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 (unit) the accompanying text. In the meantime, the new concept of roentgen equivalent man (rem) had been developed. Starting in 1957, the ICRP began to publish their recommendations in terms of rem, and the roentgen fell into disuse. The medical imaging community still has a need for ionization measurements, but they gradually converted to using C/kg as legacy equipment was replaced. The ICRU recommended redefining the roentgen to be exactly 2.58 × 10 C/kg in 1971. In 1971 the European Economic Community, in Directive 71/354/EEC, catalogued the units of measure that could be used "for ... public health ... purposes". The