Radiobiology

Radiation interaction with water is the principal radiation interaction in the body. However, the ultimate damage is to the target molecule deoxyribonucleic acid (DNA), which controls cellular metabolism and reproduction. DNA is the most sensitive target.

In vitro irradiation- macromolecules are irradiated outside the body or living cell.

In vivo irradiation- macromolecules are irradiated inside a living cell.

Main-chain scission- the breakage in the long chain molecule that divides a long, single molecule into many molecules.

Some macromolecules have spurs. These spurs can be extended as a consequence of irradiation and as a result will attach to a neighboring molecule or a segment of the same molecule. This process is called cross-linking.

Molecular cross-linking increases the viscosity of a macromolecular solution.

The irradiation of macromolecules can also result in disruption of single chemical bonds, producing point lesions- changes that result in impairment, or loss of function.

All metabolic functions occur in the cytoplasm which include anabolism and catabolism.

Catabolism- the breaking down of nutrient molecules into energy for the cell.

Anabolism- the production of large molecules for form and function.

LD 50/30 - the lethal radiation dose required to kill 50% of a population in 30 days.

The range for LD 50/30 in humans is 250-450 R. This figure will be higher if medical treatment is available.

LD 50/60 – the whole body dose that will result in death within 60 days to 50% of the irradiated subjects. The LD 50/60 for humans is estimated to be approximately 350 R. With medical treatment, humans can tolerate higher doses.

LD 90/60 – indicates a 90% lethality within 60 days.

Mean survival time- the average time between exposure and death.

Linear, non-threshhold- a dose-response relationship that intersects the dose axis at or below zero.

Linear, threshold- a dose-reponse relationship that intercepts the dose axis at a value greater than zero.

Photodisintegration occurs above 10 MeV. In phototdisintigration, a high-energy photon is absorbed by the nucleus.

Carbohydrates make up about 1% of the cell, provide most of the cell’s energy, and are composed of carbon, hydrogen and oxygen.

Erythema- a sunburn-like reddening of the skin.

Epilation- the loss of hair.

Tissue is more sensitive to radiation when irradiated in the oxygenated, or aerobic, state than when irradiated under anoxic (without oxygen) or hypoxic (low oxygen) conditions.

A radiation dose relationship is a mathematic relationship between various radiation dose levels and the magnitude of the observed response.

Genetic cells or germ cells are the oogonium of the female and the and spermatogonium of the male which replicate by meiosis. All other cells in the body are somatic cells which replicate by mitosis.

A solution causing a cell to swell is called hypotonic.

A solution causing a cell to shrink is called hypertonic.

Half value layer (HFV) is the amount of attenuator necessary to remove half of the photons from the source of radiation. It is usually associated with filtration of the x-ray beam.

Pair production occurs at energy levels above 1.02 MeV. In this interaction, the photon approaches and interacts with the nucleus of an atom. The energy of the photon is the converted into two particles (E=mc2) Two electrons are produced with this interaction. One has a negative charge and is called a negatron. The other particle is a positive electron, or positron. Positrons are a form of antimatter and do not exist freely in nature. Positrons cannot exist near matter and will interact very quickly with the first electron they encounter. They interact with and destroy each other while converting their matter back into energy in the process Both particles disappear releasing two photons with an energy of 0.51 meV. This is known as an annihilation reaction. These gamma photons further interact with matter through either pair production or Compton scatter.

Increasing atomic number increases the probability of pair production

Nuclear detonations can produce large numbers of blast, burn, and projectile injuries that initially must be managed by individuals trained in critical first aid procedures.

The pocket dosimeter (pocket ionization chamber) is most often used for visitors to the x-ray department or when a short-term reading is necessary such as emergency first responders in a radiation contaminated area.

An x-ray technologist has an ethical obligation to act in the best interest of the patient.

As for patients, the risks of exposure are outweighed by the greater benefit of an accurate diagnosis or the successful treatment of a disease or condition.

