Radiation Absorbed Dose

Units:
During the early days of radiological experience, there was no precise unit of radiation dose that was suitable either for radiation protection or for radiation therapy. For purposes of radiation protection, a common “dosimeter” used was a piece of dental film with a paper clip attached. A daily exposure great enough just to produce a detectable shadow, called a “paper-clip” unit, was considered a maximum permissible dose. For greater doses and for therapy purposes, the dose unit was frequently the “skin erythema unit.” Because of the great energy dependence of these dose units as well as other inherent shortcomings, neither of these two units could be biologically meaningful or useful either in the quantitative study of the biological effects of radiation or for radiation safety purposes.
Furthermore, since the fraction of the energy in a radiation field that is absorbed by the body is energy dependent, it is necessary to distinguish between radiation exposure and radiation absorbed dose.

Gray & rad:
Although the gray is the SI and newer unit and has replaced the rad in literature and most countries, the rad and its derivatives nevertheless continue to be useful units and are used in the official radiation safety regulations in the United States. It is important to understand that radiation absorbed dose concept, the gray and the rad, is a macroscopic construct and is not intended for microdosimetry on the cellular or subcellular levels. Radiation absorbed dose has been found to be correlated with biomedical
effects on the tissue, organ, and organism levels and thus is appropriate for radiation safety measurements and for medical diagnostic and therapeutic uses of radiation. The radiation absorbed dose concept implies that the absorbed energy is uniformly distributed throughout the entire mass of the tissue of interest. On the cellular and subcellular levels that are of interest to molecular biologists, the biological effects are proportional to the number and types of intramolecular bonds that are broken rather than to the concentration of absorbed energy within the cell. On the tissue level, the number of such intramolecular breaks in the tissue is proportional to the radiation absorbed dose. The distinction between microdosimetry and radiation absorbed dose may be illustrated with the following thought experiment.Although the gray is the SI and newer unit and has replaced the rad in literature and most countries, the rad and its derivatives nevertheless continue to be useful units and are used in the official radiation safety regulations in the United States.
It is important to understand that radiation absorbed dose concept, the gray and the rad, is a macroscopic construct and is not intended for microdosimetry on the cellular or subcellular levels. Radiation absorbed dose has been found to be correlated with biomedical effects on the tissue, organ, and organism levels and thus is appropriate for radiation safety measurements and for medical diagnostic and therapeutic uses of radiation. The radiation absorbed dose concept implies that the absorbed energy is uniformly distributed throughout the entire mass of the tissue of interest. On the cellular and subcellular levels that are of interest to molecular biologists, the biological effects are proportional to the number and types of intramolecular bonds that are broken rather than to the concentration of absorbed energy within the cell. On the tissue level, the number of such intramolecular breaks in the tissue is proportional to the radiation absorbed dose. The distinction between microdosimetry and radiation absorbed dose may be illustrated with the following thought experiment.