Radiation Biology – Lecture Review

These flashcards cover key definitions, mechanisms, historical notes, cellular processes, dose-response concepts, and clinical effects discussed in the lecture on Radiation Biology. They are formatted in question-and-answer style to facilitate active recall for exam preparation.

What is the focus of Radiation Biology?

The study of the effects of ionizing radiation on biologic tissue.

Which radiation effects are considered deterministic (early) effects in humans?

Hematologic syndrome, gastrointestinal (GI) syndrome, central nervous system (CNS) syndrome, tissue damage such as erythema and desquamation, hematologic damage to blood cells, and cytogenic (chromosomal) damage.

Name three late (probabilistic) effects of radiation exposure.

Leukemia, bone cancer, and thyroid cancer (other correct answers include breast cancer, lung cancer, local tissue damage, lifespan shortening, genetic damage, and cytogenic damage).

What are the five main categories of fetal irradiation effects?

Prenatal death, neonatal death, congenital malformations, childhood malignancy, and diminished growth and development.

Which occupational group historically showed a high incidence of lung cancer from radiation exposure?

Uranium miners.

State the Law of Bergonie and Tribondeau in simple terms.

Radiosensitivity of living tissue is greater in cells that are less mature, stem‐like, divide rapidly, and have high metabolic activity.

According to Bergonie & Tribondeau, which trimester of pregnancy is most radiosensitive?

The first trimester.

What is Linear Energy Transfer (LET)?

The rate at which energy is transferred from ionizing radiation to soft tissue, expressed in keV/µm.

How does LET relate to Relative Biologic Effectiveness (RBE)?

As LET increases, RBE generally increases, meaning radiation becomes more biologically damaging.

What is the typical RBE value assigned to diagnostic x-rays?

1

Define dose protraction.

Delivering the total radiation dose continuously but at a lower dose rate over a longer time, reducing biological effect.

Define fractionation in radiation exposure.

Dividing the total radiation dose into several smaller doses delivered over days or weeks, allowing normal tissue repair between exposures.

List two biological factors that increase radiosensitivity.

Presence of oxygen (oxygen effect) and young age (embryo/fetus).

What is the Oxygen Enhancement Ratio (OER)?

A numerical description of how much more sensitive tissue is to radiation when oxygenated versus hypoxic/anoxic.

Give one example of a radiosensitizer.

Halogenated pyrimidines incorporated into DNA (another correct answer: Vitamin K).

Why are most radioprotectors not used clinically?

They must be given at toxic levels to be effective and can be more harmful than the radiation itself.

What does the radiation hormesis hypothesis suggest?

Low doses of radiation may be beneficial, potentially stimulating cell repair mechanisms and increasing longevity.

Differentiate between a linear non-threshold (LNT) and a linear threshold dose-response relationship.

LNT assumes any dose, no matter how small, carries some risk, whereas a linear threshold model assumes no effect occurs below a specific dose level.

At which mitotic sub-phase are radiation-induced chromosome damages best analyzed?

Metaphase.

List the four sub-phases of mitosis in order.

Prophase, metaphase, anaphase, telophase.

During which cell-cycle phase does DNA replication occur?

S-phase (synthesis phase) of interphase.

What is the most radiosensitive molecule in the cell?

DNA.

Name three major radiation-induced alterations of macromolecules.

Main-chain scission, cross-linking, and point lesions.

What product of water radiolysis is highly toxic to cells?

Hydrogen peroxide (H₂O₂).

Define a free radical.

An uncharged molecule containing a single unpaired electron, highly reactive and short-lived.

Contrast direct and indirect radiation effects.

Direct effect occurs when radiation ionizes the target molecule (e.g., DNA) directly; indirect effect occurs when radiation ionizes a non-critical molecule (usually water), producing free radicals that then damage the target.

What does the Target Theory propose?

Certain critical cellular molecules (targets) must be hit by radiation for cell death or serious injury to occur.

Which category of cells is most radiosensitive: VIM, DIM, or fixed postmitotic?

Vegetative intermitotic (VIM) cells.

Give an example of a VIM cell.

Basal skin cells (other correct answers: crypt cells of the intestine, erythroblasts, type A spermatogonia).

Which human cell type is considered radioresistant?

Nerve cells (also muscle cells).

What percentage of the human body is water by molecule count?

Approximately 80 %.

List the four primary organic macromolecules with their approximate body percentages.

Proteins (15 %), lipids (2 %), carbohydrates (1 %), nucleic acids (1 %).

Identify the purine bases in DNA.

Adenine and guanine.

Identify the pyrimidine bases in DNA.

