Human Response to Radiation – Exam Notes

Deterministic vs. Stochastic Effects

Deterministic Effects: These effects happen only if a certain dose of radiation (a threshold) is reached. If the dose is below this threshold, they won't occur. The severity of these effects increases as the radiation dose goes up, and they usually appear relatively quickly, within days or weeks after exposure.

• Acute Radiation Syndrome (ARS): This is a severe set of symptoms that occur after a high dose of radiation affects the whole body. It has several distinct forms:

  • Hematologic Syndrome: This involves damage to the bone marrow, which makes blood cells. It leads to a severe drop in all types of blood cells, making a person vulnerable to infections and bleeding.

  • Gastrointestinal Syndrome: This results from damage to the lining of the digestive system, causing severe nausea, vomiting, diarrhea, and dehydration.

  • Central Nervous System (CNS) Syndrome: This is the most serious form, occurring after extremely high doses. It quickly leads to the failure of the circulatory and nervous systems, resulting in rapid death.

• Local Tissue Injury: This refers to damage observed in specific parts of the body, like:

  • Skin: Reddening (erythema), peeling (desquamation), or hair loss (epilation).

  • Gonads: Damage to reproductive organs.

  • Extremities: Injury to arms or legs.

• Hematologic Depression: A general decrease in the number of blood cells, especially white blood cells (lymphocytes are very sensitive), red blood cells, and platelets.

• Cytogenetic Damage: Visible changes or abnormalities in the structure of chromosomes within cells, such as breaks or abnormal arrangements.

Stochastic Effects: These effects are different because there's no threshold dose; even a very small amount of radiation carries a slight chance of causing them. The probability (or chance) of these effects happening increases with the dose, but if they do occur, their severity doesn't depend on how high the dose was. There's also a long waiting period (latency) of months to many years before they show up.

• Malignancies (Cancers): The main concern, as radiation can increase the risk of various cancers like leukemia, bone cancer, lung cancer, thyroid cancer, and breast cancer. The risk is typically described as a probability (e.g., a certain chance per unit of dose).

• Local Late Damage: Long-term problems in specific tissues, such as skin thinning (atrophy), or cataracts (clouding of the eye lens), and long-lasting damage to the reproductive organs.

• Life-span Shortening: A general observation that groups exposed to radiation, on average, tend to live slightly shorter lives, even if they don't develop a specific radiation-induced disease.

• Genetic Effects: Damage to the DNA in sperm or egg cells, which could potentially lead to mutations or health problems in future offspring. While theoretically possible, strong evidence for genetic effects in humans from low-level radiation exposure is limited.

Fetal Exposure Risks: If a developing fetus is exposed to radiation, the risks depend on the stage of pregnancy and the dose. Potential problems include:

• Prenatal or neonatal death (death before or shortly after birth).

• Severe congenital malformations (birth defects, especially during the stage when organs are forming).

• Growth retardation (slower development).

• Increased risk of childhood cancers, particularly leukemia.

Human Evidence Groups: Our understanding of how radiation affects humans comes from studying groups of people who have been exposed to radiation historically. These include:

• American radiologists: Early medical professionals who received significant occupational exposures.

• Japanese A-bomb survivors: Provided crucial long-term data on cancer incidence and other health effects.

• Accident victims: Individuals involved in radiation accidents (e.g., Chernobyl, Goiania).

• Marshall Islanders: Exposed to radioactive fallout from nuclear testing.

• Uranium miners: Showed an increased risk of lung cancer due to radon exposure.

• Radium painters: Developed bone cancer from ingesting radium.

• Medical cohorts: Patients who received therapeutic radiation for medical conditions.

Atomic & Molecular Composition

Our bodies are made up of different atoms and molecules:

Atoms in body:

• Hydrogen (H): 60.0\%

• Oxygen (O): 25.7\%

• Carbon (C): 10.7\%

• Nitrogen (N): 2.4\%

• Calcium (Ca): 0.2\%

• Phosphorus (P): 0.1\%

• Sulfur (S): 0.1\%

• Trace elements (other minor elements): 0.8\%

Molecules in tissue:

• Water: 80\%

• Protein: 15\%

• Lipid (fats): 2\%

• Carbohydrate (sugars/starches): 1\%

• Nucleic acid (DNA/RNA): 1\%

• Other organic and inorganic molecules: 1\%

Key Macromolecules

These are the large, complex molecules essential for life:

• Water: The most abundant molecule in cells, making up about 80\%. It's vital for regulating body temperature, acts as a solvent (dissolving many substances), and participates in countless biochemical reactions within the body.

• Proteins: These are complex molecules built from smaller units called amino acids, linked together in long chains. Proteins do a huge variety of jobs:

  • Enzymes: Speed up chemical reactions in the body.

  • Hormones: Act as chemical messengers, signaling between cells.

  • Antibodies: Part of the immune system, helping to fight off infections.

  • They also provide structural support to cells and tissues.

• Lipids: This group includes fats, oils, and related substances. They are important for:

  • Building cell membranes (the outer layer of cells).

  • Storing energy for long periods.

  • Providing insulation to keep the body warm.

• Carbohydrates: These are the body's main source of quick energy. Glucose, a simple sugar, is the primary metabolic fuel cells use to produce ATP (the cell's energy currency). They also form storage molecules like glycogen.

• DNA (Deoxyribonucleic Acid): This is the most crucial molecule in terms of radiation sensitivity. DNA carries the entire genetic code, which contains all the instructions for how a cell functions and how an organism develops. It's organized into a double helix structure and is responsible for passing traits from one generation to the next.

