🔋College Physics I – Introduction Unit 32 – Nuclear Physics in Medicine

Fundamentals of Nuclear Physics

• Atomic structure consists of a dense nucleus containing protons and neutrons surrounded by electrons in orbitals
  • Protons have a positive charge, neutrons are neutral, and electrons have a negative charge
  • The number of protons in the nucleus determines the element's atomic number (Z)
  • Isotopes are atoms of the same element with different numbers of neutrons
• Nuclear forces are the strong and weak interactions that hold the nucleus together
  • The strong force binds quarks together to form protons and neutrons and holds the nucleus together
  • The weak force is responsible for radioactive decay processes
• Mass-energy equivalence is described by Einstein's equation $E=mc^2$, linking mass and energy
  • In nuclear reactions, small changes in mass can result in large amounts of energy released or absorbed
• Binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons
  • Nuclei with higher binding energy per nucleon are more stable (iron-56 is the most stable)
• Radioactive decay is the spontaneous emission of particles or energy from an unstable nucleus
  • Types of decay include alpha (α), beta (β), and gamma (γ) decay
  • Half-life is the time required for half of a radioactive sample to decay

Radioactivity and Decay Processes

• Alpha decay involves the emission of an alpha particle (two protons and two neutrons) from the nucleus
  • Alpha particles have a positive charge and are relatively heavy, limiting their penetration depth
  • Examples of alpha emitters include uranium-238 and radon-222
• Beta decay occurs when a neutron transforms into a proton, emitting an electron (beta minus, β⁻) and an antineutrino
  • Beta plus (β⁺) decay involves a proton converting into a neutron, emitting a positron and a neutrino
  • Examples of beta emitters include carbon-14 and strontium-90
• Gamma decay is the emission of high-energy photons from an excited nucleus
  • Gamma rays have no charge or mass and can penetrate deeply into matter
  • Gamma emission often accompanies alpha or beta decay (cobalt-60)
• Decay chains are series of radioactive decays that occur until a stable isotope is reached
  • The uranium-238 decay chain ends with stable lead-206
• Activity is the rate of decay, measured in becquerels (Bq) or curies (Ci)
  • One becquerel equals one decay per second
  • The activity of a sample decreases exponentially with time according to the half-life

Radiation Detection and Measurement

• Geiger-Müller counters detect ionizing radiation by measuring electrical pulses created by charged particles
  • Consists of a gas-filled tube with a central electrode and a high voltage applied
  • Ionization events trigger avalanches of secondary ionizations, producing measurable pulses
• Scintillation detectors use materials that emit light when exposed to ionizing radiation
  • Common scintillators include sodium iodide (NaI) and bismuth germanate (BGO)
  • Photomultiplier tubes convert the light into electrical signals for measurement
• Semiconductor detectors (silicon or germanium) create electron-hole pairs when exposed to radiation
  • The number of pairs is proportional to the energy deposited by the radiation
  • Provides excellent energy resolution for spectroscopy applications
• Thermoluminescent dosimeters (TLDs) store radiation exposure information in crystal defects
  • Heating the TLD releases the stored energy as light, which is measured to determine the dose
• Film badges contain radiation-sensitive film that darkens when exposed to ionizing radiation
  • The degree of darkening is proportional to the radiation dose received
  • Commonly used for personal dosimetry in medical and industrial settings

Biological Effects of Radiation

• Ionizing radiation can cause damage to biological molecules, such as DNA, proteins, and lipids
  • Direct effects occur when radiation interacts directly with critical targets (DNA strand breaks)
  • Indirect effects involve the production of reactive species (free radicals) that damage biomolecules
• Radiation dose is the energy absorbed per unit mass of tissue, measured in grays (Gy) or rads
  • One gray equals one joule of energy absorbed per kilogram of tissue
  • Equivalent dose (in sieverts or rems) accounts for the biological effectiveness of different radiation types
• Acute radiation syndrome (ARS) occurs when the body is exposed to a large dose of radiation in a short time
  • Symptoms may include nausea, vomiting, fatigue, and skin burns
  • High doses can lead to organ failure and death
• Stochastic effects are probabilistic and have no threshold dose (cancer and genetic mutations)
  • The probability of occurrence increases with dose, but the severity is independent of dose
• Deterministic effects have a threshold dose below which they do not occur (skin erythema, cataracts)
  • The severity of the effect increases with dose above the threshold
• Radiation protection aims to minimize the risk of radiation-induced health effects
  • Principles include justification, optimization (ALARA), and dose limitation
  • Time, distance, and shielding are key factors in reducing exposure

