radiobiology radiobiology radiobiology (also known as radiation biology, and uncommonly as actinobiology) is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation. ionizing radiation is generally harmful and potentially lethal to living things but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. its most common impact is the induction of cancer with a latent period of years or decades after exposure. high doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. controlled doses are used for medical imaging and radiotherapy. health effects in general, ionizing radiation is harmful and potentially lethal to living beings but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. most adverse health effects of radiation exposure may be grouped in two general categories: deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; and stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells. stochastic some effects of ionizing radiation on human health are stochastic, meaning that their probability of occurrence increases with dose, while the severity is independent of dose. radiation-induced cancer, teratogenesis, cognitive decline, and heart disease are all stochastic effects induced by ionizing radiation. its most common impact is the stochastic induction of cancer with a latent period of years or decades after exposure. the mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial. the most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. if this linear model is correct, then natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. quantitative data on the effects of ionizing radiation on human health is relatively limited compared to other medical conditions because of the low number of cases to date, and because of the stochastic nature of some of the effects. stochastic effects can only be measured through large epidemiological studies where enough data has been collected to remove confounding factors such as smoking habits and other lifestyle factors. the richest source of high-quality data comes from the study of japanese atomic bomb survivors. in vitro and animal experiments are informative, but radioresistance varies greatly across species. the added lifetime risk of developing cancer by a single abdominal ct of 8 msv is estimated to be 0.05%, or 1 in 2,000. deterministic further information: acute radiation syndrome deterministic effects are those that reliably occur above a threshold dose, and their severity increases with dose. high radiation dose gives rise to deterministic effects which reliably occur above a threshold, and their severity increases with dose. deterministic effects are not necessarily more or less serious than stochastic effects; either can ultimately lead to a temporary nuisance or a fatality. examples of deterministic effects are: acute radiation syndrome, by acute whole-body radiation radiation burns, from radiation to a particular body surface radiation-induced thyroiditis, a potential side effect from radiation treatment against hyperthyroidism chronic radiation syndrome, from long-term radiation. radiation-induced lung injury, from for example radiation therapy to the lungs cataracts and infertility. the us national academy of sciences biological effects of ionizing radiation committee "has concluded that there is no compelling evidence to indicate a dose threshold below which the risk of tumor induction is zero". phase symptom whole-body absorbed dose (gy) 1–2 gy 2–6 gy 6–8 gy 8–30 gy > 30 gy immediatenausea and vomiting5–50%50–100%75–100%90–100%100% time of onset2–6 h1–2 h10–60 min< 10 minminutes duration< 24 h24–48 h< 48 h< 48 h— (patients die in < 48 h) diarrheanonenone to mild (< 10%)heavy (> 10%)heavy (> 95%)heavy (100%) time of onset—3–8 h1–3 h< 1 h< 1 h headacheslightmild to moderate (50%)moderate (80%)severe (80–90%)severe (100%) time of onset—4–24 h3–4 h1–2 h< 1 h fevernonemoderate increase (10–100%)moderate to severe (100%)severe (100%)severe (100%) time of onset—1–3 h< 1 h< 1 h< 1 h cns function no impairment cognitive impairment 6–20 h cognitive impairment > 24 h rapid incapacitation seizures, tremor, ataxia, lethargy latent period28–31 days7–28 days< 7 daysnonenone illness mild to moderate leukopeniafatigueweakness moderate to severe leukopeniapurpurahemorrhageinfectionsalopecia after 3 gy severe leukopeniahigh feverdiarrheavomitingdizziness and disorientationhypotensionelectrolyte disturbance nauseavomitingsevere diarrheahigh feverelectrolyte disturbanceshock — (patients die in < 48h) mortalitywithout care0–5%5–95%95–100%100%100% with care0–5%5–50%50–100%99–100%100% death6–8 weeks4–6 weeks2–4 weeks2 days – 2 weeks1–2 days table source by type of radiation when alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest. this is due to the high relative biological effectiveness of alpha radiation to cause biological damage after alpha-emitting radioisotopes enter living cells. ingested alpha emitter radioisotopes such as transuranics or actinides are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous than an equivalent activity of beta emitting or gamma emitting radioisotopes. if the radiation type is not known, it can be determined by differential measurements in the presence of electrical fields, magnetic fields, or with varying amounts of shielding. external dose quantities used in radiation protection. see article on sievert on how these are calculated and used. in pregnancy the risk for developing radiation-induced cancer at some point in life is greater when exposing a fetus than an adult, both because the cells are more vulnerable when they are growing, and because there is much longer lifespan after the dose to develop cancer. if there is too much radiation exposure there could be harmful effects on the unborn child or reproductive organs. research shows that if there is more than 1 scan in 9 month it can harm your unborn child. possible deterministic effects include of radiation exposure in pregnancy include miscarriage, structural birth defects, growth restriction and intellectual disability. the deterministic effects have been studied at for example survivors of the atomic bombings of hiroshima and nagasaki and cases where radiation therapy has been necessary during pregnancy: gestational ageembryonic ageeffectsestimated threshold dose (mgy) 2 to 4 weeks0 to 2 weeksmiscarriage or none (all or nothing)50 - 100 4 to 10 weeks2 to 8 weeksstructural birth defects200 growth