comms hidnekiSMEAOAarmani Oo -10- | as . decreased with mitotic conpetence, then estimates of carcinogenesis based on a fictitious averaging of a local inhomogeneous dose over 4 much larger volume would be necessarily conservative. Since radiation carcinegensis can, and in. fact, does increase to anomalously large values while the mitotic -ompetence bacomes vanishlingly small, the. fictitious averaging of dose over larger volumes is not necessarily ‘conservative. Instead it would appear that an intense local dose of ionizing radiation can be a more efficient carcinogen than a diffuse tissue exposure with the same typ2 of ionizing radiation and the same total energy. The above then implies that averaging of dose over larger volumes may be far from conservative. It is obvious that as a local exposure becomes more intense, a stage must finally be reached where the carcinogenic efficiency of the exposure (con a per unit energy basis) is reduced. to previous arguments. This is not pertinent It would, however, be important to know the characteristics of the most carcinogenicly efficient exposures. The following excerpt taken from the BEIR report (p. 95) summarizes the state of knowledge concerning the causation of cancer (emphasis added): "Although the mechanisms of carcinogenesis, or of vadiztion carcinogenesis in particuiar, are not fully Known, available information implies that most, if not ali, types of cancer develop as a resuit of the combined effects of multiple factors. These causative factors may include: prezygotic (inherited) mutations of chromosomal aberrations, which can spread during develop- ment to many kinds of cells; somatic cell mutations or chromosomal aberrations, which can be acquired at any time after conception; changes resulting from the action of viruses; and changes in systemic growth factors (e.g., depressed immune competence, hormonal imbalance) and in local tissue requlation (disorcanization,damage), Such as may result Tron diseases other than cancer or from advancing age (1). “Altncugh point mutations, chronosanal abborations, and other chances at the celluler and molecular Icvel may require only sual] coses, tissue cisoraanization end .