Medical Imaging Techniques

• X-ray radiography uses X-rays to create 2D images of internal structures
  • X-rays pass through the body and are attenuated based on tissue density
  • Contrast agents (barium, iodine) can enhance the visibility of specific organs or blood vessels
• Computed tomography (CT) produces cross-sectional images by rotating an X-ray source and detectors around the patient
  • 3D images are reconstructed from multiple 2D projections
  • Provides excellent anatomical detail and can visualize bone, soft tissue, and blood vessels
• Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to create detailed images
  • Protons in the body align with the magnetic field and absorb and emit radio waves
  • Different tissue types have varying relaxation times, allowing for contrast in the images
• Nuclear medicine imaging uses radioactive tracers to visualize physiological processes
  • Positron emission tomography (PET) detects gamma rays from the annihilation of positrons emitted by the tracer
  • Single-photon emission computed tomography (SPECT) detects gamma rays directly emitted by the tracer
• Ultrasound imaging uses high-frequency sound waves to create real-time images of internal structures
  • Sound waves are reflected at tissue boundaries, creating echoes that are detected and processed
  • Doppler ultrasound can measure blood flow velocity by analyzing frequency shifts in the echoes

Radiation Therapy in Cancer Treatment

• External beam radiation therapy (EBRT) delivers high-energy radiation from outside the body
  • Linear accelerators (LINACs) produce high-energy X-rays or electron beams
  • Intensity-modulated radiation therapy (IMRT) uses multiple beams of varying intensities to conform the dose to the tumor
• Brachytherapy involves placing radioactive sources directly inside or near the tumor
  • Low-dose rate (LDR) brachytherapy uses sources with lower activity over a longer time (prostate cancer)
  • High-dose rate (HDR) brachytherapy uses high-activity sources for shorter durations (cervical cancer)
• Radioisotope therapy uses targeted radionuclides that concentrate in specific tissues or tumors
  • Iodine-131 is used to treat thyroid cancer and hyperthyroidism
  • Radium-223 targets bone metastases in prostate cancer
• Proton therapy uses accelerated proton beams to deliver precise doses to tumors
  • Protons deposit most of their energy at the end of their range (Bragg peak), sparing healthy tissue
• Radiation treatment planning involves simulating and optimizing the dose distribution
  • CT scans are used to create a virtual patient model and delineate the target volume and organs at risk
  • Dose constraints are set to maximize tumor coverage while minimizing normal tissue toxicity

Safety and Radiation Protection

• The ALARA (As Low As Reasonably Achievable) principle guides radiation protection practices
  • Justification: The benefits of a procedure must outweigh the risks
  • Optimization: Doses should be kept as low as possible while achieving the desired outcome
• Occupational dose limits are set to protect workers from the harmful effects of radiation
  • The effective dose limit for radiation workers is typically 20 mSv per year
  • Dose limits for the lens of the eye, skin, and extremities are higher due to lower sensitivity
• Shielding is used to reduce radiation exposure by attenuating the intensity of the radiation
  • Lead aprons and thyroid shields protect radiosensitive organs during medical procedures
  • Concrete and lead walls shield radiation areas in hospitals and nuclear facilities
• Personal dosimeters monitor individual radiation exposure
  • Film badges, thermoluminescent dosimeters (TLDs), and optically stimulated luminescence (OSL) dosimeters are commonly used
  • Dosimeters are worn on the body and regularly analyzed to track cumulative dose
• Contamination control measures prevent the spread of radioactive materials
  • Protective clothing (gloves, gowns, shoe covers) minimizes the risk of personal contamination
  • Decontamination procedures (washing, using decontaminants) remove or reduce contamination levels
• Radioactive waste management ensures the safe handling, storage, and disposal of radioactive materials
  • Short-lived waste is stored until it decays to background levels
  • Long-lived waste requires specialized containment and long-term storage or disposal facilities

Future Trends and Emerging Technologies

• Theranostics combines diagnostic imaging and targeted radionuclide therapy
  • Peptide receptor radionuclide therapy (PRRT) targets somatostatin receptors in neuroendocrine tumors
  • Prostate-specific membrane antigen (PSMA) ligands are used for imaging and treating prostate cancer
• Radiomics involves extracting quantitative features from medical images for personalized medicine
  • Texture analysis, shape descriptors, and intensity histograms can provide insights into tumor heterogeneity and prognosis
  • Machine learning algorithms can identify patterns and predict treatment response or survival
• Adaptive radiation therapy adjusts the treatment plan based on changes in the patient's anatomy or tumor
  • Daily imaging (CT, MRI) is used to monitor variations and modify the dose distribution accordingly
  • Accounts for tumor shrinkage, organ motion, and weight changes during the course of treatment
• Flash radiation therapy delivers ultra-high dose rates (>100 Gy/s) in a single, short exposure
  • Preclinical studies suggest reduced normal tissue toxicity compared to conventional dose rates
  • Potential applications include treatment of moving tumors and radiosensitive organs
• Nanoparticle-based radiosensitizers enhance the effects of radiation therapy
  • Gold nanoparticles, hafnium oxide nanoparticles, and gadolinium-based nanoparticles have shown promise
  • Nanoparticles can preferentially accumulate in tumors and amplify the local radiation dose
• Artificial intelligence (AI) and deep learning are being applied to various aspects of nuclear medicine
  • Image reconstruction, segmentation, and classification can be improved using AI algorithms
  • AI can assist in treatment planning, dose optimization, and quality assurance tasks