Basic Radiation Biology Concepts

Basic Radiation Biology Concepts

• Radiobiology is a branch of science, combining physics and biology, studying the effects of ionizing radiation on living organisms and biological tissues.
• Basic radiobiological mechanisms study biological effects from energy absorption in small volumes (e.g., single cells).
• Living organisms are composed of protoplasm, which is a solution/suspension of inorganic/organic compounds in water.
• The cell is the basic microscopic unit of all living organisms.
• A group of cells performing one or more functions is called tissue.
• A group of tissues performing one or more functions is called an organ.
• A group of organs performing one or more functions is called an organ system or organism.
• Cells contain inorganic compounds (water and minerals) and organic compounds (proteins, carbohydrates, nucleic acids, and lipids).
• The two main cell components are cytoplasm (supporting metabolism) and nucleus (containing DNA).
• Human cells are categorized as somatic cells (all other cells except sperm and egg) and germ cells (sperm and egg).
• Cells propagate through cell division: somatic cells by mitosis (creating genetically identical cells) and germ cells by meiosis (creating four sex cells with half the original chromosomes).
• Somatic cell division produces two cells, each with the same chromosome complement as the original cell.
• Cells themselves can undergo further division, producing more progeny.
• Chromosome is the thread-like, hereditary part of the cell containing genes. Humans have 23 pairs of chromosomes (22 non-sex + 1 sex chromosome pair).
• A gene is the hereditary unit occupying a specific position on a chromosome.
• Somatic cells can be classified as stem cells, transit cells, and mature cells.
• Stem cells can continually divide and differentiate into various cell/tissue types. Key characteristic: self-renewal (copying) and differentiation (specialization).

Law of Bergonié and Tribondeau

• Radio-sensitivity in tissues depends on the number of undifferentiated cells, mitotic activity, and proliferation time.
• The Law states that radio-sensitivity of tissues increases with a higher number of undifferentiated cells, greater mitotic activity and a longer proliferation time.
• Stem cells are highly radiosensitive, while mature cells are more resistant.
• Younger tissues are more radiosensitive.
• Tissues with high metabolic activity are highly radiosensitive.
• High proliferation and growth rates mean high radiosensitivity.

Radiation Interactions with Human Cells

• Ionizing radiation can either interact directly or indirectly with the cell.
• Direct interaction involves ionization radiation striking the macromolecules (like DNA or proteins) of the cell, possibly causing mutations or cell death. Single-stranded DNA damage is easier to repair than double-stranded damage.
• Indirect interaction involves ionizing radiation interacting with water, producing free radicals that damage cell structures. Cells undergoing active mitosis are potentially more prone to effects of radiation.
• The cell repair mechanism is probabilistic, and damage does not always lead to cell-death.
• Radiation effects are observed only when the cell replacement mechanism slows down or fails.
• There is a latent period after radiation exposure before any observable response. The duration of this period may be from hours to decades, depending on the radiation doses.
• Radiation is categorized as ionizing or non-ionizing. Ionizing radiation is further divided into directly-ionizing (e.g., charged particles) and indirectly-ionizing (e.g., photons).

Types of Cell Damage

• Direct hits can cause damage or complete breaks in cell DNA, leading to cell death.
• Humans have an abundant number of cells and somatic cell reproduction (mitosis) occurs to replace dead cells.
• Actively dividing cells are more sensitive to radiation than non-dividing cells.

Other important aspects of Radiation biology.

• Experiments on fruit flies and mice show that ionizing radiation can induce mutations in offspring, but also that such mutations are similar to naturally occurring spontaneous mutations.
• The effect of radiation depends on the total dose and exposure rate. A large dose given in a short timeframe is more damaging than the same dose spread over a longer period. Radiation interacts with cells probabilistically. Cellular repair often takes place, and permanent damage from radiation exposure is not guaranteed.
• Damage may occur through chemical, heat or physical means, in addition to radiation, and damage is not unique to radiation.