Fundamentals of Ionizing Radiation in Medicine

Ionizing Radiation Revision Sheet

1. 📌 Essentials

• Ionizing radiation has enough energy to ionize atoms or molecules, creating free electrons and ions.
• Types include electromagnetic (X-rays, gamma rays) and particulate (electrons, protons, alpha particles, neutrons).
• The activity (A) is the decay rate, calculated by $A = \lambda N$; half-life $T_{1/2} = \frac{\ln 2}{\lambda}$.
• Exposure is measured in Roentgen (R); absorbed dose in Gray (Gy); equivalent dose in Sievert (Sv).
• Main photon interactions: photoelectric effect (Z-dependent, dominant at low energies), Compton scattering (intermediate energies), pair production (energy >1.022 MeV).
• Charged particles cause ionization via Coulomb interactions; neutrons indirectly via nuclear reactions.
• Cellular damage occurs directly (DNA ionization) or indirectly (free radicals); repair mechanisms influence survival.
• Dose effects: below 0.1 Gy often repaired; >10 Gy causes immediate cell death.
• Stochastic effects (cancer) have no threshold; deterministic effects (e.g., cataracts) have threshold doses.
• Radiosensitivity varies: bone marrow, intestines, lens are sensitive; nerve tissue is resistant.
• Oxygen enhances radiosensitivity; OER drops below 20 mmHg oxygen tension.
• Cell survival curves follow $S = e^{-(\alpha D + \beta D^2)}$.
• Radiotherapy fractionation (~1.8–2 Gy) balances tumor control and normal tissue repair.
• High LET radiation (alpha particles) deposits dense…