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 sievert in 1980, but never accepted the use of the rem. The US National Institute of Standards and Technology (NIST) recognizes that this unit is outside the SI, but temporarily accepts its use in the US with the SI. The rem remains in widespread use as an industry standard in the US. The United States Nuclear Regulatory Commission still permits the use of the units curie, rad and rem alongside SI units. The following table shows radiation quantities in SI and non-SI units: Roentgen equivalent man The roentgen equivalent man (or rem) is an older, CGS unit of equivalent dose,