Epidemiology Radiation-induced cancer

increased risk of solid cancer dose a-bomb survivors

the associations between ionizing radiation exposure , development of cancer based on lss cohort of japanese atomic bomb survivors, largest human population ever exposed high levels of ionizing radiation. cohort exposed high heat, both initial nuclear flash of infrared light , following blast due exposure firestorm , general fires developed in both cities respectively, survivors underwent hyperthermia therapy various degrees. hyperthermia, or heat exposure following irradiation known in field of radiation therapy markedly increase severity of free-radical insults cells following irradiation. presently no attempts have been made cater confounding factor, not included or corrected in dose-response curves group.

additional data has been collected recipients of selected medical procedures , 1986 chernobyl disaster. there clear link (see unscear 2000 report, volume 2: effects) between chernobyl accident , unusually large number, approximately 1,800, of thyroid cancers reported in contaminated areas, in children.

for low levels of radiation, biological effects small may not detected in epidemiological studies. although radiation may cause cancer @ high doses , high dose rates, public health data regarding lower levels of exposure, below 10 msv (1,000 mrem), harder interpret. assess health impacts of lower radiation doses, researchers rely on models of process radiation causes cancer; several models predict differing levels of risk have emerged.

studies of occupational workers exposed chronic low levels of radiation, above normal background, have provided mixed evidence regarding cancer , transgenerational effects. cancer results, although uncertain, consistent estimates of risk based on atomic bomb survivors , suggest these workers face small increase in probability of developing leukemia , other cancers. 1 of recent , extensive studies of workers published cardis, et al. in 2005 . there evidence low level, brief radiation exposures not harmful.

modelling

alternative assumptions extrapolation of cancer risk vs. radiation dose low-dose levels, given known risk @ high dose: supra-linearity (a), linear (b), linear-quadratic (c) , hormesis (d).

the linear dose-response model suggests increase in dose, no matter how small, results in incremental increase in risk. linear no-threshold model (lnt) hypothesis accepted international commission on radiological protection (icrp) , regulators around world. according model, 1% of global population develop cancer result of natural background radiation @ point in lifetime. comparison, 13% of deaths in 2008 attributed cancer, background radiation plausibly small contributor.

many parties have criticized icrp s adoption of linear no-threshold model exaggerating effects of low radiation doses. cited alternatives “linear quadratic” model , “hormesis” model. linear quadratic model viewed in radiotherapy best model of cellular survival, , best fit leukemia data lss cohort.

in 3 cases, values of alpha , beta must determined regression human exposure data. laboratory experiments on animals , tissue samples of limited value. of high quality human data available high dose individuals, above 0.1 sv, use of models @ low doses extrapolation might under-conservative or over-conservative. there not enough human data available settle decisively of these model might accurate @ low doses. consensus has been assume linear no-threshold because simplest , conservative of three.

radiation hormesis conjecture low level of ionizing radiation (i.e., near level of earth s natural background radiation) helps immunize cells against dna damage other causes (such free radicals or larger doses of ionizing radiation), , decreases risk of cancer. theory proposes such low levels activate body s dna repair mechanisms, causing higher levels of cellular dna-repair proteins present in body, improving body s ability repair dna damage. assertion difficult prove in humans (using, example, statistical cancer studies) because effects of low ionizing radiation levels small statistically measured amid noise of normal cancer rates.

the idea of radiation hormesis considered unproven regulatory bodies. if hormesis model turns out accurate, conceivable current regulations based on lnt model prevent or limit hormetic effect, , have negative impact on health.

other non-linear effects have been observed, particularly internal doses. example, iodine-131 notable in high doses of isotope less dangerous low doses, since tend kill thyroid tissues otherwise become cancerous result of radiation. studies of very-high-dose i-131 treatment of graves disease have failed find increase in thyroid cancer, though there linear increase in thyroid cancer risk i-131 absorption @ moderate doses.