Irradiation of Macromolecules

• Irradiation of macromolecules has different effects than irradiation of water.
• Macromolecules are more radiosensitive within a living cell (in vivo) compared to outside the body or cell (in vitro).

Irradiation of Macromolecules

• When macromolecules are irradiated in solution in vitro, three major effects occur: main-chain scission, cross-linking, and point lesions.

Main-Chain Scission

• Involves the breakage of the backbone of a long-chain macromolecule.
• Results in reducing a long molecule into many smaller molecules.
• Reduces both the size and the viscosity of the solution.
• Main-chain scission is essentially radiation breaking a long molecule into smaller pieces, reducing viscosity.

Cross-Linking

• Happens when radiation makes molecules stick together by forming extra links.
• Some macromolecules have side structures, and radiation can create more, leading to more tangled molecules.
• Results in a thicker and stickier liquid (higher viscosity) as molecules join together.

Point Lesions

• Are the result of radiation interaction that disrupts single chemical bonds.
• Though undetectable, these can cause minor modifications and malfunction within the cell.
• At low radiation doses, point lesions cause cellular radiation damage, leading to stochastic effects.
• Laboratory experiments suggest these effects can be reversible through intracellular repair and recovery.
• Point lesions occur when radiation slightly damages a molecule, potentially leading to diseases like cancer, but cells can repair this damage.

Macromolecular Synthesis

• Proteins are manufactured by translation of the genetic code from transfer RNA (tRNA), which had been transferred from messenger RNA (mRNA).
• The information carried by the mRNA was in turn transcribed from DNA.
• Radiation damage to any of these macromolecules may result in cell death or late stochastic effects

DNA Radiosensitivity

• DNA is the most radiosensitive molecule

DNA Synthesis

• The molecules combine to form a single large molecule that, during the S portion of interphase, is attached to an existing single chain of DNA.
• During G1, molecular DNA is in the familiar double-helix form.
• The ladder opens up during the S phase, becoming a single chain without base pairing.
• A combined base sugar-phosphate molecule attaches to the single-strand DNA sequence, determined by permitted base pairing.
• One double-helix DNA molecule becomes two similar molecules, each a duplicate of the original.
• Parent DNA is replicated into two duplicate DNA daughter molecules.

Radiation Effects on DNA

• Radiation-damaged human chromosomes can exhibit terminal deletion, dicentric formation, and ring formation.
• Types of damage include one or both side rails severed, crosslinking, and rung breakage.
• DNA molecule damage can occur without visible chromosome aberration, potentially leading to cell death.
• Widespread damage to cells of the same type leads to deterministic effects.
• DNA damage can cause abnormal metabolic activity, leading to uncontrolled cell proliferation, characteristic of radiation-induced malignant disease, which describes the cause of a stochastic effect.
• Damage to germ cell DNA leads to potential genetic effects in future generations.
• Loss of a base destroys the triplet code, resulting in a molecular lesion called point mutation.

Radiolysis of Water

• Human body's aqueous nature (80% water) means irradiation of water is the primary radiation interaction.
• Water irradiation causes dissociation into other molecular products, called radiolysis of water.

Direct and Indirect Effects

• In vivo irradiation's harmful effects primarily come from damage to sensitive molecules like DNA.
• Direct effect: radiation ionizes the target molecule.
• Indirect effect: ionization occurs on a noncritical molecule, which then transfers energy to the target via free radicals.
• The principal effect of radiation on humans is indirect; distinguishing between direct and indirect is impossible.
• Due to the body's composition (80% water, less than 1% DNA), irradiation effects in vivo result from indirect effects.
• Oxygen amplifies indirect effects due to additional free radicals.

Cellular Radiobiology

• Most cell effects have no response due to recovery and repair.
• Cell death is the primary focus as a radiation response.
• Target theory: cell death occurs only if the target molecule (DNA in the cell nucleus) is inactivated.
• A hit occurs when radiation interacts with the target or other molecules, and direct/indirect hits are indistinguishable.
• Indirect hits make the target appear larger due to free radical mobility.
• Oxygen increases the probability of a hit by forming free radicals, enlarging the effectiveness volume.

