Radiobiology

Radiobiology (also known as radiation biology) 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.[1]

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.[2] Radiation-induced cancer, teratogenesis, cognitive decline, and heart disease are all examples of stochastic effects.

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.[3] 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. Other stochastic effects of ionizing radiation are teratogenesis, cognitive decline, and heart disease.

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 one in 2,000.[4]

Deterministic

Deterministic effects are those that reliably occur above a threshold dose, and their severity increases with dose.[2]

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:

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"[5]

Phase Symptom Whole-body absorbed dose (Gy)
1–2 Gy 2–6 Gy 6–8 Gy 8–30 Gy > 30 Gy
Immediate Nausea and vomiting 5–50% 50–100% 75–100% 90–100% 100%
Time of onset 2–6 h 1–2 h 10–60 min < 10 min Minutes
Duration < 24 h 24–48 h < 48 h < 48 h N/A (patients die in < 48 h)
Diarrhea None None to mild (< 10%) Heavy (> 10%) Heavy (> 95%) Heavy (100%)
Time of onset 3–8 h 1–3 h < 1 h < 1 h
Headache Slight Mild to moderate (50%) Moderate (80%) Severe (80–90%) Severe (100%)
Time of onset 4–24 h 3–4 h 1–2 h < 1 h
Fever None Moderate 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 period 28–31 days 7–28 days < 7 days None None
Symptom Mild to moderate Leukopenia
Fatigue
Weakness
Moderate to severe Leukopenia
Purpura
Hemorrhage
Infections
Alopecia after 3 Gy
Severe leukopenia
High fever
Diarrhea
Vomiting
Dizziness and disorientation
Hypotension
Electrolyte disturbance
Nausea
Vomiting
Severe diarrhea
High fever
Electrolyte disturbance
Shock
N/A (patients die in < 48h)
Mortality Without care 0–5% 5–95% 95–100% 100% 100%
With care 0–5% 5–50% 50–100% 99–100% 100%
Death 6–8 weeks 4–6 weeks 2–4 weeks 2 days – 2 weeks 1–2 days
Table Source[6]

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 then it can be determined by differential measurements in the presence of electrical fields, magnetic fields, or 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.

Possible deterministic effects include of radiation exposure in pregnancy include miscarriage, structural birth defects, Growth restriction and intellectual disability.[7] The determinstistic 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 age Embryonic age Effects Estimated threshold dose (mGy)
2 to 4 weeks 0 to 2 weeks Miscarriage or none (all or nothing) 50 - 100[7]
4 to 10 weeks 2 to 8 weeks Structural birth defects 200[7]
Growth restriction 200 - 250[7]
10 to 17 weeks 8 to 15 weeks Severe intellectual disability 60 - 310[7]
18 to 27 weeks 16 to 25 weeks Severe intellectual disability (lower risk) 250 - 280[7]

The intellectual deficit has been estimated to be about 25 IQ-points per 1,000 mGy at 10 to 17 weeks of gestational age.[7]

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.[8]

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