wk4 intro to radiation and radiation protection

Learning Outcomes

• Describe ionising radiation and its effects on humans.

• Compare different types of radiation exposure.

• Explain the basic principles of radiation protection and how they are used effectively in nuclear medicine practice.

Radiation Basics

• Radiation used in Medical Radiation Science is located on the Electromagnetic (EM) spectrum.

• Characteristics of this radiation determine its ability to penetrate tissue and cause biological damage:     * High energies.     * Small wavelengths.

Ionising Radiation

• Verbatim Definition of Ionisation: The process in which an electron is given enough energy to break away from an atom – results in the formation of $2$ charged particles or ions.

• Biological Damage: The energy produced during ionisation can cause chemical changes by breaking chemical bonds, which can lead to damage in living tissue.

• Sources: Produced from both natural and artificial sources.

Types of Ionising Radiation

• Electromagnetic Radiation (EMR):     * X-Rays: Used in diagnostic radiography and radiation therapy. They are produced by electrons external to the nucleus or when electrons strike a target. An electron moves from a higher energy state to a lower one, causing excess energy to be released.     * Gamma rays: Used in nuclear medicine imaging (General NM). They are emitted from an excited nucleus after it undergoes radioactive decay. They possess high energy and are able to pass through many different materials.

• Charged Particles:     * Alpha particles ($\alpha$): Used in nuclear medicine therapy applications. They are particles composed of $2$ protons and $2$ neutrons tightly bound, emitted from the nucleus during radioactive decay. They are highly ionising but have very low penetration depth.     * Beta particles ($\beta^{-}$): Primarily used in nuclear medicine therapy applications. These are high-speed electrons ejected from the nucleus during radioactive decay when there are too many neutrons. They are less ionising than alpha particles but more penetrating.     * Positrons ($\beta^{+}$): Used in nuclear medicine imaging (PET imaging). These are emitted from unstable radioisotopes that have too few neutrons. When a positron and electron come too close, they "annihilate" each other, converting into energy in the form of $2$ gamma rays emitted in opposite directions.

• Penetration Capabilities Summary:     * Alpha rays: Stopped by paper.     * Beta rays: Stopped by skin and body tissues.     * Gamma rays: Stopped by lead.

Effects of Ionising Radiation

• General Dosage Effects:     * High doses cause harm; there is uncertainty regarding the effects of low doses.     * Extreme doses (> 10\,Sv): Received in a short period, these may result in death within days or weeks.     * Very high doses ($1\,Sv$ to $10\,Sv$): Severely impair the function of organs.

• Stochastic (Probabilistic) Effects:     * Definition: The probability (rather than the severity) that exposure to radiation will result in damage to the exposed organism.     * Included effects are radiation-induced hereditary effects and cancer induction.     * The likelihood of developing cancer is dose-related; a greater dose makes cancer development more likely.     * These are statistical probabilities; it is impossible to predict specifically who will develop cancer following exposure.     * Linear, no-threshold (LNT) hypothesis: Presumes that the proportion of risk and dose observed at high levels continues through all lower levels of dose down to zero.     * Radiation Hormesis: The theory that low doses of radiation are not necessarily harmful to the organism.

• Non-Stochastic (Deterministic) Effects:     * These do not result from exposure alone but are strictly dose-related.     * There are established thresholds above which radiation damage is expected in all individuals.     * The threshold value varies from person to person.     * Once the threshold is exceeded, the severity of the effect increases as the radiation dose increases.     * Effects depend on the dose, dose rate, and type of radiation.

• Effects of Total Body Irradiation (Acute Syndromes):     * Hemopoietic syndrome (bone marrow): Results in immunosuppression and bleeding.     * Gastrointestinal syndrome: Includes usual GI symptoms, the formation of intestinal ulcers, and the gut becoming nonfunctional.     * Cerebrovascular syndrome: Results in severe nausea, vomiting, a burning sensation on the skin, coordination issues, coma, and death within $2$ to $3$ days.

• Long-term Effects of Irradiation:     * Carcinogenesis: Radiation-induced mutations of cells; Leukemia is the most common form.     * Radiation damage to the skin.     * Sterility.     * Cataracts.     * Genetic effects.

Radiobiology and History

• Law of Bergonie and Tripondeau (1906): Following experiments exposing rabbit testes to radiation, the law states that "radiosensitivity is a function of the metabolic state of the cell being irradiated."     * Increased radiosensitivity: Found in cells with high metabolic rates and in young or immature cells.     * Lower radiosensitivity: Found in cells with increased maturity.

• Historical Milestones and Disasters:     * 1904: Clarence Dally became the first fatality directly from X-ray exposure while developing a fluorescent X-ray lamp.     * 1906: Henri Becquerel accidentally burned himself while carrying radioactive materials in his pocket.     * 1945: Atomic bombs dropped over Hiroshima and Nagasaki killed approximately $200,000$ people from explosive blasts, firestorms, and acute radiation poisoning.     * 1960s/1970s: French nuclear testing occurred in the South Pacific at Mururoa Atoll.     * 1986: Chernobyl disaster involved an uncontrolled nuclear chain reaction.     * 2011: Fukushima disaster; a tsunami flooded emergency generators, leading to nuclear meltdowns and radioactive material release.

Measuring Radiation

• Activity: Measured in Becquerel ($Bq$). It describes "how brightly" a source (e.g., Cesium) glows.

• Absorbed Dose ($D$):     * Unit: Gray ($Gy$).     * Definition: The amount of energy transfer.

• Dose Equivalent ($H$):     * Unit: Sievert ($Sv$).     * Definition: A measure of biological effect.     * Calculation: Multiply the absorbed dose by a factor specific to the type of radiation.

