SI units in medical physics

Abstract

Abstract Summary. The introduction of SI units in medical physics will present some difficulties, particularly in relation to radiological units. A short account is given of the history of those units with their emphasis on the measurement of energy absorbed in biological systems. The present unit of activity of a radioactive material, the curie, will be replaced in the SI system by the becquerel (s−l). Difficulties will arise from the very small size of this unit, both in medicine and nuclear engineering. The use of a multiple prefix is often necessary and leads to a rather cumbersome nomenclature. Some examples of the use in practice of the new unit are given. The present unit of absorbed dose will be replaced by the gray (J kg−1). Both joule and kilogram with suitable prefixes are very acceptable units and the change has some clear advantages. A major problem arises in the necessity for a change by a factor of 100 in the statement of dose in clinical radiological practice. In the statement of dose-rate the obvious SI unit (W kg−1) is dangerous in that the unit of time is not stated explicitly. It is essential that this be done since irradiation may occur in complex time patterns over periods varying from seconds to years. A short account is given of problems arising from the complexity of biological response to the same energy absorbed from different kinds of radiation, as for example alpha, beta, gamma rays, neutrons, protons, or mesons. Finally, it is maintained that in spite of some dangers and difficulties and little clinical advantage it is essential that SI units be gradually adopted so that physics as applied to medicine remains in the closest possible contact with its parent subjects, physics, chemistry and biology.