually requires two to eight weeks (with treatments five days a week). This adds up to ten to forty treatments over that period of time.
Fractionation has a number of advantages. First, a small dose of radiation given each day allows the normal cells in the sur-rounding area to recover. Second, since cells are more sensitive to the damaging effects of radiation when they are dividing, small daily treatments provide more opportunities to catch the frequently dividing cancer cells at that vulnerable time. Third, cancer cells are more vulnerable to the damaging effects of radiation when they have a good supply of oxygen carried by the blood.
As the tumor shrinks over the weeks of treatment, the remaining tumor cells can get more blood, more oxygen, and thus become more sensitive to the effects of radiation.

Radiation Delivery
when the source of radiation is outside your body, it is called external beam irradiation. You are not radioactive. The radiation passes through you. It does not stay in the tumor, your blood, or any of your secretions.
The radiation oncologist, who specializes in the use of radiation to treat cancer, plans your treatment carefully. He or she uses diagnostic scans and computers to plan the beam arrangement to deliver most of the radiation to the tumor and avoid normal cells in the vicinity. This allows very precise placement of the beams.
When the source of the radiation is placed within your body, it is called brachytherapy. This kind of radiation treatment uses placement of the radiation source close to the tumor. Radioactive elements such as cesium, iridium, iodine, gold, phosphorus, and palladium have been used for brachytherapy.
When this type of therapy was first developed, the radioactive sources were put in the form of pellets, wires, or ribbons. A specific applicator was used to fit to the part of the body being treated, and then the radioactive source was inserted by the radiation oncologist. Treatment usually required two or more days, during which the patient was said to by "loaded", or radioactive, since the radioactive source was temporarily inside the body. The patient had to remain in the hospital isolated from other patients. This treatment was called low-dose brachytherapy.
In the last fifteen years high-dose (rate) brachytherapy (HDR) has largely replaced low-dose (rate) brachytherapy. HDR uses a highly active source, such as iridium, that gives off a large amount of radiation over a short time. This treatment may only take ten minutes instead of several days. The iridium source is programmed by a computer to be placed in the body then withdrawn automatically. This can be done as an outpatient in the radiation therapy department. Like other kinds of radiation, the total dose is divided so that a fraction is given each day. That allows time for normal tissue to recover between treatments. HDR is widely used for treatment of cancers of the cervix, endometrium, prostate, lung, esophagus, head, and neck. nw