Radiation Biology and Effects Quiz

Radiation Interactions with Biological Matter - Radiobiology

• Radiobiology is the study of the action of ionizing radiation on living things.
• Prerequisites include radiation physics and biology.
• The history of radiology began with the discovery of X-rays in 1895 by Wilhelm Conrad Röntgen.
• The science of diagnostic radiology emerged in the late 18th century.
• Antoine-Henri Becquerel discovered radioactivity in 1896.
• Radiotherapy emerged a few years later.

Interaction of Radiation with Matter - General

• Ionizing radiation (potentially harmful or beneficial to humans) includes cosmic, gamma, X-rays.
• The energy spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, and ionizing radiation.
• Ionizing radiation has high frequency and high energy.
• The formula df = c = 3 x 10⁸ m/s defines the relationship between frequency (f) and wavelength (c) of electromagnetic radiation at the speed of light.

The Photon Energy

• Formula for the energy of a photon (E): E = hf (where h is Planck's constant).
• Planck's constant, h = 6.6261 x 10⁻³⁴ m².kg/s
• Formula 2 for photon energy (E) in keV: E [keV] = 12.4 / λ[Å°]
• 1 electron volt (eV) = 1.6022 x 10⁻¹⁹ J
• 1 Angstrom (Å) = 10⁻¹⁰ m

Electromagnetic radiation

• Electromagnetic (EM) spectrum's wavelengths vary greatly, from kilometers (radio waves) to picometers (X-rays and gamma rays).
• Energy of X-rays and gamma-rays comes in packets (photons) with sufficient energy to break chemical bonds and cause biological change, unlike non-ionizing waves.

Ionizing Radiations

• Absorption of energy from radiation in biological materials may lead to excitation (raising electron energy levels without ejection) or ionization (ejection of one or more electrons).
• Ionization happens when radiation energy surpasses a threshold, termed "Ionizing Radiation".
• The high energy dissipated during an ionization event can break chemical bonds in biological matter, a key factor in biological effects.

Types of Ionizing Radiations

• Two main types: Electromagnetic (x-rays, gamma rays) and Particulate (electrons, protons, alpha particles, neutrons).
• X-rays are produced extranuclearly (outside of the nucleus) by accelerating electrons into a target.
• Gamma rays originate from radioactive decay processes inside the atomic nucleus.

Ionizing Photon Energy

• Electromagnetic radiation is considered ionizing if its photon energy exceeds 124 eV.
• This corresponds to wavelengths shorter than approximately 10⁻⁸ m, shorter than ultraviolet light on the electromagnetic spectrum.

Units of Radiation

• Radiation exposure can be measured in Roentgen (R), Rad, or Gray (Gy).
• Roentgen (R) measures the ability of x- or γ-rays to ionize air.
• Gray (Gy) is the newer SI unit for absorbed dose, representing energy absorption of 1 J/kg.
• 1 rad = 100 erg/g and 1 Gray (Gy) = 100 rad

Particulate Radiations

• Particulate radiation arises from electrons, positrons, protons, alpha particles, neutrons, and heavy charged ions.
• Electrons and positrons are negatively and positively charged particles respectively.
• Protons are positively charged particles with a significantly higher mass than electrons.
• Alpha particles are helium nuclei, comprising two protons and two neutrons.
• Neutrons are uncharged particles within the same mass range as protons.
• Heavy charged particles include various atomic nuclei (carbon, neon, argon, iron) with stripped electrons.

Ionization of Biologic Materials

• Direct ionization: Particulate radiation directly disrupts atomic structure, causing chemical and biological changes.
• Indirect ionization: Electromagnetic radiation (like X-rays) transfers energy to the medium, creating fast-moving charged particles (mostly electrons) and free radicals that cause damage.

Interaction of Radiation with Matter

• Photons interact with matter through elastic scattering, photoelectric effect, Compton scattering, pair production, and photonuclear interactions.
• Elastic scattering has no loss of photon energy.
• Photoelectric effect is dominating at lower energies.
• Compton effect is dominating at higher energies.
• Pair production is involved when energy of photons involved exceeds a certain amount.
• The total attenuation coefficient, denoted as μ/ρ, represents the probability of a photon interacting with the medium.

The Photoelectric Effect

• A dominant process at lower energies.
• An incoming photon interacts with a bound electron in an atom, transferring all its energy to that electron, ejecting it from the atom.
• The ejected electron is called a photoelectron.
• The missing orbital electron leads to the production of characteristic x-rays.