High-LET Radiation

• The probability of a direct effect is high
• Adding oxygen does not increase the number of hits
• The maximum number of hits is produced by direct effect

Low-LET Radiation

• Presence of oxygen amplifies the indirect effect
• The volume of action is enlarged
• For high-LET radiation, the effective volume of action remains unchanged

Cell-Survival Kinetics

• Cell cloning involves planting cells in a Petri dish and allowing them to divide into visible colonies
• Two cell survival models derived mathetmatically from target theory are the single-target and multitarget
• Single-target model applies to simple cells and enzymes
• Multitarget applies to more complicated systems, such as human cells.

Single-Target, Single-Hit Model

• A square on the runway is considered wet when one or more raindrops fall on it.
• Probability of a square becoming wet is governed by the Poisson distribution
• When the number of raindrops equals the number of squares, 63% are wet and 37% are dry.

Single-Target, Single-Hit Model: D37 Definition

• D37 is the radiation dose where 63% of cells are killed (37% survival) and measures a cell's radiosensitivity.
• Low D37 indicates high radiosensitivity; high D37 indicates radioresistance
• If no hits were wasted, D37 would kill 100% of the cells.

Multitarget, Single-Hit Model

• Analogy: Each square on a runway is divided in half; both halves must be hit to be considered wet.
• A threshold is represented because many raindrops have to fall to wet both halves of a square to be considered wet

Radiation Dose Effect

• At very low radiation, cell survival is almost 100%
• As radiation increases, fewer cells with both targets hit survive
• At high radiation doses, cells survive with one target hit
• Dose-response would appear as a single-target, single-hit model
• D0 represents the constant to radiosensitivity of the cell and equals D37 in the linear portion of the graph
• D0 is the dose that results in one hit per target if no radiation were wasted

Dose and Recovery

• Subsequent experiments range extrapolation numbers from 2 to 12, making n unknown.
• DQ is called the threshold dose and measures of the multitarget
• DQ relates to the capacity of the cell to recover from sublethal damage.
• cells can readily recover if there is a large DQ

Recovery

• Shoulder of the graph shows mammalian cells must accumulate some sublethal damage before dying
• Wider shoulder means more sublethal damage and a higher value of DQ.
• DQ measures the capacity to accumulate sublethal damage and the ability to recover from it

CELL-CYCLE EFFECTS

• Cell-cycle time is the average time between mitosis during cell replication.
• Most proliferating human cells have generation times of ~24 hours.
• Specialized cells' times extend to 100s of hours
• Neurons do not replicate
• Increased time results primarily from the lengthening of G1 phase of cell cycle.
• G1 is the the most variable phase of the cell cycle

Cell Cycle Effects

• The age-response function is change in radiosensitivity as a function of phase in the cell cycle
• Humans cells are the most radiosensitive in M
• Humans cells are the most radioresistant in late S
• Oxygen maximizes the effect of low-LET radiation and is the most studied modifier.
• Anoxic cells require a considerably higher dose to produce a given effect

Deterministic Effects of Radiation

• During the 1920s and 1930s, radiation techs would get weekly blood examinations to monitor x-ray workers
• Periodic blood examination of x-ray workers was before personnel radiation monitors

Effects of Radiation

• Dose causes the severity of deterministic radiation responses, along with a dose threshold and nonlinear relationship

ACUTE RADIATION LETHALITY

• Death is the most devastating human response to radiation exposure, no cases of death after diagnostic x-ray exposure has ever been recorded
• Deaths that occurred from stochastic are after high radiation doses.
• Acute radiation-induced human lethality is of only academic interest in diagnostic radiology
• Diagnostic x-ray beams are neither intense enough nor large enough to cause death

Effects of Radiation on the Body

• Diagnostic x-ray beams always result in partial-body exposure which is less harmful than whole-body exposure
• The acute radiation syndrome is the sequence of events following high-level radiation exposure leading to death that are dose-related
• The acute radiation syndrome include hematologic death, gastrointestinal (GI) death, and central nervous system (CNS) death

Periods associated with acute radiation lethality

• Prodromal Period and Latent Period exist in addition to the three syndromes
• Prodromal is the acute clinical symptom that occurs within hours to a day or two of exposure
• After the prodromal period, there may be a latent period.

