Radiation Safety in Nuclear Medicine

Principles & ALARA - Dose Low, Safety High

• Radiation Types:
  • $\alpha$: High LET, short range (internal hazard).
  • $\beta$: Med LET, med range (skin/internal).
  • $\gamma$/X-ray: Low LET, long range (external).
• Key Units:
  • Activity: Becquerel (Bq), Curie (Ci). $1 \text{ Ci} = 3.7 \times 10^{10} \text{ Bq}$.
  • Absorbed Dose: Gray (Gy).
  • Equivalent Dose ($H_T$): Sievert (Sv) (Absorbed Dose $\times W_R$).
  • Effective Dose (E): Sievert (Sv) (sum of weighted $H_T$).
• ALARA Principle: As Low As Reasonably Achievable.
  • 📌 TDS: Time (minimize ↓), Distance (maximize ↑, $I \propto 1/d^2$), Shielding (use).
• Biological Effects:
  • Deterministic: Threshold dose. Severity $\propto$ dose (e.g., skin erythema >2 Gy).
  • Stochastic: No threshold. Probability $\propto$ dose (e.g., cancer).

⭐ Annual occupational effective dose limit (whole body): 20 mSv/year (avg. over 5 yrs), max 50 mSv in any year.

Biological Effects & Dose Limits - Harm Alert, Limits Set

• Biological Effects of Radiation:
  • Stochastic Effects: Probability increases with dose; NO threshold.
    • Examples: Carcinogenesis, genetic mutations.
  • Deterministic Effects (Non-Stochastic): Severity increases with dose; HAS a threshold dose.
    • Examples: Skin erythema, cataracts, sterility, organ dysfunction.
    • Cataract threshold: 0.5 Gy (acute exposure); 5 Gy (protracted exposure).
    • Temporary male sterility: ~0.15 Gy.
  • Tissue Radiosensitivity (Law of Bergonie & Tribondeau): Cells are more sensitive if they have:
    • High mitotic rate (actively dividing).
    • Long mitotic future.
    • Less differentiation (immature).
    • Most sensitive: Lymphoid organs, bone marrow, gonads.
• Dose Limits (AERB Guidelines):
  • Occupational Worker:
    • Effective Dose: 20 mSv/year (averaged over 5 years); not to exceed 50 mSv in any single year.
    • Lens of Eye: 20 mSv/year.
    • Skin/Hands/Feet (Extremities): 500 mSv/year.
  • Public:
    • Effective Dose: 1 mSv/year.
  • Pregnant Worker (post-declaration):
    • Dose to Foetus: 1 mSv for the remainder of pregnancy.

⭐ Stochastic effects have no dose threshold and their probability is dose-dependent, whereas deterministic effects have a threshold dose below which they do not occur, and their severity increases with dose above this threshold.

Practical Protection Measures - Shield, Distance, Time Trio

📌 TDS Principle: Key to ALARA (As Low As Reasonably Achievable).

• Time: Minimize exposure duration.
  • Dose directly proportional to time.
  • Pre-plan; work efficiently.
• Distance: Maximize distance from source.
  • Inverse Square Law: $I \propto 1/d^2$.
  • Use forceps, tongs, remote devices.
• Shielding: Use appropriate barriers.
  • Gamma/X-rays: Lead (Pb) (aprons, syringe/vial shields, L-bench).
  • Beta (β⁻) particles: Low Z (Perspex/plastic) first (↓Bremsstrahlung), then Pb if needed.
  • HVL: Thickness for 50% attenuation.
  • TVL: Thickness for 90% attenuation (TVL ≈ 3.3 HVL).

⭐ For β⁻ emitters, use low Z (e.g., plastic) primary shielding to minimize Bremsstrahlung; high Z (e.g., lead) for secondary shielding if required.

Special Scenarios & AERB - Pregnancy, Spills, Rules

• Pregnancy & Lactation:
  • Strict justification: Benefit vs. Risk. Aim for fetal dose < 1 mGy.
  • Breastfeeding interruption (consult specific guidelines):
    • Tc-99m (most common agents): 4 hrs to 24 hrs (confirm specific agent).
    • I-131 (therapy): Complete cessation.
    • I-123/I-131 (diagnostic): Days to weeks.
  • ⭐ > AERB: Therapeutic abortion not indicated for fetal radiation exposure < 100 mGy.
• Radioactive Spills: (📌 S.W.I.M. Protocol)
  • Stop spill, Warn others, Isolate area, Minimize exposure.
  • Notify Radiation Safety Officer (RSO). Decontaminate: Patient first, then area.
• AERB Key Guidelines:
  • Occupational dose limits: Annual effective dose 20 mSv (averaged over 5 yrs); single year max 30 mSv. Fetus of worker: 1 mSv during declared pregnancy.
  • Public dose limit: 1 mSv/year.

High‑Yield Points - ⚡ Biggest Takeaways

• ALARA principle (As Low As Reasonably Achievable) is the cornerstone of radiation protection.
• The three cardinal principles are Time (minimize), Distance (maximize), and Shielding (use appropriate).
• Distance is highly effective; radiation exposure decreases by the inverse square law.
• Lead (Pb) aprons and thyroid shields are vital for personnel protection against gamma rays.
• Personnel monitoring devices like TLD badges are mandatory for occupational exposure assessment.
• The annual effective dose limit for radiation workers is 20 mSv, averaged over 5 years.
• Pregnancy must be declared; the dose limit to the foetus is 1 mSv during the entire pregnancy period after declaration.