restriction200 - 250 10 to 17 weeks8 to 15 weekssevere intellectual disability60 - 310 18 to 27 weeks16 to 25 weekssevere intellectual disability (lower risk)250 - 280 the intellectual deficit has been estimated to be about 25 iq points per 1,000 mgy at 10 to 17 weeks of gestational age. these effects are sometimes relevant when deciding about medical imaging in pregnancy, since projectional radiography and ct scanning exposes the fetus to radiation. also, the risk for the mother of later acquiring radiation-induced breast cancer seems to be particularly high for radiation doses during pregnancy. measurement the human body cannot sense ionizing radiation except in very high doses, but the effects of ionization can be used to characterize the radiation. parameters of interest include disintegration rate, particle flux, particle type, beam energy, kerma, dose rate, and radiation dose. the monitoring and calculation of doses to safeguard human health is called dosimetry and is undertaken within the science of health physics. key measurement tools are the use of dosimeters to give the external effective dose uptake and the use of bio-assay for ingested dose. the article on the sievert summarises the recommendations of the icru and icrp on the use of dose quantities and includes a guide to the effects of ionizing radiation as measured in sieverts, and gives examples of approximate figures of dose uptake in certain situations. the committed dose is a measure of the stochastic health risk due to an intake of radioactive material into the human body. the icrp states "for internal exposure, committed effective doses are generally determined from an assessment of the intakes of radionuclides from bioassay measurements or other quantities. the radiation dose is determined from the intake using recommended dose coefficients". absorbed, equivalent and effective dose the absorbed dose is a physical dose quantity d representing the mean energy imparted to matter per unit mass by ionizing radiation. in the si system of units, the unit of measure is joules per kilogram, and its special name is gray (gy). the non-si cgs unit rad is sometimes also used, predominantly in the usa. to represent stochastic risk the equivalent dose h t and effective dose e are used, and appropriate dose factors and coefficients are used to calculate these from the absorbed dose. equivalent and effective dose quantities are expressed in units of the sievert or rem which implies that biological effects have been taken into account. these are usually in accordance with the recommendations of the international committee on radiation protection (icrp) and international commission on radiation units and measurements (icru). the coherent system of radiological protection quantities developed by them is shown in the accompanying diagram. organizations the international commission on radiological protection (icrp) manages the international system of radiological protection, which sets recommended limits for dose uptake. dose values may represent absorbed, equivalent, effective, or committed dose. other important organizations studying the topic include international commission on radiation units and measurements (icru) united nations scientific committee on the effects of atomic radiation (unscear) us national council on radiation protection and measurements (ncrp) uk public health england us national academy of sciences (nas through the beir studies) french institut de radioprotection et de sûreté nucléaire (irsn) european committee on radiation risk (ecrr) the stage of radiation depends on the stage the body parts are affected exposure pathways external a schematic diagram showing a rectangle being irradiated by an external source (in red) of radiation (shown in yellow) a schematic diagram showing a rectangle being irradiated by radioactive contamination (shown in red) which is present on an external surface such as the skin; this emits radiation (shown in yellow), which can enter the animal's body external exposure is exposure which occurs when the radioactive source (or other radiation source) is outside (and remains outside) the organism which is exposed. examples of external exposure include: a person who places a sealed radioactive source in his pocket a space traveller who is irradiated by cosmic rays a person who is treated for cancer by either teletherapy or brachytherapy. while in brachytherapy the source is inside the person it is still considered external exposure because it does not result in a committed dose. a nuclear worker whose hands have been dirtied with radioactive dust. assuming that his hands are cleaned before any radioactive material can be absorbed, inhaled or ingested, skin contamination is considered to be external exposure. external exposure is relatively easy to estimate, and the irradiated organism does not become radioactive, except for a case where the radiation is an intense neutron beam which causes activation. by type of medical imaging effective dose by medical imaging type target organsexam typeeffective dose in adultsequivalent time of background radiation ct of the headsingle series2 msv8 months with + without radiocontrast4 msv16 months chestct of the chest7 msv2 years ct of the chest, lung cancer screening protocol1.5 msv6 months chest x-ray0.1 msv10 days heartcoronary ct angiography12 msv4 years coronary ct calcium scan3 msv1 year abdominalct of abdomen and pelvis10 msv3 years ct of abdomen and pelvis, low dose protocol3 msv1 year ct of abdomen and pelvis, with + without radiocontrast20 msv7 years ct colonography6 msv2 years intravenous pyelogram3 msv1 year upper gastrointestinal series6 msv2 years lower gastrointestinal series8 msv3 years spinespine x-ray1.5 msv6 months ct of the spine6 msv2 years extremitiesx-ray of extremity0.001 msv3 hours lower extremity ct angiography0.3 - 1.6 msv5 weeks - 6 months dental x-ray0.005 msv1 day dexa (bone density)0.001 msv3 hours pet-ct combination25 msv8 years mammography0.4 msv7 weeks internal internal exposure occurs when the radioactive material enters the organism, and the radioactive atoms become incorporated into the organism. this can occur through inhalation, ingestion, or injection. below are a series of examples of internal exposure. the exposure caused by potassium-40 present within a normal person. the exposure to the ingestion of a soluble radioactive substance, such as 89sr in cows' milk. a person who