MAP.17 Radiation safety and dosimetry

MAP.17 Radiation Safety and Dosimetry

• Presenter: Dr. Andy Ma

Learning Outcomes

• Discuss interactions of decay products with biological tissue.

• Differentiate between the photoelectric effect, Compton effect, pair production & pair annihilation.

• Demonstrate the role of the radioactive decay equation in calculations of half-life.

• Differentiate between activity, Becquerel, Curie, absorbed dose, and quality factor.

• Differentiate equivalent dose (dose equivalent) and effective dose.

• Describe the physiological effects of radiation.

• Restate commonly used maximum permissible doses.

• Discuss environmental radioactivity.

Ionizing Radiation & Interactions with Matter

• Definition: Ionizing radiation has sufficient energy to free electrons within tissue.

• Common Sources: UV, IR, electromagnetic (e-m) radiation.

• Types of Ionizing Radiation:

  • Alpha (α) particles

  • Beta (β) particles

  • X-rays

  • Gamma (γ) rays

Biological Effects of Radiation

• Radiation can leave trails of secondary ionization that disrupt sensitive biological systems.

• Categories of Interest:

  • Positive ions (α-particles)

  • Electrons (β-particles)

  • Photons (X-rays, γ-rays)

  • Neutrons

• All radiation types can produce biological damage through ionization.

Positive Ions (α-particles)

• Characteristics:

  • Short range in matter (5 MeV α-particle travels about 4 cm in air).

  • Cannot penetrate a thin sheet of paper.

• Interaction: High degree of ionization due to relatively large mass and charge.

• Energy Deposition:

  • Energy loss found at approx. 100 keV/mm in tissue.

Electrons (β-particles)

• Characteristics: Energies range from a few keV to ~1 MeV.

• Velocity: Higher than that of α-particles for similar energies.

• Energy Loss: Approximately 0.25 keV/mm, much less than α-particles.

  • Example: 1 MeV β-particle can travel ~4 mm in tissue.

Photons (X-rays, γ-rays)

• Origin: γ-rays arise from nuclear processes; typically higher energies than X-rays.

• Ionization: Primarily indirect ionization by generating secondary electrons.

  • Energy loss processes include:

    • Photoelectric Effect (Energy < 0.1 MeV)

    • Compton Scattering (Energy up to 1 MeV)

    • Pair Production (Energy > 1.02 MeV)

• Penetration: High-energy photons penetrate tissues more deeply than α- or β-particles.

Neutrons

• Characteristics: Uncharged, do not ionize directly but can cause ionization indirectly through collisions with nuclei.

• Interaction Effects:

  • Recoil nuclei causing ionization.

  • High probability capture by nuclei at low energy.

Radiation Penetration

• General Rule:

  • α-particles: Stop in air/ thin paper.

  • β-particles: Travel meters in air or mm in aluminum.

  • γ-rays: Can penetrate several cm in high-density materials (e.g., lead).

Radioactive Half-Life

• Definition: Time required for half of radioactive nuclei to decay.

• Example: 131I has a half-life of 8 days (varies greatly across isotopes).

Biological Half-Life

• Time for the body to remove half of a substance.

• Effective Half-Life: Takes into account both radioactive and biological decay.

• Formula:

  • TEffective = 1 / (1/T1/2(radioactive) + 1/T1/2(biological))

  • Example for 131I: 5.21 days after combining radioactive and biological half-lives.

Radioactive Decay Rate

• Dependent on the number of parent nuclei present.

• Formula:

  • A = λN (Activity; decays per second)

• SI unit of activity: Becquerel (1 Bq = 1 disintegration/second).

Radiation Exposure and Dose

• Absorbed Dose: Amount of energy per unit mass absorbed (measured in RAD/Gy).

• Biological Effects: Dependency on radiation type, energy, and tissue distribution.

Equivalent Dose and Effective Dose

• Definition: Equivalent Dose = Absorbed Dose × wR (Radiation Weighting Factor).

• Measured in sievert (Sv).

• The effective dose considers tissue sensitivity (e.g., gonads vs skin).

Maximum Permissible Dose (MPD)

• Guidelines established by ICRP:

  • Public: 1 mSv/year.

  • Radiation workers: 20 mSv/year.

  • Localized populations (e.g., near Chernobyl): 10 mSv/year.

• Keep exposures ALARA (As Low As Reasonably Achievable).

Linear-Non-Threshold Theory (LNT)

• Underlying model for radiation protection; any dose can increase cancer risk directly proportionately.

Clinical Application Doses

• Chest X-ray: ~0.017 mSv.

• CT scan: ~8 mSv.

• Diagnostic nuclear medicine: 1-7 mSv.

• Cumulative exposure effects can occur. Exercise caution!

Risks from Radiation Exposure

• Average risk factor is 0.05 Sv-1; 1 Sv annual dose yields a 5% cancer risk.

• High-level exposures have severe consequences (e.g., nuclear accidents).

Immediate Effects of Radiation

• Effects range from negligible effects at low doses to severe at high doses (e.g., LD50 at 450 Sv).

• Potential outcomes include temporary sterility, nausea, or even lethal responses depending on exposure levels.

Natural Radiation Exposure Sources

• Cosmic radiation: 27 mSv/year.

• Domestic pilots: 2 mSv/year from natural exposure.

• Annual averages are presented for different population exposures.

Learning Outcomes (Reiteration)

• Emphasize understanding of radiation interactions, dosimetry, biological effects, and safety standards.

Contact Information

• Presenter: Dr. Andy Ma

• Email: ama@rcsi.com