Thymine and cytosine.

Which nitrogenous bases pair together in DNA?

Adenine with thymine, and cytosine with guanine.

Name the two main structural parts of a basic cell.

Nucleus and cytoplasm.

Which cellular organelle is known as the energy 'engine' of the cell?

Mitochondria.

Where does protein synthesis occur within a cell?

On ribosomes found free in the cytoplasm or attached to rough endoplasmic reticulum.

What is the function of lysosomes?

They contain digestive enzymes that break down cellular waste and contaminants.

Which phase follows mitosis in the cell cycle?

Interphase.

Define vegetative intermitotic (VIM) cells.

Rapidly dividing, undifferentiated stem cells with short life spans and high radiosensitivity.

Which cell is considered the most radiosensitive in the human body?

Lymphocyte.

How many chromosomes do normal human somatic cells contain?

46 chromosomes (23 pairs).

Meiosis results in gametes containing how many chromosomes each?

23 chromosomes.

Differentiate somatic and genetic cells.

Somatic cells compose body tissues and divide by mitosis; genetic cells (gametes) are reproductive cells and divide by meiosis.

Which historical scientist first named the 'cell' in 1665?

Robert Hooke.

Who described the molecular structure of DNA in 1953?

James Watson and Francis Crick.

What is a codon?

A sequence of three nucleotide bases on mRNA that codes for a specific amino acid during protein synthesis.

Name the three main types of RNA involved in protein synthesis.

Messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

What term describes the shrinkage of tissue due to cell loss from radiation?

Atrophy.

Define hypertrophy.

Increase in the size of tissue or organ due to enlargement of individual cells.

What is hyperplasia?

An increase in cell number leading to an enlarged tissue or organ.

What percentage of body atoms is hydrogen?

About 60 %.

Which macromolecule class serves as the primary long-term energy store?

Lipids.

What is the simplest form of lipid composed of?

Glycerol and fatty acids.

State the function of hormones.

Regulate body functions such as growth and development; produced by endocrine glands.

What is the role of antibodies?

Provide the body's primary defense against infection and disease.

Which organelles allow communication between the nucleus and cytoplasm?

Endoplasmic reticulum channels (rough and smooth ER).

What is main chain scission and its effect on macromolecules?

Breakage of the backbone of long-chain macromolecules, decreasing their viscosity and splitting them into smaller chains.

Describe cross-linking as a radiation effect on macromolecules.

Formation of spur-like side structures that link different parts of a molecule or adjacent molecules, increasing viscosity.

Define point lesion.

Disruption of a single chemical bond in a macromolecule, often undetectable but capable of causing functional damage.

What chromosome aberration results from two chromosome fragments rejoining to form a chromosome with two centromeres?

Dicentric formation.

Which chromosome aberration involves end-to-end union creating a circular structure?

Ring formation.

Explain 'terminal deletion' in chromosome damage.

Loss of a chromosome segment after a radiation-induced break near an end of the chromosome.

What is meant by 'overkill' on an LET vs. RBE graph?

A region where very high LET radiation deposits more energy than needed to kill the cell, so additional energy does not increase biological effect.

What radiation dose (in Gy) can lethally damage a human cell?

Less than 1 Gy (100 rad) can kill some human cells.

Why are stem cells generally more radiosensitive than mature cells?

They divide more rapidly, are less differentiated, and have higher metabolic activity.

Which tissue type—muscle or endocrine—is more radioresistant?

Muscle tissue.

Name two organ systems with highly radiosensitive tissues.

Hematopoietic (blood-forming) and gastrointestinal systems.

What are macromolecules collectively involved in building large molecules from small units called?

Anabolism (anabolic processes).

What does catabolism refer to?

Breaking down large molecules into smaller units, releasing energy and producing water and carbon dioxide.

Which physical measurable property of a solution decreases with main-chain scission of macromolecules?

Viscosity.

Why are in-vivo macromolecules more radiosensitive than those irradiated in-vitro?

Cellular environment and metabolic processes in living tissue amplify radiation damage compared to isolated molecules in solution.

What is the minimum energy (in Mrad or kGy) required to detect a physical change in isolated macromolecules?

Approximately 1 Mrad (10 kGy).

List three sources of human population data used to study radiation effects.

American radiologists, uranium miners, and radium watch-dial painters (additional examples: patients treated with I-131, Thorotrast patients, cyclotron workers).

What is the principal interaction with the body when it is irradiated?

Radiolysis of water, since the body is ~80 % water.

Explain the term 'cytotoxic'.

Having the ability to kill cells.