Prodromal Period

• Signs appear within minutes to hours if 1 Gyt (100 rad) is delivered to the total body.
• Nausea, vomiting, diarrhea, and leukopenia are the early radiation sickness symptoms that present immediately

Latent Period

• Apparent well-being the occurs after initial radiation sickness
• Latent extends from hours with doses greater than 50 Gyt
• Latent extends to weeks with doses of 1 to 5 Gyt.
• The period where there is no sign of radiation sickness

The manifest illness period

• The latent period is mistaken with early recovery from moderate radiation.
• During a second wave there is nausea, vomiting, followed by diarrhea
• The victim experiences a loss of appetite (anorexia) and may become lethargic

Consequences of Radiation

• Diarrhea can lead to loose and bloody stools
• Death can not be stopped with supportive therapy, from rapid symptom progression, within 4 to 10 days
• GI death occurs because of damage to the cells lining the intestines
• Radiation kills the most sensitive cells of the stem
• When the intestinal lining becomes denude of functional cells, fluid gets destroyed, which lead to infection

LD50/60 radiation

• The percentage of animals that die from 1 to 10 Gyt of radiation is plotted that illustrates the radiation dose-response relationship for acute human lethalit.
• Whole body dose that kills 50% of irradiated subjects within 60 days

Mean Survival

• As the whole-body radiation increases, the average time between exposure and death decreases

LOCAL TISSUE DAMAGE

• A higher that is required when only part of the body is irradiated.
• It can lead to cell death results in shrinkage of the organ or tissue.
• The effect can lead to total lack of function organ or tissue or followed be recovery
• Cell death is called Atrophy, which is the shrinkage.

Affects of Radiation

• The tissue with which we have had the most experience is the skin.
• Radiation also affect the Hemopoietic System.

Hemopoietic System

• Consists of bone marrow, circulating blood, and lymphoid tissue that included (the lymph nodes, spleen, and thymus)
• The principal effect is a depressed number of blood cells in the peripheral circulation.
• Time- and dose-related effects on the various types of circulating blood cells are determined by the normal growth and maturation.

Pluripotential stem cell creates various cell types

• Principally, lymphocytes (immune response), granulocytes (fight bacteria), thrombocytes (clotting) and erythrocytes (transportation agents for oxygen) are cell lines that develop the bone marrow at different rates
• All are released to the peripheral blood as mature cells.

he principal response of the hemopoietic system to radiation exposure is a decrease in the numbers of all types of blood cells.

• The cell response to radiation can deplete mature circulation cells.
• The depletion can be given from samples of low, moderate and high doses of the circulating three cell types.
• The responses can be seen in accident victims or after animal and radiotherapy procedures
• Immediately after the exposure, the first cells affected are the lymphocytes.
• When lympthocytes get reduced there is slow recovery
• the lymphocytes has a direct effect because the response is is immediate.

Radiosensitive Cells

• The lymphocytes and the spermatogonia are the most radiosensitive cells in the body.
• When Granulocytes experience a rapid rise in number (granulocytosis) with further reduces (granulocytopenia).
• With moderate results in an abortive rise in granulocyte 15 to 20 days after irradiation
• When a depletion of platelets has a slow recover (thrombocytopenia) and is sensitive during maturity
• Thrombocytes reach a minimum in about 30 days and recover in approximately 2 months and have a similar response to granulocytes
• Erythrocytes are less sensitive to injuries and cell recover for 6 months to a year after depletion

CYTOGENETIC EFFECTS

• Developed in the 1950s to contribute to human genetic analysis and radiation genetics.
• The chromosomes of each cell can be easily observed and studied.
• This has resulted in effects on radiation-induced chromosome damage.

Karyotype

• Karyotype is the process where photographs are taken and paired with its sister into a chromosome map
• Structural damage is visualized in the normal karyotype
• Damaged radiation can be single- and double-hit chromosome aberrations.
• Reciprocal translocations require with point genetic mutations is undetectable with karyotype construction.