Radiation Physics & Radiobiology

RADIATION PHYSICS & RADIOBIOLOGY

ELECTROMAGNETIC RADIATION

  • Definition: Wave-like fluctuations of electric and magnetic fields.

  • Examples: Gamma Rays, X-rays, visible light, microwaves, and radiowaves.

ELECTROMAGNETIC SPECTRUM

  • Description: A range of electromagnetic waves categorized by wavelength and frequency.

  • Key Components:

    • Energy: Measures penetrating and ionizing power.

    • Frequency: Number of cycles per second, measured in Hertz (Hz).

    • Wavelength: The distance between two consecutive wave crests, inversely related to frequency.

VELOCITY IN THE SPECTRUM

  • Constant: The velocity of all electromagnetic radiation is constant in a vacuum.

RELATIONSHIP BETWEEN FREQUENCY AND WAVELENGTH

  • Inversely Related: As frequency increases, wavelength decreases and vice versa.

PRODUCTION OF X-RAYS

Components of an X-ray Tube

  • Parts:

    • Protective housing

    • Collimator

    • Filter

    • Tungsten target

    • Adjustable lead shutters

    • X-ray beam

Electron-Target Interactions

  • Three Main Interactions:

    1. Heat

    2. Characteristic X-rays

    3. Bremsstrahlung X-rays

Location of Interactions

  • Occurrence: All interactions happen inside the X-ray tube.

Mechanism of X-ray Production

  • Process:

    • High-speed electrons are suddenly decelerated by hitting the tungsten target.

    • Electrons are sourced from the heated cathode filament (thermionic emission) and then focused on the tungsten target.

TYPES OF RADIATION

Bremsstrahlung Radiation

  • Description: Produced when a projectile electron passes near a nucleus, slows down, and changes direction, losing kinetic energy.

  • Significance: Most commonly produced X-rays in diagnostic settings.

Characteristic Radiation

  • Description: Occurs when a projectile electron collides with an inner-shell electron of a target atom, resulting in ionization. The vacancy created is filled by an outer-shell electron, emitting a characteristic X-ray.

  • Photon Energy: Each emitted photon has energy equal to the difference in binding energies of the shells involved.

X-RAY INTERACTIONS IN THE BODY

  • Key Interactions:

    • Photoelectric effect

    • Compton effect

Define Attenuation

  • Definition: The combination of absorption and scattering of radiation.

PHOTOELECTRIC EFFECT

  • Description: Incident X-ray radiation interacts with an inner-shell electron, ionizing the atom and ejecting a photoelectron. The incident X-ray is absorbed, not scattered, culminating in the emission of characteristic X-ray from an outer-shell electron.

COMPTON EFFECT

  • Also Known As: Compton scattering or modified scatter.

  • Process: The incident X-ray interacts with an outer-shell electron, ejecting it and causing the photon to change direction with reduced energy.

  • Ejected Electron: Known as secondary electron or recoil electron.

CLASSICAL SCATTERING

  • Also Known As: Coherent, Thomson, Rayleigh scattering.

  • Description: The incident X-ray changes direction slightly without losing energy, primarily caused by low-energy X-ray photons (below 10 keV).

BEAM CHARACTERISTICS

  • Quality: Reflected by kilovolt peak (kVp) which indicates penetrability.

  • Quantity: Measured in milliampere-seconds (mAs), representing the number of electrons.

BIOLOGICAL EFFECTS OF RADIATION

  • Measurement Units:

    • Absorbed dose: Gray (Gy)

    • Dose equivalent: Sievert (Sv)

    • Exposure: Coulomb per kilogram (C/kg) - measures electron charge produced in air.

    • Effective Dose: Sievert (Sv)

    • Air Kerma: Gray (Gy) - energy transferred to air.

DOSE-RESPONSE RELATIONSHIPS

  • Definition: Graphic representation of the relationship between the absorbed radiation dose and the biological damage (response).

DOSE RESPONSE TERMINOLOGY

  • Linear Dose Response: Directly proportional to dose.

    • Example: Higher doses result in a greater response.

  • Non-linear Dose Response: Varies with different doses, not proportional.

