Radiobiology 2 MCQ

Cell Cycle Phases

• The cell cycle is a series of events that lead to cell growth and division
• The cell cycle has several phases: G0, G1, S, G2, and mitosis.
• G0: Cell cycle arrest; the cell is not actively dividing.
• G1: Cellular contents, excluding the chromosomes, are duplicated.
• S: Each of the 46 chromosomes is duplicated by the cell.
• G2: The cell "double checks" the duplicated chromosomes for error, making any needed repairs.
• Mitosis: Cell division
• Cytokinesis: The splitting of the cell.

Cell Cycle Times

• CHO hamster cells have a cell cycle time ranging from 11 hours to 24 hours.
• HeLa human cells have a cell cycle time ranging from approximately 24 hours to 48 to 72 hours.

Radiosensitivity

• More DNA is present in one area at the S phase.
• Increased chromatin in cancer cells makes them more radiosensitive than normal cells.
• The S phase is considered the most radiosensitive time in the cell cycle, as more DNA is present in one area at that point in the cycle. Increased chromatin in cancer cells, as a result of unusually high mitotic rates, also leads to increased radiosensitivity.

Indirect Interaction

• Radiation interacts with water molecules rather than the macromolecules in the cell.
• This water interaction results in the hydrolysis of water.
• The products of this interaction are hydrogen atoms and hydroxyl radicals.
• Hydroxyl molecules recombine to form hydrogen peroxide, which in the cell readily combines with other organic compounds to form stable organic hydrogen peroxide molecules.
• This process may result in the loss of essential enzymes, leading to cell death or future mutations.

Antioxidants

• Antioxidants block the recombination of hydroxyl radicals, preventing stable organic hydrogen peroxide compounds from forming.
• This process by which the body can defend itself from indirect radiation interactions is why antioxidants have received so much attention lately as a potential cancer prevention agent.

Hydrolysis of Water

• Ionizing radiation interacting with water results in the formation of hydrogen ions and hydroxyl radicals.
• Antioxidants can recombine to create water, or a stable compound, but without them, unstable hydrogen peroxide molecules can be formed from hydroxyl free radicals.
• If these molecules combine with an organic molecule, permanent damage can occur.
• The resulting damage leads to the lack of essential enzymes or eventual cell death.

Cellular Injury

• Ionizing radiation exposure can cause three types of cellular injury: division delay, reproductive failure, and interphase death.
• Division delay involves a temporary delay in cell division, with the delay dependent on the dose.
• Reproductive failure appears when cells stop successful mitosis completion.
• Interphase cell death is a fast, relatively prompt cell death event initiated by apoptosis.
• Some cancer cells also show apoptosis in response to radiation.

RBE (Relative Biological Effectiveness)

• A measure of the effectiveness of different radiation types in producing a biological effect.
• Lower LET radiation (like gamma rays, X-rays) have a lower RBE because they deposit energy more sparsely in the cell.
• Higher LET radiation (like alpha particles) have a higher RBE because they deposit energy more densely in the cell.
• To adjust for differences in radiation quality, radiation dose is often adjusted using equivalent doses.
• Equal doses of different radiation types do not produce equal biologic effects.
• For low LET radiation RBE is approximately equivalent to LET

Linear Energy Transfer (LET)

• The rate at which energy is deposited as a charged particle travels through matter.
• LET depends on differing qualities of radiation type.
• LET is determined by quality of radiation, its quantity, the dose of radiation, and the exposure conditions. For example alpha, neutrons, protons, and electrons have significantly different LETs, thereby indicating different energy transfer capabilities. The units for LET are typically keV/µm (kilo electron volts per micrometer).
• The type of particle and energy determine the amount of energy deposited per unit path length.
• This is related to the probability of producing a double strand break, and to biological effects.

Dose-Response Models

• There are different theoretical dose-response models to explain radiation effects.
• These range from a linear-no-threshold model (where any dose of radiation is harmful), to a linear-threshold model (where some threshold dose is needed before cell injury occurs); or to a linear quadratic model (that introduces a quadratic component to cell damage).
• These models are used to extrapolate the effects from high-dose radiation to low-dose radiation exposure to estimate outcomes and to quantify the probability of harmful effects.

Optimal LET

• The optimal or most effective LET for causing biological effects is about 100 keV/µm.
• At this density, ionizing events occur roughly equal to the DNA double helix diameter.
• This causes damage in the form of double-strand DNA breaks, which is the basis of many biological effects.
• An X-ray's lower LET would have a lower RBE, resulting in less cell damage with equal dose as a higher LET particle.

Explore the different phases of the cell cycle, including G0, G1, S, G2, and mitosis, as well as the time it takes for CHO and HeLa cells to complete their cycles. Understand the concept of radiosensitivity during these phases, particularly focusing on the S phase. This quiz will deepen your knowledge about cellular processes and their implications in cancer cells.