The Compton Effect

• A dominant process at higher energies.
• An incoming photon interacts with a loosely bound electron, transferring part of its energy to the electron.
• The scattered photon travels in a different direction with reduced energy.
• The electron is ejected with kinetic energy.

The Compton Effect - Summary

• Dominating process at high energies, a characteristic feature of radiation therapy systems.
• Depends on electron density (pe).
• Independent of Z (atomic number).
• Interaction of a photon with unbound electrons causes scattering and partial energy absorption.
• Change in photon wavelength (A out - A in) depends on the angle of the scattered photon.
• A direct hit by a photon results in a scattered photon and an electron with maximum energy in the opposite direction of the scattered photon, and minimum energy, if the scattered photon is directed backward.

Difference between Compton and Photoelectric Absorption Processes - Summary

• Compton process deals with high-energy photons, whereas Photoelectric process deals with low-energy photons.
• Compton process only transfers a small fraction of energy to the orbital electrons by the incoming photon during an interaction.
• In the Photoelectric process, a significant portion of the photon's energy is transferred to orbital electrons.

Difference between Compton and Photoelectric Absorption Processes - Biophysics

• Compton process attenuation coefficient (a.k.a. mass absorption coefficient) is independent of the atomic number of the absorbing material.
• Photoelectric process attenuation coefficient is a cubic function of that atomic number (Z³).
• Bone has a higher atomic number compared to soft tissue due to its calcium content. Thus, X-rays are absorbed to a greater extent and produce contrast in radiology images.

Pair Production - Type 1&2

• Pair Production results in a positron-electron pair being produced along with an absorbed photon.

Photonuclear Interactions

• A high-energy photon interacts with an atomic nucleus resulting in the emission of a proton (p) or a neutron (n).
• Not a significant process in radiobiology.

Direct and Indirect Actions of Radiation

• Damage to DNA is the primary mechanism for biological effects of radiation.
• Direct action occurs when radiation interacts directly with DNA molecule atoms.
• Indirect action occurs when radiation interacts with other cell molecules, producing free radicals.
• Free radicals damage DNA molecules.

Free Radical

• A free radical is an atom or molecule with an unpaired electron in its outer shell.
• Free radical reactivity is high.
• In water, radiation can cause ionization and the ejection of an electron, resulting in a highly-reactive free hydroxyl radical (OH) which reacts with and damages other cellular molecules, including DNA.
• Hydroxyl radicals are implicated in approximately two-thirds of the DNA damage from X-rays in mammalian cells

Chain of Events Leading to Biological Effects

• Ionizing radiation may lead to ionization, excitation, breaking of bonds between molecules, and subsequent biological effects.

Absorption of Neutrons

• Neutrons interact differently with tissue than X-rays and gamma-rays:
  • X-rays interact with electrons.
  • Neutrons interact with atomic nuclei.
• At low to moderate energies, neutron interaction with nuclei is primarily elastic scattering.
• At high energies, the interaction is primarily inelastic.
• Interaction between incident neutrons and hydrogen nuclei is the dominant process of energy transfer in soft tissues.

Interaction of Neutrons with Hydrogen Nucleus

• A fast neutron impacts a hydrogen nucleus, resulting in deflected, lower-energy neutrons and a recoil proton.

Interaction of Neutrons with Carbon or Oxygen Nuclei (Spallation)

• High-energy neutrons can collide with Carbon and Oxygen Nuclei.
• The interactions release alpha particles.

Differences Between Photons and Neutrons

• X-rays and gamma rays are indirectly ionizing and create fast-moving electrons.
• Neutrons are capable of being either directly or indirectly ionizing, and also create recoil protons, alpha particles, and heavier nuclear fragments.
• Heavy particles created by neutrons are much more massive and interact directly than the electrons created during x-ray or gamma-ray interactions.
• Indirect biologic effects by X and gamma rays is dominant while direct actions of neutrons are important.

DNA Strand Breaks

• Single-strand breaks (SSBs) occur in one DNA strand and are often repaired.
• Double-strand breaks (DSBs) occur in both DNA strands and are more severe, potentially leading to cell death or mutations.

Measuring DNA Strand Breaks

• Electrophoresis techniques, such as pulsed-field gel electrophoresis and the comet assay, are used to measure DNA strand breaks.
• Methods are based on separating DNA fragments by size and charge using an electric field.

