Title:
An approach to assessing stochastic radiogenic risk in medical imaging.

Abstract:
This letter suggests a formalism, the medical effective dose (MED), that is suitable for assessing stochastic radiogenic risks in diagnostic medical procedures.
The MED is derived from radiobiological and probabilistic first principals, including: (1) The independence of radiation-induced biological effects in neighboring voxels at low doses; (2) the linear no-threshold assumption for stochastic radiation injury (although other dose-response relationships could be incorporated, instead); (3) the best human radiation dose-response data currently available; and (4) the built-in possibility that the carcinogenic risk to an irradiated organ may depend on its volume. The MED involves a dose-risk summation over irradiated voxels at high spatial resolution; it reduces to the traditional effective dose when every organ is irradiated uniformly and when the dependence of risk on organ volumes is ignored. Standard relative-risk tissue weighting factors can be used with the MED approach until more refined data become available.
The MED is intended for clinical and phantom dosimetry, and it provides an estimate of overall relative radiogenic stochastic risk for any given dose distribution. A result of the MED derivation is that the stochastic risk may increase with the volume of tissue (i.e., the number of cells) irradiated, a feature that can be activated when forthcoming radiobiological research warrants it. In this regard, the MED resembles neither the standard effective dose (E) nor the CT dose index (CTDI), but it is somewhat like the CT dose-length product (DLP).
The MED is a novel, probabilistically and biologically based means of estimating stochastic-risk-weighted doses associated with medical imaging. Built in, ab initio, is the ability to link radiogenic risk to organ volume and other clinical factors. It is straightforward to implement when medical dose distributions are available, provided that one is content, for the time being, to accept the relative tissue weighting factors published by the International Commission of Radiological Protection (ICRP). It requires no new radiobiological data and avoids major problems encountered by the E, CTDI, and CT-E formalisms. It makes possible relative inter-patient dosimetry, and also realistic intercomparisons of stochastic risks from different protocols that yield images of comparable quality.