Lecture 3 ~~~ Notes

X-ray Interactions

• Two types of interactions occur at the atomic level: photoelectric effect and Compton scattering
• Photoelectric effect: Occurs when X-ray interacts with an inner electron shell. The X-ray is fully absorbed, and an electron is ejected. Atoms with more electron shells absorb X-rays better and contribute to contrast in the radiograph. The atom becomes ionized.
• Compton scattering: Occurs when X-ray interacts with an outer electron shell. The electron is ejected but not all the X-ray energy is used; some energy is redirected. This leads to less contrast in the image and more image noise. Atoms are still ionized contributing to dose.

Molecular Level

• X-rays can damage DNA via two mechanisms:
  • Direct ionization (rare): Direct interaction between X-rays and DNA molecules, creating ions that can disrupt chemical structure and lead to mutations or DNA strand breaks
  • Indirect ionization (majority): Water molecules are ionised, resulting in instability and free radicals. These free radicals then damage DNA.

Cellular Level

• If DNA is damaged:
  • Repair: The DNA repair mechanism may fail to fix the damaged DNA, leading to apoptosis(programmed cell death).
  • Mutation: If apoptosis doesn't occur, but DNA isn't repaired, mutations can occur.

Tissue Level

• Deterministic Effects: Radiation burns (reddening, hair loss, cataracts) occur 100% above a certain threshold dose. The severity of these effects depends on the dose
• Stochastic Effects: Radiation-induced cancer is a chance effect that can occur with any dose, depending on the probability. The effect's probability is very low at low dose levels. It is not dependent on dose level.