• Effective Dose ($E$):     * Unit: Sievert ($Sv$).     * Definition: Accounts for the radiosensitivity of specific organs.     * Calculation: The sum of the Dose Equivalent multiplied by factors for individual organs.

Types of Radiation Exposure

• Natural Background Radiation:     * Cosmic radiation: Residual background radiation in space left over from the Big Bang. Dose increases with altitude.     * Terrestrial radiation: Natural radioactivity found in rocks and soil.     * Inhalation: Radon gas seeping from the ground.     * Variable Factors: Dose depends on soil/rock type, altitude, latitude, and diet.     * Average Dose: Approximately $1.5\,mSv/yr$ (inclusive of cosmic radiation).

• Cosmic Radiation Dose Rates by Altitude:     * Sea level: $0.0\,mSv$ per hour.     * Mexico City ($2,240\,m$): $0.08\,mSv$ per hour.     * La Paz, Bolivia ($3,900\,m$): $0.20\,mSv$ per hour.     * International air travel ($10,000\,m$ to $12,000\,m$): Approximately $3.7\,mSv$ per hour.     * Supersonic air travel ($15,000\,m$): $10\,mSv$ per hour.

• Case Study: Air New Zealand Crew:     * International Routes: $1,000$ hours per year ($90\%$ at $12\,km$ altitude) results in $6.5\,mSv$ annual dose.     * Domestic Routes: $1,000$ hours per year ($70\%$ at $11\,km$ altitude) results in $3.5\,mSv$ annual dose.

• High Background Radiation Areas (HBRA):     * Yangjiang (China), Kerala (India), Guarapari (Brazil), and Ramsar (Iran).     * Ramsar, Iran: The highest known background radiation in the world. Effective dose equivalents are several times higher than ICRP-recommended limits for radiation workers due to $^{226}Radium$ brought to the surface by hot springs.

• Sources of Radiation Exposure (NCRP Report No. 160):     * Radon & Thoron (Background): $37\%$     * Computed Tomography (Medical): $24\%$     * Nuclear Medicine (Medical): $12\%$     * Interventional Fluoroscopy (Medical): $7\%$     * Conventional Radiography/Fluoroscopy (Medical): $5\%$     * Internal (Background): $5\%$     * Space (Background): $5\%$     * Terrestrial (Background): $3\%$     * Consumer products: $2\%$     * Occupational: < 0.1\%     * Industrial: < 0.1\%

Basic Principles of Radiation Protection

The system recommended by the ICRP (International Commission on Radiological Protection) relies on three principles:

1. Justification: The benefit of the procedure must outweigh the risk (Benefit > Risk).

2. Optimization: Using ALARA (As Low As Reasonably Achievable) to minimize exposure.

3. Dose Limitation: Strict adherence to numerical dose limits.

Occupational Exposure and Safety

• Current Dose Limits (ICRP 60/103):     * Public: $1\,mSv$ per year.     * Occupational: $20\,mSv$ per year (averaged over $5$ years, with a maximum of $50\,mSv$ in any single year).

• Nuclear Medicine Technologists (NMT) receive the highest occupational exposures among health workers, primarily during dose preparation, dose administration, and patient contact.

• Basic Radiation Safety Precautions:     * Time: Dose received is directly proportional to time of exposure. Minimize time in hot labs and near radioactive patients.     * Distance: Follows Newton’s Inverse Square Law ($I \propto \frac{1}{d^2}$). Doubling distance reduces exposure by a factor of $4$. (Example: Moving from $1\,m$ to $2\,m$ from a $10\,\mu Sv/hr$ source reduces exposure to $2.5\,\mu Sv/hr$).     * Shielding: Use high atomic number ($Z$) and high-density materials (Lead, Tungsten, Concrete, Steel).

• Half Value Layer (HVL): The thickness of material required to reduce the number of photons to one-half of the initial number.

• Syringe Shielding Specifics:     * Tungsten (W) is denser than lead (Pb).     * $1.5\,mm$ Tungsten shield: Decreases finger dose from $^{99m}Tc$ by $94\%$.     * $14\,mm$ Tungsten PET shield: Decreases finger dose from $511\,keV$ photons by approximately $97\%$.

• Lead Aprons: Effectiveness depends on radiation energy.     * $0.5\,mm\,Pb$ apron: Reduces exposure from $^{99m}Tc$ by a factor of roughly $4$.     * $0.3\,mm\,Pb$ apron: Reduces exposure from $^{99m}Tc$ by a factor of roughly $2.5$.     * Note: Lead aprons are not used routinely in NM due to weight and effectiveness misconceptions.

• Pregnancy and NM Staff:     * Once a pregnancy is declared, the dose limit to the surface of the abdomen is $1\,mSv$.     * The embryo is treated with the same protection level as a member of the public.     * NMTs and students should declare pregnancy to supervisors as soon as possible to implement additional precautions.

Measuring Occupational Exposure

• Thermoluminescent Dosimeter (TLD):     * Used by medical radiation workers.     * Radiation excites electrons in the TLD material, which stores the energy.     * TLD badges are sent for reading and heated; excited electrons return to the ground state and emit light.     * Light output is proportional to the radiation exposure.

• Ring TLD dosimeter: Used for extremity monitoring; predominantly worn by NMTs, especially in PET settings.

Resources

• ARPANSA: http://www.arpansa.gov.au/RadiationProtection/index.cfm

• ICRP: http://www.icrp.org/

• UNSCEAR: http://www.unscear.org/unscear/en/faq.html

• Course Reading: "Basic Review of Radiation Biology and Terminology" on Canvas.