Choice of the measuring quantity for therapy-level dosemeters.

With the introduction of SI units in radiology the quantity exposure will probably be abandoned for therapy level dosemeters because of the inconvenient conversion factors which have, in SI units, to be applied for the change into absorbed dose. Instead of exposure, a quantity is envisaged which, like absorbed dose, is measured in the unit gray (= joule per kilogram). A difficulty lies in the fact that in the immediate future only primary standards of exposure will be available, necessitating the introduction of conversion factors at the interface between primary and secondary standards or field instruments. Expert opinion differs as to whether a kerma quantity, a half-way stage to the desired situation, should be used, or whether the full step should be taken involving the calibration of field instruments in terms of 'absorbed dose in water under specified conditions'. This paper discusses the problem and recommends the full step.