The maximum permissible dose for radiation workers is set at 5 rem per year.

After about 5000 rad to the whole body, the central nervous system syndrome results. Death usually results in hours, although it may take several days.

After the prodomal syndrome, which lasts up to one day, a latent period of 3 to 5 days results.

Linear energy transfer (LET)- the way x-rays travel through tissue creating a track of ionized molecules. X radiation in the diagnostic range have a low of 1 LET.

Relative biologic effectiveness (RBE)- the ratio of the energy required to produce a given biologic effect.

As LET increases, so does RBE.

Low LET radiation such as x- and gamma radiation have a low RBE.

High LET radiation such as alpha and beta particles have a high RBE.

Deterministic effect- increases in severity with dose, and a threshold is assumed.

In the photoelectric effect, all of the energy of the incoming photon is totally transferred to the atom. As a result, the photon no longer exists. The incoming electron interacts with an orbital electron.

Direct effects of irradiation directly ionize a macromolecule. Physical or chemical changes can result.

Indirect effects of irradiation- interacts with water in the cell causing the formation of free radical (a chemical compound that damages the cell).

Although DNA is the most sensitive target, radiolysis of water is the primary effect from radiation exposure to a cell. When ionizing radiation interacts with water, the water is ionized, producing an ion pair HOH+ and e-. The electron produced may reconnect with the HOH+ ion or it may attach itself to another uncharged water molecule creating a free radical.

Free radicals contain a single unpaired electron piggybacking in their outer shell making them chemically unstable.

Messenger RNA (mRNA)- carries the code for specific amino acid sequences from the DNA to structures in the cytoplasm for protein synthesis.

Transfer RNA (tRNA)- transfers amino acid groups to the ribosome for protein synthesis.

Ribosomal RNA (rRNA) exists in the ribosomes and is thought to assist in protein synthesis.

1 rad x 0.01 = Grays
1 rem x 0.01 = sieverts

The cell membrane is composed of lipids and proteins in a flexible structure.

The function of the cell nucleus is to contain the genetic and metabolic information of the cell.

Chromosomes are linear threads in the nucleus of a cell. Chromosomes are composed of proteins and deoxyribonucleic acid (DNA).

Lipids or fats make up about 2% of a cell on the average. They are not soluble in water but are soluble in certain solvents such as alcohol, ether, oil, and chloroform.

Compton scattering involves partial absorption of the incident x-ray photon by an outer shell electron. The electron absorbs enough energy to break the binding energy bond, and is ejected while the remaining photon energy exits the atom. The ejected electron is called a Compton or recoil electron.

BERT- Background equivalent radiation time- compares x-ray exposure with natural background radiation.

Acute radiation lethality follows a nonlinear, threshold dose-response relationship.

Lymphocytes and spermatogonia are the most radiosensitive cells in the body.

Genetic significant dose (GSD) is a measure of the genetic exposure to the population from diagnostic and other forms of ionizing radiation. It indicates the genetic load on a population.

Ethics are not rules of behavior. They involve general guidelines that translate into practice.

Coherent scattering also called classical or Thompson scatter occurs primarily with low energy x-rays (below 10 KeV). The photon causes excitation rather than ionization of the target atom. This excess energy is given off as scatter in a different, but usually forward, direction in the form of an x-ray photon. The exiting photon has the same energy and wavelength as the incident photon.

Lead should not be used as shielding for beta-emitting radionuclides used in nuclear medicine. Placing a beta source in a lead shield causes the production of bremsstrahlung radiation, which results from the deceleration of beta particles as they approach the nuclei of the lead. As the beta particles slow down, they lose energy that is emitted in the form of x-radiation. Therefore, placing a beta-emitting source in a lead shield can actually increase the amount of radiation emitted.

Law of Bergonie and Tribondeau- Stem cells are radiosensitive; mature cells are radioresistant. Younger tissues and organs are radiosensitive. Tissues with high metabolic activity are radiosensitive. A high proliferation rate for cells and a high growth rate for tissues result in increased radiosensitivity.