    • Example: Doubling the dose does not double the response.

  • Threshold Response: A certain dose must be reached to elicit a response.

  • Non-threshold Response: Any dose can potentially produce a response.

THRESHOLD & NONTHRESHOLD RESPONSES

  • Linear, Non-threshold: Responses such as radiation-induced leukemia or cancer; any dose presents a risk.

  • Linear, Threshold: Responses only occur above a certain dose.

  • Non-linear, Non-threshold: Any radiation dose can produce effects, often related to stochastic effects.

  • Non-linear, Threshold: No effect until a specific dose is exceeded, linked to deterministic effects.

TYPES OF RISK

  • Deterministic Effects: Early effects of radiation, typically have a threshold.

  • Stochastic Effects: Late and chronic effects, do not have a threshold.

RADIOBIOLOGY

  • Definition: The study of the effects of ionizing radiation on biological tissue.

BIOLOGICAL EFFECTS OF IONIZING RADIATION

Law of Bergonie & Tribondeau

  • Influence on Radiosensitivity:

    1. Stem Cells: Undifferentiated cells are more radiosensitive.

    2. Young Tissues: Highly mitotic tissues are more sensitive.

L.E.T. (Linear Energy Transfer)

  • Definition: Measure of rate at which energy is transferred from ionizing radiation to biological tissues.

  • Value: Higher LET indicates increased biological damage potential.

R.B.E. (Relative Biological Effectiveness)

  • Definition: Ratio of the dose of standard radiation to produce a given effect versus the dose of test radiation needed for the same effect.

  • Relationship: RBE is directly related to LET; as LET increases, RBE increases.

TARGET MOLECULE IN CELLS

  • Identified Target: DNA molecules within cells.

Target Theory

  • Concept: Cell death occurs if target molecules (DNA) are inactivated by radiation exposure.

  • Hits: Occur through direct and indirect effects.

Radiolysis of Water

  • Process: Water molecule separation into ion pairs and free radicals upon irradiation.

    • Consequences:

    1. Ion pairs may either rejoin to form stable water or lead to secondary reactions.

    2. Result is production of reactive free radicals that damage cell bonds.

MOLECULAR EFFECTS OF IONIZING RADIATION

  • Direct Effect: Ionizing event occurs on the target molecule itself.

  • Indirect Effect: Ionizing event occurs on a distant molecule, leading to the production of free radicals that damage the target molecule.

TYPES OF DNA DAMAGE

  • Damage Types:

    1. Main-chain Scission: Dual side rail break (possible cell death) or single side rail break (repairable).

    2. Cross-linking: Long-chain molecule develops side-arms that attach to other parts.

    3. Base Damage: Point lesions or mutations in base pairs.

RADIATION EFFECTS ON MOLECULAR STRUCTURE

  • Main Chain Scission: Breakdown of long molecules leading to viscosity changes.

  • Cross-Linking: Side-arms development in molecules affecting their structure.

  • Point Lesions: Alteration in base sequences can produce genetic errors in daughter cells.

CELLULAR & TISSUE RADIOSENSITIVITY

Tissue Radiosensitivity Order

  • Most Sensitive Cells:

    • Lymphocytes

    • Reproductive Cells

    • Erythrocytes

    • Epithelial cells

    • Endothelial cells

    • Connective tissue cells

    • Bone cells

    • Nerve cells

    • Brain cells

    • Muscle cells

Protraction and Fractionation Definitions

  • Protraction: Small doses delivered over long periods (low dose rate).

  • Fractionation: A higher rate dose divided into smaller doses, allowing for cellular repair and tissue recovery.

OXYGEN AND RADIOSENSITIVITY

  • Oxygen Enhancement Ratio (OER): A measure comparing radiation effects under oxygenated versus non-oxygenated conditions.

  • OER Formula:
    OER=Dose required for effect without oxygenDose required for effect with oxygenOER = \frac{\text{Dose required for effect without oxygen}}{\text{Dose required for effect with oxygen}}

EFFECT OF AGE ON RADIOSENSITIVITY

  • Observations: Fetal tissues are more radiosensitive; sensitivity increases again in advanced age.

