Difficulties in estimating the effect of low radiation doses in man

Any reliable estimation of the frequency with which low doses of ionising radiation cause cancer requires a detailed study of records from very large exposed populations, over long periods of time following exposure, and with an exact comparison of cancer frequencies with those of equally large or larger populations which have not been similarly exposed, but in which the cancer incidences or mortalities have been ascertained with equal efficiency. Special problems occur in finding appropriate comparison groups for occupationally exposed workers and for patients treated or investigated radiologically. These are discussed, together with the difficulties in ensuring comprehensive recording of total cancer incidence, rather than of mortality.