Cell Structure & Function

Cells are the basic units of life, and they have distinct parts:

• Nucleus: This is like the control center of the cell. It contains all the cell's genetic material (DNA), organized into structures called chromosomes. It also contains RNA, various proteins, and water. The nucleus is enclosed by a double membrane called the nuclear envelope, and within it is the nucleolus, which is involved in making ribosomes (protein factories).

• Cytoplasm: This refers to everything inside the cell membrane except for the nucleus. It consists of two main parts:

  • Cytosol: The jelly-like substance that fills the cell.

  • Organelles: Various small structures suspended in the cytosol, each with a specific job.

• Organelles: These are like miniature organs within the cell:

  • Endoplasmic Reticulum (ER): A network of membranes that forms a communication system throughout the cell. It's involved in making lipids and proteins. There are two types:

    • Rough ER: Has ribosomes attached and is involved in synthesizing and modifying proteins.

    • Smooth ER: Involved in lipid synthesis and detoxifying harmful substances.

  • Mitochondria: Often called the "powerhouses" of the cell. They are responsible for producing most of the cell's energy in the form of ATP (adenosine triphosphate) through cellular respiration.

  • Ribosomes: Tiny structures responsible for protein synthesis (making proteins).

  • Lysosomes: Contain digestive enzymes that break down waste materials and cellular debris.

• Protein synthesis: This is the process of making proteins, involving two main steps:

  • Transcription: The DNA's genetic code is copied into a messenger RNA (mRNA) molecule in the nucleus.

  • Translation: The mRNA then travels to a ribosome, where transfer RNA (tRNA) molecules deliver the correct amino acids, assembling them into a protein chain according to the mRNA's code.

Cell Reproduction

Cells multiply in two main ways:

• Somatic cells (body cells): Reproduce through mitosis. This process creates two identical daughter cells from one parent cell. It's essential for growth, repair, and replacement of old cells. The cell cycle for mitosis has specific phases:

  • G*1 (Gap 1): The cell grows and carries out its normal functions, preparing for DNA replication.

  • S (Synthesis): The cell synthesizes (copies) its DNA, so each chromosome now consists of two identical sister chromatids.

  • G*2 (Gap 2): The cell continues to grow and synthesizes proteins necessary for cell division, preparing for mitosis.

  • M (Mitosis): The actual cell division phase, which includes:

    • Prophase: Chromosomes condense and become visible.

    • Metaphase: Chromosomes align in the middle of the cell.

    • Anaphase: Sister chromatids separate and move to opposite poles.

    • Telophase: New nuclear envelopes form around the separated chromosomes, and the cell begins to divide into two.

• Genetic cells (germ cells, like sperm and egg): Reproduce through meiosis. This process creates four genetically unique cells, each with half the number of chromosomes of the parent cell. It's crucial for sexual reproduction.

  • First meiotic division: A single cell divides into 2 cells, each containing 46 chromosomes (still duplicated, or 2 chromatids per chromosome).

  • Second meiotic division: These 2 cells then divide again, resulting in 4 cells, each with 23 unduplicated chromosomes. This division also includes crossing-over, a process where genetic material is exchanged between homologous chromosomes, adding significant genetic variability to the offspring.

Tissues, Organs & Radiosensitivity

Our bodies are organized into different levels:

• Tissue categories: Groups of similar cells working together.

  • Epithelium: Tissues that cover body surfaces, line internal organs and cavities, and form glands (e.g., skin, lining of intestines).

  • Connective/Supporting tissue: Provides support and connects other tissues (e.g., bone, cartilage, blood, fat).

  • Muscle tissue: Responsible for movement.

  • Nervous tissue: Transmits electrical signals throughout the body.

• Organ parts: Organs are made of:

  • Parenchyma: The functional cells of an organ (e.g., liver cells in the liver).

  • Stroma: The supporting framework of an organ, including connective tissue, blood vessels, and nerves.

• System examples: Different organs working together for specific functions:

  • Nervous system, Reproductive system, Digestive system, Respiratory system, Endocrine system, etc.

• Radiosensitivity ranking: Not all cells and tissues respond to radiation in the same way. Some are much more sensitive than others:

  • High Sensitivity: Cells that divide rapidly and are immature.

    • Lymphocytes (a type of white blood cell).

    • Spermatogonia (precursor cells to sperm).

    • Erythroblasts (precursor cells to red blood cells).

    • Intestinal crypt cells (cells lining the intestines that divide rapidly).

  • Intermediate Sensitivity:

    • Endothelial cells (lining blood vessels).

    • Osteoblasts (bone-forming cells).

    • Spermatids (developing sperm cells).

    • Fibroblasts (cells that produce connective tissue).

  • Low Sensitivity: Cells that divide slowly or not at all, and are highly specialized.

    • Muscle cells.

    • Nerve cells.

• Rule of Radiosensitivity: Generally, stem cells (immature, rapidly dividing cells) are much more radiosensitive than mature cells (fully developed, specialized cells).

Protection Imperatives

• When using diagnostic imaging, the diagnostic benefit to the patient must always be greater than the potential risk from the radiation exposure, and this exposure should always be minimized.

• The entire radiologic team (doctors, technologists, physicists) has a responsibility to ensure patient and staff safety. This includes:

  • Appropriate shielding (e.g., lead aprons).

  • Technique optimization (using the lowest radiation dose settings that still provide a good image).

  • Monitoring radiation exposure levels for staff and equipment.