[en] There is a continuing need to understand dose-response relationships for ionizing radiation in order to protect the health of the public and nuclear workers from undue exposures. However, relatively few human populations have been exposed to doses of radiation high enough to cause observable, long-term health effects from which to derive dose-response relationships. This is particularly true for internally deposited radionuclides, although much effort has been devoted to epidemiological studies of the few types of exposures available, including lung cancers in uranium miners form the inhalation of the radioactive decay products of Ra, liver cancers in patients injected with Thorotrast X-ray contrast medium containing Th, bone cancers in radium dial painters who ingested Ra, and bone cancers in patients who received therapeutic doses of Ra. These four types of exposures to internally deposited radionuclides provide a basis for understanding the health effects of many other radionuclides for which a potential for exposure exists. However, potential exposures to internally deposited radionuclides may differ in many modifying factors, such as route of exposure, population differences, and physical, chemical, and elemental forms of radionuclides. The only means available to study many of these modifying factors has been in laboratory animals, and to then extrapolate the results to humans. Three conclusions can be drawn from this example