Physics Basics

There are five concerns of energy absorbed into living tissue:

Production of X-ray

All about radioactivity

Linear energy transferred (LET)

  • The rate at which energy is transferred from ionizing radiations to soft tissue.
  • The unit —> kiloelectron volts/cm or micrometer (keV/μm)
  • There are highly penetrating and less penetrating radiations.
  • Pareticulate radiation are less penetrating.
    • Particulate radiation, photoelectrons, alpha particles, and beta radiation
    • The alpha particle has a +2 charge on emission and will very aggressively ionize adjacent atoms to acquire two electrons returning it to a stable electrically neutral helium atom.
    • This process causes primary and secondary ionization events. The alpha particle loses an average of 34 eV per ionization event and therefore a 34 meV alpha particle could cause up to 100,000 ionizations creating 100,000 ion pairs before coming to rest in a few centimeters of air.
  • High energy photons are highly penetrating and hence the LET is low:
    • Diagnostic x-ray —> 3.0 keV/μm
    • 25-MeV photon —> 0.2 keV/μm

What LET tells us is that the number of ionization events increase as the LET increases and decrease as the LET decreases.

Although the RBE expresses the relative effectiveness of two different types of radiation; the factor used in radiation protection to express this effectiveness is the quality factor written Q. The quality factor or Q is a symbol for expressing the LET dependent response by a biological system. If the biological response per rad of two different radiations is the same then their Q is the same. X-rays, gamma rays, and beta particles all have the same Q, which is equal to 1.

Relative Biological Effectiveness (RBE)

  • RBE is without a unit.

The relative biological effectiveness is a term for quantifying specific radiation effects not general or relative risks. It includes the various effects caused by different types of ionizing radiation, the tissue type into which the energy is imparted, the biological effect under investigation, and the rate at which that dose is delivered.

The RBE always compares the amount of orthovoltage radiation to another type of radiation (e.g. alpha or beta radiation), and a specific biological effect produced by those tested radiations, such as cataract. Orthovoltage radiation is electromagnetic radiation with a range of 200-250 kVp. If it takes 15 rad of 250 keV x-rays to produce cataracts and only 5 rad of alpha particles the RBE is said to be 3.

Oxygen enhancement



Law of Bergonie and Tribondeau.