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In modern times, radiation has an important place in medicine. Radiation is used in cancer treatment, as well as imaging techniques such as PET scans and X-Rays. When used in a controlled, well understood manner, nuclear medicine is highly safe and effective. One of the oldest uses of radioactive compounds is as tracers. Radioactive isotopes are injected, ingested, or otherwise introduced into the body. Their passage throughout the body can then be measured by observing the activity from their decay. How the body processes these elements provides a variety of useful information; one of the most well known uses of radioactive elements is the use of Iodine-131 in diagnosis and treatment of thyroid illnesses. Thorotrast was one of the earliest radioactive tracer compounds. A suspension of thorium oxide, Thorotrast was first developed in the early 1930s, a time when radiation was not as well understood as today. Thorium is notable for its high ability to absorb X-rays. Areas where the body concentrated thorium, such as tumors, would show up well on X-Rays following the ingestion of Thorotrast. Its long biological half life also made it useful. Additionally, while not perfectly safe, thorium is far less toxic than other heavy metals such as lead or uranium. However, Thorium-232, the most common isotope, is an alpha emitter. Alpha particles (essentially helium-4 nuclei) have very little penetration, thanks to their high electric charge. But the massive size of the alpha particle (compared to other charged particles such as electrons) gives it a high momentum and the ability to cause massive damage if it gets inside the body. According to some studies, alpha particles have anywhere from twice to 100 times the relative biological effectiveness of fast neutrons. Notably, Polonium-210, favored weapon of the FSB, is an alpha emitter. As Thorotrast accumulated in various locations in the body, those areas would receive massive radiation doses. And since thorium has a long half life, these effects went unnoticed initially. It is estimated that some patients received doses of up to 3,000 rads in the liver (where Thorotrast would concentrate). This is about eight times the (whole body) dose needed to cause severe ARS. Studies showed significantly higher cancer risks for patients who received Thorotrast; The use of Thorotrast was discontinued in the 1940s and 1950s once the risks were realized. However, thousands were exposed. While the exact death toll is unknown, it is probable that hundreds or even thousands of additional cancers were caused by Thorotrast usage. https://www.researchgate.net/publication/12838805_Thorotrast-induced_liver_cancer_results_of_the_German_thorotrast_study