The immediate response of radiation sickness is the prodomal period occuring within hours of exposure and lasting a day or two.

The latent period is the time after exposure during which there are no outward sign of radiation sickness.

Manifest illness- the dose related period characterized by three separate syndromes: hematologic, GI, and CNS

*Radition sickness level I
0-50 R
Less than half the people exposed to this much radiation experience nausea and vomiting within 24 hours. Afterwards, some people may tire easily, but otherwise there are no further symptoms.

*Radiation sickness level II
200-450 R
More than half the people exposed to 200-450 R experience nausea and vomiting and are ill for a few days. This illness is followed by a period of one to three weeks when there are few if any symptoms (a latent period). At the end of this latent period more than half of those exposed experience loss of hair. A moderately severe ill ness develops which is often characterized by sore throat. Radiation damage to blood producing organs results in a loss of white blood cells, increasing the chance of illness from infections. Most of the people in this group need medical care, but more than half will survive without treatment.

*Radiation sickness level III
450-600R
Most of the people exposed to 450-600 R experience severe nausea and vomiting and are very ill for several days. The latent period is shortened to one or two weeks. The main episode of illness which follows, is characterized by severe bleeding from the mouth, throat, and skin, as well as loss of hair. Infections such as sore throat, pneumonia, and enteritis are common. People in this group need intensive medical care and fewer than half will survive in spite of the best care.

*Radiation sickness level IV
600-1000R
This level produces an accelerated version of the illness described in level III. All the people in this group begin to experience severe nausea and vomiting. Without treatment, this condition can continue for several days until death. Death can occur within two weeks without appearance of bleeding or loss of hair. It is highly unlikely, even with extensive medical care, that many can survive.

*Radiation sickness level V
Several thousand R
Symptoms of rapidly progressing shock come on almost as soon as the dose has been received. Death occurs in a period of a few hours to a few days.

*source: FEMA CPG 2-6.4, September 23, 1983- Radiation safety in shelters

Chapter 2
EMPLOYMENT CONSIDERATIONS
WEAPONS EFFECTS AND THE COMBAT ENVIRONMENT
Nuclear weapons add significantly to the physical and psychological environment of combat. They cause intense, violent effects which severely affect unit movement, employment, and protection. Commanders at all levels must understand the operational and tactical implications of the nuclear environment and its effect on operations. The basic effects of a nuclear detonation are blast, thermal radiation, residual ionizing radiation, initial radiation, and electromagnetic pulse (EMP). These effects can destroy or neutralize targets as well as impair, through physical injury, the operational capability of personnel. Flash blindness, radiation sickness, eardrum rupture, and second-degree burns are some of the injuries persons might experience. Weather, terrain, surface conditions, and man-made structures modify nuclear-weapons effects. Also, conditions existing naturally on the battlefield at any given moment can enhance or mitigate such effects.

Blast
The blast wave (static overpressure and dynamic pressure) from a nuclear air burst mostly causes material damage. Surface and subsurface bursts generally produce less air-blast damage and more cratering. Most data on blast effects describe blasts as observed on flat or gently rolling terrain. There is no quick and simple method for calculating changes in blast pressures in hilly, mountainous, or forested terrain. In general, compared to the same distance on flat terrain, pressures are greater on the forward slopes of steep hills and lower on reverse slopes. Line-of-sight (LOS) shielding is not dependable; blast waves can bend or diffract around obstacles. Hills may decrease dynamic pressure and offer some local protection from flying debris. However, small hills or folds in the ground are considered negligible for target analysis. Wooded hills lessen dynamic pressure, but do not significantly affect overpressure. Wooded hills will also produce significant wood splintering, tree blowdown, and forest fires. The reflecting nature of a surface over which a weapon detonates significantly influences the distance to which blast effects extend. Smooth, reflecting surfaces such as ice, snow, sand, moist soil, and water reflect most of the blast energy, maximizing its effects. Conversely, surfaces with thick, low, combustible vegetation; dry soils with sparse vegetation; and desert sand minimize such effects. Built-up areas do not significantly affect a blast wave’s effects. And, even though urban structures may provide some local shielding from flying debris, they can also increase pressures by channeling a blast wave. Weather conditions also affect blast damage. Rain and fog lessen the force of the blast wave by increasing air density and moisture. These conditions help dissipate the energy of the blast wave as it moves through the heavier air.