Chromosomes and Telomeres

• Chromosomes are thread-like structures that contain DNA and proteins.
• They are found in the nucleus of cells.
• Humans have 46 chromosomes in 23 pairs.
• Telomeres are protective segments at the ends of chromosomes that shorten with each cell replication.
• Telomeres help regulate cell replication.

Cell Cycle

• A series of events culminating in cell growth and division into two daughter cells.
• Non-dividing cells are not considered in the cycle.
• The stages are G1, S, G2, and M.

Cell Mitosis

• Cell division process consisting of phases Prophase, Metaphase and other stages resulting in two identical daughter cells.

Radiation-induced Chromosome Aberrations

• Radiation-induced damage to chromosomes, leading to various types of structural changes.
• Aberrations can be classified as asymmetric or symmetric, with asymmetric aberrations generally leading to cell death.
• Symmetric aberrations often survive subsequent mitosis; and can lead to cell mutation, cancer, and hereditary effects.

Radiation-induced Asymmetric Chromosomal Aberrations - Lethal Aberrations

• Lethal aberrations cause significant changes in chromosome structure during inter-chromosomal interactions.
• They usually lead to cell death during subsequent mitosis.

Radiation-induced Symmetric Chromosomal Aberrations - Carcinogenesis Aberrations

• Symmetric aberrations do not immediately kill cells but can induce carcinogenesis or hereditary effects.
• They often result in symmetric chromosomes that survive mitosis, thus known as "stable" aberrations.

Radiation-induced DNA Damage - Summary

• Ionizing radiation can damage DNA by causing single-strand and double-strand breaks and chromosomal aberrations.
• Single-strand breaks are often repaired, causing no biological effects.
• Double-strand breaks and chromosomal aberrations, if not repaired, can cause cell death, cell mutation, cancer, and hereditary effects.

Radiation Biology - A Complicated Field of Study

• Radiation effects depend on the cell type, cell cycle stage, and the type and dose of radiation.

Linear Energy Transfer (LET)

• LET is the energy transferred per unit length of the track of the particle in the matter.
• LET is measured in keV/μm.
• LET is an important factor in assessing the biological effects of radiation, determining radiation density.
• The biological effect of radiation correlates well with average LET.

Relative Biologic Effectiveness (RBE)

• RBE accounts for differences in biological effects of radiation types at equal doses.
• RBE is calculated as the ratio of the dose of a standard radiation (250-kV X-rays) required to produce a biological effect to the dose required to produce the same effect using another type of radiation.
• RBE varies based on type of cells/tissues and a range of biologic endpoints.

RBE and Fractionated Doses

• RBE is influenced by the biologic endpoint and the dose.
• RBE varies for different cell types and tissues.
• Higher radiation energies lead to a reduction in the variation of RBE across tissues and cells.

RBE as a Function of LET

• RBE increases with LET for most mammalian cells up to an optimal LET value (around 100 keV/μm).
• Beyond the optimum LET, RBE decreases.

The Optimal LET

• The optimal LET value is a key factor for biological effects of radiation.
• An LET value of about 100 keV/μm causes the highest probability of double-strand breaks in DNA.

Factors that Determine RBE

• RBE depends on LET (linear energy transfer).
• The dose (energy).
• The dose rate.
• The number of dose fractions.
• Type of cell or tissue.
• The biologic endpoint.

Equivalent Dose

• Equivalent dose is a quantity used for radiation protection that simplifies the consideration of the different biological effects of different types of radiation.
• It is calculated by multiplying the absorbed dose by a radiation weighting factor (WR).
• The equivalent dose is measured in units of Sievert (Sv) or Rem (rad equivalent man).
• The WR value is 1 for low linear energy transfer (LET) radiations (X-rays and γ-rays) and increases to a maximum value of 20 for high-LET neutrons and alpha particles.

Values of WR (Radiation Weighting Factors)

• The value for WR varies for all low-LET radiations (X-rays and γ-rays) from 1 to a maximum 20 for high-energy neutrons and a particles.

Equivalent Dose - Examples 1&2

• Worked examples demonstrating the calculation of equivalent dose using absorbed dose and WR values.

Test your knowledge on the biological effects of radiation, including the types of radiative particles, DNA structure, and chromosomal aberrations. This quiz covers important concepts in radiation biology and the factors influencing bioeffects, helping you understand the implications of ionizing radiation on living cells.