Fundamentals of Ionizing Radiation in Medicine

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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=λNA = \lambda N; half-life T1/2=ln⁡2λT_{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−(αD+βD2)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…
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Преглед на теста

1. Who discovered X-rays and in which year?

2. What is the primary purpose of the 'activity' (A) measurement in ionizing radiation?

3. What is the main difference between directly ionizing and indirectly ionizing radiation?

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Преглед на флашкартите

What is ionizing radiation?

Ionizing radiation has enough energy to create ion pairs by ejecting electrons from atoms or molecules. It includes electromagnetic types like X-rays and gamma rays, as well as particles such as electrons, protons, alphas, and neutrons.

Ionizing radiation — definition?

Energy capable of ionizing atoms or molecules.

Name and briefly describe the three main photon interaction mechanisms relevant in medical physics.

The photoelectric effect involves photon absorption where an inner-shell electron is ejected, dominant at low energies with high atomic number materials. Compton scattering is when photons scatter off electrons, transferring part of their energy, important at intermediate energies (~4-25 MeV). Pair production occurs when a photon with energy above 1.022 MeV converts into an electron-positron pair near a nucleus, relevant at energies >10 MeV.

Activity (A) — formula?

A = λN (decay rate).

Explain the concept of Linear Energy Transfer (LET) and its significance in radiobiology.

LET is the energy deposited by radiation per unit length traveled in tissue, measured in keV/μm. High LET radiation (e.g., alpha particles) causes dense, localized damage that is often irreparable, leading to higher biological effectiveness, while low LET radiation (e.g., X-rays) results in more spread-out, reparable damage.

Photoelectric effect — energy dependence?

More likely with higher Z and lower energy.

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