Thermal Radiation
A fireball’s intense heat possesses high thermal energy that, as thermal radiation, is transmitted from the point of detonation over a wide area. Thermal radiation travels at wavelengths from ultraviolet to infrared. The atmosphere absorbs some of the ultra-violet radiation; therefore, the prime source of thermal radiation is the infrared. Thermal radiation can ignite material and cause serious burns. However, the effect of thermal radiation on a target is influenced by many factors, including the state of the atmosphere and the target’s thermal absorption qualities (color, thickness, consistency, and reflective properties). For example, when a weapon detonates below an overcast sky, the underside of the cloud layer acts as a reflector.

2-1

FM 100-30
The reflected energy is then added to that coming directly from the point of explosion. The differing levels of energy released from the various-yield weapons further complicates the use of thermal effects for targeting. The level of energy released is not the only effect; the rate at which it is released also has impact. Smaller weapons release thermal energy relatively quicker than larger ones. Also, larger weapons generate heat more slowly, taking longer to dissipate or be conducted away. Therefore, the total amount of thermal energy avail-able for a given type of weapon is directly proportional to its yield. Although not a basic effect, flash blindness is a phenomenon that soldiers might experience from the thermal effect from a nuclear explosion. Flash blindness takes two forms—dazzle and retinal burns. Dazzle is the most common form of flash blind-ness. Its effect is similar to the temporary blindness that camera flash bulbs or bright car headlights at night cause. The difference is in intensity. Dazzle effects from a flash bulb are a temporary inconvenience. Effects from a nuclear burst are prolonged and cause far greater loss of vision. Looking directly at a burst causes severe impairment of vision for from 2 to 3 minutes by day to over 10 minutes at night when the pupils are fully dilated. The second and more serious form of flash blind-ness results from retinal burns received when the lens of the eyes focus the image of the fireball onto the back of the eyes. Estimates of the risk of retinal burns vary. Small pinpoint retinal burns may heal in time, but greater damage is unlikely to do so and will leave a permanent blind spot in the affected eye. Some sources believe that only a small percentage of troops will receive such injuries; others believe this could be a more serious threat.

Residual Ionizing Radiation, Initial Radiation, and the Operational Exposure Guide (OEG)

Residual ionizing radiation typically occurs after the first minute of detonation. It primarily consists of energized impurity particles and debris falling back to earth because of air movement and/or rainout. Residual ionizing radiation could be a lingering and widespread operational hazard. Within the first minute after a nuclear-weapon detonates, initial radiation, in the form of x-rays, gamma rays, and neutrons, is emitted. Initial radiation travels at nearly the speed of light and can penetrate and damage materiel and injure personnel. Initial radiation can help defeat the enemy, but it can also endanger friendly forces and the local civilian population. Denser air at sea level absorbs more initial radiation than thinner air at higher altitudes. As the height of burst (HOB) or the temperature of the air in-creases, the air density decreases. This allows initial radiation to extend farther because it is less absorbed by air molecules. An important factor influencing the amount of initial radiation a target receives is shielding. For example, the surrounding ground, acting as an absorber or shield, will sharply reduce the initial radiation from surface and subsurface bursts. Terrain features can greatly influence initial radiation effects. Minor irregularities, such as ditches, gullies, and small folds in the ground, offer some protection. Major terrain features, such as large hills and forests, can provide significant protection for equipment and personnel, depending on the height of burst. People inside buildings, tanks, or individual fighting positions receive lower initial radiation doses than people in the open and at the same distance from the nuclear detonation. How much less depends on how much initial radiation the intervening material absorbs. All material absorbs some nuclear radiation. However, because of the high penetrating power of neutrons and gamma rays, the shielding material must be quite thick to provide significant protection.

