Roentgen (unit) One roentgen deposits of absorbed dose in dry air, or in soft tissue. One roentgen of X-rays may deposit anywhere from in bone depending on the beam energy. This tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the ionizing energy of a reference medium, which is ambiguous in the latest NIST definition. The roentgen has its roots in the Villard unit defined in 1908 by the American Roentgen Ray Society as "the quantity of radiation which liberates by ionisation one esu of electricity per cm of air under normal conditions
Roentgen equivalent man as the unit of equivalent dose in 1962 to measure the way different types of radiation distribute energy in tissue, and began recommending values of relative biological effectiveness (RBE) for various types of radiation. In practice, the unit of rem was used to denote that an RBE factor had been applied to a number which was originally in units of rad or roentgen. In 1977 the rem was redefined by the ICRP as 0.01 sievert or 0.01 J/kg, with the intention that the sievert would come to replace the rem. The International Committee for Weights and Measures (CIPM) adopted the