INTRODUCTION Radiation is one of the few, health sciences that spans the entire range from constituting an ubiquitous environmental agent of concern, the control of cancer. if not the only agent of interest in the to being an effective therapeutic agent for These characteristics place the former in the realm of public health including accident statistics and epidemiology (Ph); latter in the discipline of pharmacology, toxicology, the and medicine (Md). The same sets of characteristics that separate low-level exposure (LLE) to radiation from high-level exposure (HLE) require that the primary independent variable be the amount of exposure to agent~carrying objects (charged particles) in the environment of cells for the first; but mean dose to the organ or other cell system for the second. The basic radiation quantities and units in current use and defined by the ICRU (J) were developed during that era in which a central theme was ‘therapeutic uses and thus early acute effects on an organ or a tumor: clearly in the Md realm. Thus, the description and quantification of these effects of HLE could, and still can be comfortably accommodated by those quantities and units adopted early during this period. variable was, and continues depends The principal to be organ or tumor absorbed dose, on which the fraction of organs or tumors responding quantally (1.e., an all-or-nothing change of state, from one of functional, to essentially permanent dysfunction or death). However, this state of affairs was not achieved without considerable discussion and disagreements about how the “amount” or quantity of radiation was to be defined. In the physicist's eye, this quantity was either the number of energy-carrying particles per unit area per unit time flowing from the source, or alternatively the total energy flow from a source, per unit area, i.e., either the particle or total energy fluence, or a parameter of these variables. standpoint, However, from the physician's these quantities expressing the strength of either the 4 radiation source or field were considered to be irrelevant: what mattered . ¢: was that energy actually absorbed in tissue. Se In fact, the “skin erythema dose” unit of radiation "amount" had already been invented and used, which 1,. by-passed any physical measurement beyond the duration of time spent in & iny Dns radiation field calibrated against such a “biological dosimeter”. > -204- S0124bb