Biological Effects of Radiation - Radiobiology

Vocabulary practice flashcards defining key terminology, principles, and processes in radiation biology, covering radiation types, LET, RBE, water radiolysis, and DNA lesions.

Radiobiology

The study of the response of biological tissues to ionising radiation, combining the disciplines of radiation physics and biology.

Electromagnetic Radiation

A class of ionising radiation composed of photons with no mass and no electrical charge, specifically comprising x-rays and gamma rays.

Particle Radiation

A category of ionising radiation consisting of subatomic particles that possess mass and charge (with the exception of neutrons, which have mass but no charge).

Directly Ionising Radiation

Types of radiation possessing both mass and an electric charge (such as protons, electrons, and alpha particles) that directly interact with atomic electrons.

Indirectly Ionising Radiation

Types of radiation lacking both mass and electrical charge (such as x-rays and gamma rays) that produce ionisation via intermediate interactions.

Alpha Particle ($\alpha$-particle)

A particulate radiation entity composed of $2$ protons and $2$ neutrons that possesses a positive charge and can be accelerated.

Linear Energy Transfer (LET)

The rate at which energy is deposited as an ionising particle travels through matter, expressed in kilo-electron volts per micrometer ($\text{keV}/\mu\text{m}$).

Low LET Radiation

Radiation types (such as x-rays and gamma rays) where energy deposition along the track in tissue is sparse or far apart, causing less biological damage for a given dose.

High LET Radiation

Radiation types (such as alpha particles and heavy ions) where energy deposition along the track in tissue is dense or close together, producing greater biological damage.

Relative Biological Effectiveness (RBE)

The ratio comparing the biologic effectiveness of different radiation types, defined as: $\text{RBE} = \frac{\text{Dose from } 250\,\text{keV x-rays necessary to produce a given effect}}{\text{Dose of test radiation necessary to produce the same effect}}$.

Overkill Effect

The reduction in Relative Biological Effectiveness (RBE) observed at very high LET values (above approximately $100\,\text{keV}/\mu\text{m}$), occurring because energy deposited in a target cell exceeds what is needed to kill it, thereby wasting dose.

Direct Action (Direct Interaction)

A mechanism where radiation deposits its energy directly into DNA atoms, leading to excitation or ionisation, chemical bond breakage, strand breaks, and possible cell death; it occurs predominantly with high-LET radiation.

Indirect Action (Indirect Interaction)

A damage mechanism where ionising radiation is absorbed by water molecules, initiating the radiolysis of water to create free radicals that subsequently damage DNA; this accounts for most interactions because the body is $70\%$ to $80\%$ water.

Radiolysis of Water

The breakdown of water molecules ($\text{H}_2\text{O}$) caused by ionising radiation, producing an unstable ion and free electron ($\text{H}_2\text{O} \rightarrow \text{H}_2\text{O}^+ + e^-$) which dissociate into hydrogen ions and free radicals.

Hydroxyl Radical ($\text{OH}^\bullet$)

The main reactive free radical generated during water radiolysis that diffuses a short distance ($\sim 6\,\text{nm}$) to attack DNA and other cellular macromolecules.

Base Damage

A DNA lesion involving the alteration or loss of a nitrogenous base (adenine, thymine, guanine, or cytosine), which can result in mutations if not correctly repaired.

Single-Strand Break (SSB)

A radiation-induced lesion where only one of the sugar-phosphate strands of the DNA double helix is severed; this is usually repaired successfully by cellular mechanisms.

Double-Strand Break (DSB)

A severe lesion involving ruptures in both complementary strands of the DNA double helix in close proximity; it is difficult for the cell to repair and frequently causes mutation or cell death.

DNA Crosslinks

Unwanted covalent bonds produced by radiation between bases on the same strand, between opposing strands, or between DNA and protein molecules, disrupting normal DNA function.

Photoelectric Effect

A photon absorption interaction where an incident x-ray photon transfers all of its energy to an inner-shell electron, resulting in the ejection of a photoelectron.

Compton Effect

A photon interaction where an incident x-ray strikes an outer-shell electron, ejecting a Compton electron and producing a scattered x-ray photon of lower energy and longer wavelength ($\lambda < \lambda'$).