The use of radiation can be documented as far back as a hundred years ago when X rays were first used to treat a cancer patient in December of 1895, four weeks after their discovery by Rontgen. In March of 1896 only a few months later Henri Berquearel discovered radioactivity. Less than two years after that discovery, Marie and Pierre Curie discovered the existence of two radioactive substance polonium and radium. The benefits of these discoveries are still being researched and improved upon today, a century later.

With the rise of radiobiology there came such theories as the target theory. Which has provided a quantitative treatmen

Radiobiology is an extensive field that branches off into many directions. From the study of radiation and radioactivity to radiophysics and radiotherapy. It will always be growing due to new advancement and methods and fewer changing due to new problems arising to new solutions being discovered.

No one cal tells when a radioactive nucleus will decay. But in even a small amount of matter there are millions of nuclei. After a certain length of time half of them have decayed. This length of time is called the half-life of raduim-226 is 1,620 years. While the half-life of other substance may be a fraction of a second per millions of years.

Along with radiodiagnosis and nuclear medicine, radiotherapy forms the third branch of radiology. It was founded in 1896, but the discovery of radium in 1903 gave it new dimensions. The progress made over the past years has made it one of the principal treatments for cancer. In any effective cancer program radiotherapy by itself or in association with chemotherapy and surgery is needed for over half the cancer patients.

The target theory states that the biological effects of radiation's such as X-rays from ionization by individual photons of radiation that are absorbed at sensitive points (targets) in a cell. It is theorized that to produce a given effect there must be one or more hits on a target. Ionization of a target molecule of genetic material produces a direct effect on the constitution of the cell, which may be passed on to the cell's progeny.

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