Deterministic and Stochastic Effects

In radiobiology, most adverse health effects of radiation exposure are usually divided into two broad classes:

• Deterministic effects are threshold health effects, that are related directly to the absorbed radiation dose and the severity of the effect increases as the dose increases.
• Stochastic effects occur by chance, generally occurring without a threshold level of dose. Probability of occurrence of stochastic effects is proportional to the dose but the severity of the effect is independent of the dose received.

Deterministic Effects

In radiobiology, deterministic effects (or non-stochastic health effects) are health effects, that are related directly to the absorbed radiation dose and the severity of the effect increases as the dose increases. Deterministic effects have a threshold below which no detectable clinical effects do occur. The threshold may be very low (of the order of magnitude of 0.1 Gy or higher) and may vary from person to person. For doses between 0.25 Gy and 0.5 Gy slight blood changes may be detected by medical evaluations and for doses between 0.5 Gy and 1.5 Gy blood changes will be noted and symptoms of nausea, fatigue, vomiting occur.

Once the threshold has been exceeded, the severity of an effect increases with dose. The reason for the presence of this threshold dose is that radiation damage (serious malfunction or death) of a critical population of cells (high doses tend to kill cells) in a given tissue needs to be sustained before injury is expressed in a clinically relevant form. Therefore, deterministic effects are also termed tissue reaction. They are also called non-stochastic effects to contrast with chance-like stochastic effects (e.g. cancer induction).

Deterministic effects are not necessarily more or less serious than stochastic effects. High doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. Acute doses below 250 mGy are unlikely to have any observable effects. Acute doses of about 3 to 5 Gy have a 50% chance of killing a person some weeks after the exposure, if a person receives no medical treatment. Deterministic effects can ultimately lead to a temporary nuisance or also to a fatality. Examples of deterministic effects:

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

Lethal Doses of Radiation

The lethal dose of radiation (LD) is an indication of the lethal amount radiation. In radiation protection, the median lethal dose, LD_XY is usually used. For example, the dose of radiation expected to cause death to 50 % of the irradiated persons within 30 days is LD_50/30.  LD₁ is the dose expected to cause death to 1% of the irradiated persons, consequently, LD₉₉ is lethal for all (99%) persons irradiated. It is also very important, whether a person receives some medical treatment or not. The greater an acute radiation dose is, the greater is the possibility of it killing the individual. For a healthy adult, the LD₅₀ is estimated to be somewhere between 3 and 5 Gy.

• 2.5 Sv – Dose that kills a human with a 1% risk (LD₁), if the dose is received over a very short duration.
• 5 Sv – Dose that kills a human with a 50% risk within 30 days (LD_50/30), if the dose is received over a very short duration. Cause of death will be loss of bone marrow function.
• 8 Sv – Dose that kills a human with a 99% risk (LD₉₉), if the dose is received over a very short duration. At around 10 Gy, acute inflammation of the lungs can occur and lead to death.

The lethal dose data given above apply to acute gamma doses delivered in a very short time, e.g., a few minutes. More dose is required to produce the effects listed above, if the dose is received over a period of hours or longer.

Stochastic Effects

In radiobiology, stochastic effects of ionizing radiation occur by chance, generally occurring without a threshold level of dose. Probability of occurrence of stochastic effects is proportional to the dose but the severity of the effect is independent of the dose received. The biological effects of radiation on people can be grouped into somatic and hereditary effects. Somatic effects are those suffered by the exposed person. Hereditary effects are those suffered by the offspring of the individual exposed. Cancer risk is usually mentioned as the main stochastic effect of ionizing radiation, but also hereditary disorders are stochastic effects.

According to ICRP:

(83) On the basis of these calculations the Commission proposes nominal probability coefficients for detriment-adjusted cancer risk as 5.5 x 10^-2 Sv^-1 for the whole population and 4.1 x 10^-2 Sv^-1 for adult workers. For heritable effects, the detriment-adjusted nominal risk in the whole population is estimated as 0.2 x 10^-2 Sv^-1 and in adult workers as 0.1 x 10^-2 Sv^-1 .

Special Reference: ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4).

The SI unit for effective dose, the sievert, represents the equivalent biological effect of the deposit of a joule of gamma rays energy in a kilogram of human tissue. As a result, one sievert represents a 5.5% chance of developing cancer. Note that, the effective dose is not intended as a measure of deterministic health effects, which is the severity of acute tissue damage that is certain to happen, that is measured by the quantity absorbed dose.

There are three general categories of stochastic effects resulting from exposure to low doses of radiation. These are:

• Genetic effects. The genetic effect is suffered by the offspring of the individual exposed. It involves the mutation of very specific cells, namely the sperm or egg cells. Radiation is an example of a physical mutagenic agent. Note that, there are also many chemical agents as well as biological agents (such as viruses) that cause mutations. One very important fact to remember is that radiation increases the spontaneous mutation rate, but does not produce any new mutations.
• Somatic effects. Somatic effects are those suffered by the exposed person. The most common impact of irradiation is the stochastic induction of cancer with a latent period of years or decades after exposure. Since cancer is the primary result, it is sometimes called the carcinogenic effect. Radiation is an example of a physical carcinogenic, while cigarettes are an example of a chemical cancer causing agent. Viruses are examples of biological carcinogenic agents.
• In-Utero effects involve the production of malformations in developing embryos. However, this is actually a special case of the somatic effect, since the embryo/fetus is the one exposed to the radiation.

Somatic effects as a result of exposure to radiation are thought by most to occur in a stochastic manner. 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. This model is known as the linear no-threshold model (LNT). This model assumes, that there is no threshold point and risk increases linearly with a dose. 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. The LNT is not universally accepted with some proposing an adaptive dose–response relationship where low doses are protective and high doses are detrimental. It must be emphasized, that a number of organisations disagree with using the linear no-threshold model to estimate risk from environmental and occupational low-level radiation exposure.