Radiobiology: Radiation Physics and Biological Effects

Welcome to a deep dive into Radiobiology: Radiation Physics and Biological Effects, a crucial field for understanding how radiation interacts with matter and, more importantly, with living systems. This article will break down the fundamental concepts, from the physics of radiation to its profound impact on cellular structures and survival.

Understanding Radiation Physics: Interactions with Matter

Radiation's journey through matter begins with fundamental interactions at the atomic level.

Ionization and Excitation: The Core Processes

When radiation interacts with atoms or molecules, two primary events can occur:

• Ionization: This is the separation of charges, specifically the ejection of one or more orbital electrons from an atom or molecule. It requires sufficient energy to overcome the electron's binding energy.
• Excitation: This involves raising an orbital electron to a higher energy level within an atom or molecule without ejecting it. This requires less energy than ionization.

The energy dissipated per ionizing event in tissue is approximately 33 eV, which corresponds to the photon energy at a wavelength of about 40 nm.

Ionizing vs. Non-Ionizing Radiation

Radiation is categorized based on its energy level and ability to cause ionization:

• Ionizing Radiation: This type of radiation possesses sufficient energy to cause ionization. Examples include X-rays, gamma (γ) radiation, alpha (α)- and beta (β)-radiation, protons, neutrons, and accelerated ions.
• Non-ionizing Radiation: This radiation lacks sufficient energy for ionization, causing only excitation of electrons. Examples include ultraviolet, visible light (including lasers), infrared, microwaves, and radiowaves.

How Photons Interact with Matter

Photons (like X-rays and gamma rays) interact with matter through three main processes, depending on their energy and the absorbing material:

• Photoelectric Effect: The incident photon interacts with a tightly bound orbital electron, giving up all its energy. The electron is ejected as a photoelectron, with kinetic energy equal to the photon's energy minus the electron's binding energy. The resulting vacancy is filled by an outer electron, emitting a characteristic X-ray.
• Compton Effect: A photon interacts with a loosely bound planetary electron. Only part of the photon's energy is transferred to the electron (now a Compton electron), which is ejected. The photon, deflected from its original direction, continues with reduced energy.
• Pair Creation: For very high-energy photons (above 1.022 MeV), interaction with an atomic nucleus can create an electron-positron pair. The positron then immediately annihilates with another electron, producing two gamma photons.

The relative importance of these effects varies with photon energy, as shown by the mass energy-absorption coefficient.

Interaction of High-Energy Electrons

Incoming high-energy electrons can interact in two ways:

• Ionization: They transfer energy to orbital electrons, causing ionization. The vacancy is filled, leading to characteristic X-rays.
• Bremsstrahlung: They are slowed down in the electric field of an atomic nucleus, emitting their energy as X-rays with a continuous spectrum. This is also known as "braking radiation."

Interaction of Heavy Charged Particles and Neutrons

• Heavy Charged Particles (e.g., protons, alpha particles): Similar to electrons, but due to their greater mass, energy transfer to orbital electrons (ionization) is the most dominant process.
• Neutrons: Being uncharged, fast neutrons interact primarily with the nuclei of atoms, especially hydrogen. They transfer part of their energy to a proton, which then acts as a directly ionizing particle.

Directly and Indirectly Ionizing Radiation

This classification is based on whether the radiation itself directly causes ionization:

• Directly Ionizing Radiations: These are charged particles (electrons, positrons, protons, alpha particles, heavy ions) with sufficient energy to directly ionize or excite atoms and molecules, disrupting atomic structures. Examples include electrons, protons, and alpha-particles.
• Indirectly Ionizing Radiations: These are uncharged particles (photons, neutrons) that do not directly produce changes in atomic structure. Instead, they set in motion directly ionizing charged particles or initiate nuclear transformations. Examples include photons and neutrons.

Electron cascades are created by different kinds of radiation, illustrating the complex paths of energy deposition.

Biological Effects: Radiation's Impact on Living Systems

Radiation's interaction with biological matter, particularly water, leads to a cascade of events affecting DNA.

Radiolysis of Water: The Chemical Precursor to Damage

Since living cells are primarily water, the interaction of radiation with water is critical:

• Key Processes:
• H₂O → H₂O⁺ + e⁻
• H₂O⁺ → H⁺ + OH· (hydroxyl radical)
• e⁻ + H⁺ → H· (hydrogen radical)
• Key Species: The primary reactive species formed are the OH· radical, H· radical, and the hydrated electron (e⁻_aq). The G-value (number of radicals generated per 100 eV absorbed energy) is typically 2.3 for OH·, 0.6 for H·, and 2.3 for hydrated electrons.

Direct and Indirect Action: How DNA Gets Damaged

• Direct Action: The secondary electron (resulting from radiation absorption) directly interacts with and damages vital macromolecules like DNA. This is common for densely ionizing radiation.
• Indirect Action: The secondary electron interacts with water molecules, producing highly reactive free radicals (like OH·). These radicals then diffuse and cause damage to DNA. Indirect action is dominant for sparsely ionizing radiation, such as X-rays, as free radicals produced within a 2 nm diameter cylinder around the DNA helix can inflict damage.

DNA Damage: Single- and Double-Strand Breaks

Strand breaks are the most common type of DNA damage from ionizing radiation, often accompanied by base damage. At a dose of 1 Gy, a single cell typically experiences:

• 1000 sites of base damage

• ~ 1000 single-strand breaks
• ~ 40 double-strand breaks

These figures are largely independent of radiation quality. While single-strand breaks are readily repaired, double-strand breaks (DSBs), especially if opposite or closely separated, are far more significant and can lead to chromatin snapping into two pieces. They are considered the most biologically significant lesion.

Locally Multiply Damaged Sites

Radiation can cause complex damage where multiple lesions (base damage, strand breaks) occur within a very small region, creating locally multiply damaged sites (LMDS). These are more challenging for cells to repair.

Cellular Responses and Repair Mechanisms

Cells possess sophisticated mechanisms to repair radiation-induced DNA damage.

Double-Strand Break Repair Pathways

Cells primarily use two main pathways to repair DSBs:

• Homologous Recombination (HR): This error-free process occurs mainly after DNA replication (G2 or late S-phase), when a homologous sister chromatid is available as a template. A single strand from the homologous DNA invades the damaged double strand, allowing for accurate gap filling.
• Non-Homologous End Joining (NHEJ): This process involves directly joining the broken ends of a DSB. It is considered error-prone, as information can be lost from the DNA during this