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CAUSALITY OF A GIVEN CANCER AFTER KNOWN RADIATION EXPOSURE 28 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. sometimes used interchangeably to indicate the severity of biological damage (for example, the severity of harm may be used as a ''biological dosimeter''). Thus, the probability of a quantal response may be estimated in terms of either dose or severity of harm (see Table 1). When the physician evaluates the risk of a quantal response as a function of dose or severity of organ harm, the reference population used is quite different from that used in the public health subdisciplines (see Table 1). For the physician, the individuals in the population of reference all have the common characteristic that they have been harmed, and all manifest the particular illness or injury in varying degrees. If this population is divided into groups, each having nominally the same dose or severity of harm, one can evaluate the probability of a quantal response as a function of the dose or severity of harm. From this relationship can be seen the distribution of sensitivities and the amount of harm required to cause a quantal response in the average individual. If the illness is due to a particular harmful agent, then the population could be regarded as having been assaulted by that agent and could be subdivided into groups having received nominally the same amount of agent. Thus, the physician's assessment of the probability (risk) that any given individual will show a quantal response is based upon knowledge of the response to harm for the average individual in a population, modified by such considerations as age and sex, which may affect an individual's sensitivity. RADIOTHERAHY OR ACCIDENTAL HIGH-LEVEL RADIATION EXPOSURE Knowing the circumstances under which exposure to radiation may occur is critical to understanding the associated quantal response(s). Thus, for each circumstance, it is necessary to look at the steps that lead from exposure to a possible quantal response. Consider the circumstance in which organ harm is clearly present, which can occur only in cases of high-level exposure. Although such harm is seen almost exclusively in the radiotherapy of malignant tumors, it has been observed in accidents. Radiotherapy may involve large amounts of radiation, frequently thousands of rads delivered over relatively brief intervals (hours to weeks) to limited regions of the body. Because some normal tissues must be included in a radiotherapy beam, quantal responses may appear within days, weeks, or months, in the form of function-impairing lesions of the skin or deeper tissues. Although a few such responses are expected and unavoidable, questions about cause and effect remain, particularly if the observed response may have been aggravated or even caused by factors other than radiation, such as medication or injury from burns and other forms of trauma.
