Radiation Oncology/Physics/Physics Basics

Physics Basics

Production of X-ray

What happens when electron hit a target?

• Interacts with an orbital electron ==> ionization ==> vacancy ==> another electron moves to that vacancy ==> Production of characteristic X-ray
  • Discrete energy
    • Difference of two orbital electron binding eneregy
  • Occasionally —> the energy of 'moving eletron to vacancy' is transfered to another electron —> Auger electron
• Or it interacts with the electric field close to nucleus
  • It deflects ==> loose energy ==> this energy reappears in form x-ray photon
  • A continuous range of energy
  • Highest energy in the range = energy of the electron

All about radioactivity

• Think why nucleus is stable even though the electrostatic force between protons with same charges is repulsion
• There is a strong nuclear force between protons that keep them together
  • This force is not completely understood
• This force needs protons to be close and is distance dependent
• Neutrons play a stabilizing role in nucleus
• In larger nucleus the ratio of N/P is 1.5 —> This ratio is 1 in smaller nucleus
• Radioactive nucleus are those unstable nuclide that the ratio of P/N is unfavorable

• Decay Constant
  • Fraction of atoms that decay per unit of time
  • Half life and decay constant:
    • Half life = 0.693/decay constant

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.

Equipment

Flattening filter

• To compensate for the lack of scatter at the edge of the field by deliberately designing a profile that increases toward the edges.

Radiation Detectors

All detectors make use of ionization and excitation processes.

Gas Ionization detectors

• ionization chambers
• Geiger-Mueller(G-M) counters
• Proportional counters

Basic parts!

• A chamber with fixed volume of gas
  • gas can be air, methane
  • chamber=probe
• Two electrodes ( positive and negative )

• When photons pass through the chamber ==> ion pairs produced ==> ionization current produced
• Because chamber is polarized ==> ions travel to the oppposite charged electrode
• Some ions recombine
  • Collection efficiency is the fraction of charges actually collected
  • Above 300volts —> ionization chamber region ( efficiency almost 100% )

Photons

Maximum Depth Dose

Try to imagine what happens when electron hit surface of any thing :

• At a very first layer —> Photon excite an electron
• This electron travels through its path which has a known range according to the energy it has acquired from the photon
  • This electron range is the the depth of Dmax
  • Let's say it has the energy of the photon
• As the electron travels though it's path, it excite secondary electrons…
• Until the end of the range of the primary electron, it reaches to it's equilibrium
  • Electron equilibrium happens in a point/level in which energy loss is compensated by energy gained.

See Dmax

Electron

• When field size decrease ==> D90 shifts toward surface
• rule of thumb is that Eneregy/4 to get D90 depth