GENETIC EFFECTS OF RADIATION

  • Impact on Reproductive Capacity: Damage to reproductive cells that can lead to mutations affecting future generations.

PREGNANCY AND RADIATION

  • Guideline: Radiation should be avoided during pregnancy, particularly during major organogenesis (2 – 12 weeks).

  • Effects of Fetal Irradiation: 250 mGy (25 rads) can lead to spontaneous abortion during the first two weeks of gestation.

SAFETY CONSIDERATIONS FOR FEMALES

  • Elective/10-Day Rule: Optimal time to schedule radiographic procedures to reduce embryo exposure is during the first 10 days following menses.

  • Patient Questionnaire: To assess risks associated with potential pregnancy.

SAFETY CONSIDERATIONS FOR MALES

  • Reproductive Differences: Male testes are easier to shield compared to female ovaries which are housed inside the body cavity.

CHILDREN'S RADIOSENSITIVITY

  • High Vulnerability: Children, particularly oogonia and spermatogonia, exhibit extreme radiosensitivity.

GENETICALLY SIGNIFICANT DOSE (G.S.D)

  • Definition: Average annual gonadal dose to the general population of childbearing years.

  • Estimated Value: 0.2 mSv (20 mrem).

SOMATIC EFFECTS OF RADIATION

Early vs. Late Effects

  • Early Effects: Manifested within minutes to weeks after high doses of ionizing radiation (Acute Radiation Syndrome - ARS).

  • Late Effects: Develop years after initial exposure and are typically due to low chronic exposures.

EXAMPLES OF SOMATIC EFFECTS

  • Carcinogenesis: Cancer formation from normal cells transforming into cancer cells.

  • Cataractogenesis: Cataracts develop in individuals exposed to radiation doses as low as 0.5 Gy (500 mGy).

  • Life-Span Shortening: Historically observed shorter lifespans in radiologists, now less evident.

  • Reproductive Risks: High sensitivity with doses of 2 Gy (200 rad) causing temporary sterility and 5 Gy (500 rad) potentially leading to permanent sterility.

  • Embryonic Effects: Potential for spontaneous abortions and developmental anomalies.

SKIN EFFECTS FROM RADIATION EXPOSURE

  • Threshold Dose: 2 Gy (200 rads) can cause skin reaction, ranging from mild erythema to desquamation.

    • Mild Erythema: Occurs within 1-2 days.

    • Moist Desquamation: Occurs around two weeks, leading to peeling and exposed dermis.

    • Dry Desquamation: Results in itchy or discolored skin.

ACUTE RADIATION SYNDROME (ARS)

  • Definition: Severe illness or death from whole-body doses of 1 Gy (100 rad) or more quickly.

  • Common Terms: Radiation poisoning, radiation sickness, or radiation toxicity.

TYPES OF ARS

  1. Hematopoietic Syndrome: Develops after doses of 0.7 - 10 Gy (70–1000 rads), primarily affects bone marrow, leading to infection and hemorrhage.

  2. Gastrointestinal Syndrome: Appears at doses >10 Gy (1000 rads), symptoms include severe nausea and dehydration, leading to death within 2 weeks.

  3. Central Nervous System Syndrome: Caused by doses >50 Gy (5000 rads), leads to rapid termination of bodily functions and death within days.

STAGES OF ARS

  1. Prodromal Stage: Symptoms like nausea and vomiting occur shortly after exposure.

  2. Latent Stage: Symptoms lessen but can be deceptive as damage occurs internally.

  3. Manifest Illness Stage: Symptoms depending on syndrome appear for hours to months.

  4. Recovery or Death: Recovery can take weeks to years; death may occur months after exposure.

OVERVIEW OF ARS STAGES

  • Prodromal Stage: Symptoms arise 1 hour to 2 days post exposure, mainly nausea and vomiting.

  • Latent Stage: Relief of symptoms for a period.

  • Manifest Illness Stage: Severity depends on syndrome type; medical treatment essential.

  • Recovery/Death Stage: Recovery spans weeks to years, with potential fatalities occurring months later.

Final Note

  • End of Lecture: "Wow… that lecture was exhausting!"

  • Good Luck on your Quiz!