BIOLOGICAL EFFECTS OF RADIATION

Radiation can cause biological damages on cells either by indirect and direct action. If radiation falls on a human body, it produces moving electrons. The electrons cause further ionization, excitation, resulting in chemical and molecular changes . Radiation can also produce free radicals, which are unpaired electrons that are chemically reactive.For example, radiation can interact with water molecule and produce hydroxyl (OH) and hydrogen (H) radicals. These free radicals can further interact with DNA, RNA or protein molecule and cause damage tissue. As a result, the constituents of the body cell will be affected, there by impairing its normal functions. Since human body consists of 70% water, most of the radiation damage is caused by water, especially by OH radicals (indirect action). Presence of oxygen may enhance the effect of radical damage. Chromosome breaks and aberrations are examples of biological damage caused by radiation. Chromosome aberrations include single break, ring formation, translocation, and dicentric, etc. It occurs spontaneously and scoring of human lymphocytes is used as biological dosimeter. The minimum whole body radiation that can be detected by this method is 250 mGy and 400 mGy for acute and chronic exposures. Radiation may induce structural changes in a DNA molecule that includes (i) hydrogen bond break, (ii) molecular breakage, and (iii) inter and intra molecular cross linking. Hydrogen bond break disturbs the base pairs such as adenine-thymine, resulting in genetic changes. Molecular breakage may involve single strand break and double strand breaks. Single strand breaks are mostly repairable. There are enzymes within cells that will repair the radiation damage. Double strand break are irreparable and cause loss of base or change of base called mutation,This results in chromosome aberrations, leading to carcinogenesis. High LET radiations are capable of causing double strand break. Since the DNA molecules carry the genetic code, its damage can result hereditary effects.

                                     High dose of radiation can cause cell death (deterministic effect) or cellular transformation (stochastic effect). Lymphoid tissue and rapidly proliferating tissues, such as spermatids and bone marrow stem cells are relatively radiosensitive. Nerve cells are the least radio sensitive cells. The effects of radiation on tissue depend on a number of factors such as type of radiation, dose, dose rate, dose ratiocination, cell cycle and presence of radio-protectors and radiosensitizers, etc. An acute dose, delivered in a short time, is more harmful then a chronic dose, delivered over a period of time. The radiation effects which manifest themselves soon after the irradiation are called early effects. Those effects that manifest after a period of time are called late effects.

DETERMINISTIC EFFECTS

The harmful effects of radiation maybe classified as deterministic and

stochastic effects. A deterministic effect is one in which “severity increases with increasing absorbed dose”. These effects appear at high doses > 0.5 Gy and generally result from cell death. Deterministic effects are characterized by threshold dose, below which the effect does not occur . Deterministic effects include skin erythema, epillation, organ atrophy, fibrosis, cataract induction, blood changes and reduction in sperm count, etc. Radiation accidents can cause, deterministic effects and unlikely to occur at medical X-ray procedures.

STOCHASTIC EFFECT

A stochastic effect is one in which “the probability of occurrence increases with increasing absorbed dose rather than its severity”. The stochastic effects are further classified into somatic and genetic effects. Stochastic means random and the severity of stochastic effect is independent of the radiation dose. Radiation induced cancer and

hereditary effects come under this category. It has no threshold dose, may occur even at low doses . Hence, the safety goal is to keep the

exposures as low as reasonably achievable. Stochastic effect is

the primary risk from low level radiations such as diagnostic

X-rays. Stochastic risks are independent of sex and age at the time

of exposure.

ACUTE RADIATION SYNDROME

Whole body radiation exposures involving high level radiations delivered in shorter interval can cause acute radiation syndromes. It is a group of syndromes occurring in stages over a period of days to week. It includes hematopoietic syndrome, gastrointestinal syndrome and cerebrovascular syndrome. Soon after the radiation, early symptoms appears, known as prodromal radiation syndrome. Whole body doses > 100 Gy may cause death in 24–48 hours, which is known as cerebrovascular syndrome. A dose level of 5–12 Gy may cause death in days, is called gastrointestinal syndrome. Dose of 2.5–5 Gy may cause death in weeks to months, called hematopoietic syndrome. The dose that causes 50% death over a specified time (60 days) is called lethal dose and is expressed in LD50/60, which is about 4 Gy for humans.