Dense materials such as armored vehicles offer excellent protection against gamma rays. Some readily available low-density materials offer the best protection against neutrons. Depending on its moisture content, soil may also be a good neutron shield. For example, an individual fighting position with 1 meter of overhead soil protection will shield its occupant from as much as 98 percent of the neutron radiation. Material sufficient to protect against gamma rays also provides some protection against neutrons.

As a general guideline, soldiers can construct shields of minimum thickness meant to absorb both neutrons and gamma rays by either alternating layers of high-to low-density materials or by thoroughly mixing such materials. Units may encounter nuclear contamination from sources other than weapons detonation. Possible sources include fallout caused by the destruction of an enemy’s nuclear weapons production facility, enemy stockpiled weapons, and nuclear energy re-actors (both friendly and enemy). Another source of contamination would be the deliberate spread of radioactive materiel over friendly forces or terrain. A nuclear environment can be created without the introduction or detonation of a yield-producing weapon. Therefore, commanders at all levels must be aware of this possibility as well as the possibility of the contamination from non-weapons sources that could significantly affect operations.

NOTE: See FM 3-15 and the FM 3-series manuals for a description of actions to counter these events. The operational exposure guide (OEG), ex-pressed in terms of negligible or emergency risk criteria, is the key to nuclear contamination avoidance. The OEG gives the commander a flexible system of radiation exposure control. The commander specifies OEG for his unit’s level of radiation. The level of exposure must be kept as low as possible. Based on the stated OEG, leaders can select units with low radiation exposure to perform necessary missions. Establishing and using OEG procedures helps leaders successfully employ units on a radiologically contaminated battlefield while keeping exposure to the minimum extent possible consistent with the mission. Ignoring exposure control would be disastrous.

Electromagnetic Pulse (EMP)
On impact with the earth’s atmosphere or with solid materials, initial radiation liberates free electrons. The free electrons create two additional effects: the EMP and the transient radiation effects on electronics (TREE). The EMP can severely degrade and destroy unprotected command, control, communications, computers, and intelligence operations. Electromagnetic pulse directly injures personnel only if they are physically touching metallic collectors, such as cables, at the time of an EMP surge. Hazards may also exist from indirect or secondary EMP effects. For example, damaged electronic equipment might catch fire. Also, pilots may receive incorrect information from digital instruments upset by EMP. Appropriate standing operating procedures (SOP) help mitigate secondary effects. Both EMP and TREE can burn out electronic components or upset system operations. Upset conditions can occur at low signal levels because permanent damage occurs when currents induced by EMP and TREE exceed the capacity of a particular circuit within a system. Shielding sensitive electrical and electronic components is the best protection against burnout. For example, disconnecting antenna cables when the equipment is not in use is a recommended mitigation technique for EMP in field operations. High-altitude nuclear bursts ionize the atmosphere and cause serious widespread blackout of high-frequency (HF) shortwave and synchronous satellite relay communications. Blackouts can last from a few minutes to several hours. In highly ionized regions caused by low-altitude bursts, blackout interference generally decreases as EMP frequency increases. (Most EMP energy is at frequencies below 100 megahertz.) Blackouts from low air bursts are usually not significant. Dust-laden clouds from low air bursts cause blackout effects lasting from a few seconds to several minutes at most, and then only when a fireball or dust cloud blocks transmission paths. Actual interference depends on how many nuclear bursts occur, the altitudes at which they occur, and the areas over which they occur. Units can reduce blackout by using wire communications systems. (However, a system with wires, especially long wires, is more susceptible to EMP.) Routing radio communications through a retransmission station or manual relay to bypass the blackout region. Assigning alternate frequencies. (If the signal operations (SO) officer suspects that an ionized region is producing interference, he tries higher.