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CAUSALITY OF A GIVEN CANCER AFTER KNOWN RADIATION EXPOSURE 29 original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the retained, and some typographic errors may have been accidentally inserted. Please use the print version of this publication as the authoritative version for attribution. Under these circumstances, there is rarely any question that harm to normal tissues from radiation exposure has occurred. Even if no quantal response has appeared, agents other than radiation may be suspected to have aggravated the lesion. Physicians make this determination on the basis of information gathered from the patient history, physical findings, laboratory tests, and histopathological examination of tissues obtained by biopsy or autopsy. Thus, it is clear that judgments on the type and amount of harm, the probability of a quantal response, and the evaluation of cause and effect must lie in the province of the physician. A physician is also uniquely qualified to render an authoritative opinion on the separate roles of multiple suspected causative agents. The opinion would be stated as best professional judgment, although some quantitative assessment of the evidence may be presented in probabilistic terms (for example, "It is more probable than not," or "It is beyond reasonable doubt" that the expressed opinion is in fact correct). LOW-LEVEL EXPOSURE OF NORMAL POPULATIONS Low-level radiation exposure includes that from background radiation, medical diagnostic procedures (averaging perhaps 0.1 rads per year), and occupational exposures (averaging one-tenth or less of the maximum annual exposure limit of 5 rems). In this range, quantal responses are not encountered. In fact, any possible harm at the time of exposure is so slight as to be undetectable by the physician. Even if an excess of chromosome abnormalities in the blood should be found (unlikely with low-level exposure), this has not been shown to signify any radiation-induced disease or to presage a malignancy in that individual (Awa, 1975). The only quantal effects of consequence resulting from low-level exposure are the possibility of cancer in the person exposed or genetic defects in descendants of the exposed person. Of these two responses, the genetic defects are certainly caused by harm to a "target" in a single reproductive cell. If severe enough, this injury can cause a transformation (mutation) in the cell that otherwise continues to function apparently normally. If a cell so transformed becomes one of the two cells that unite to form a new individual, then one or more organs may show abnormalities. The hypothesis that many, if not most, types of cancer also can be initiated by alteration of a single cell is strongly supported, but not proved, by available evidence (Fialkow et al., 1967; Nowell, 1967; Fialkow et al., 1977; Gould et al., 1978; Land et al., 1983; Gould, 1984; Vogelstein et al., 1985). The monoclonal nature of a number of human tumors has been demonstratedâthat is, all cells in the tumor have been found to have identical genetic characteristics. This does not necessarily mean that only one cell was malignantly transformed. However, it is strong evidence that the tumor
