Principles of radiation physics

Oxford American handbook of oncology. Second Edition. Oxford University Press (2015)


Types of radiation

Photons

  • The most common type of therapeutic radiation
  • Exponentially attenuated by tissue, due to absorption and scatter
  • Higher energies have deeper penetration and less skin dose
Super ficial x-rays 50 –150 KV
Max dose at 1–2 mm
Orthovoltage 150 –500 KV
max dose at 0.5 cm
Megavoltage >1000 KV
max dose at 1–5 cm
  • Beam travels through patient with substantial exit dose
  • Photon energies between 6 and 20 MV are most common and used for deep or centrally located tumors.

Electrons

  • Charged particles with limited penetration and full dose close to skin.
  • The effective range (penetration depth of an electron beam) is approximately half the energy.
  • The clinically useful range (depth at which dose falls to 80% of maximum) is one-third of the energy.

Range  (cm) = E/2  80% isodose (cm) = E/3

e.g.: 12 MeV electron beam:

Dose at 6 cm = 9.6%, at 4 cm = 80.0%

  • Useful for super ficial lesions while minimizing dose to deeper structures

Protons

  • Charged particle that deposits most of its dose at the end of its path
  • Can treat deep tumors with minimal exit dose
  • Requires specialized accelerator, currently has limited availability

Neutrons

  • Uncharged particle
  • Used primarily for unresectable salivar y tumors
  • Requires specialized accelerator, currently has limited availability

Oxford American Handbook of Oncology-Oxford University Press (2015) F8.1

Figure 8.1. The percentage of dose based on tissue depth for representative electron (dash-dotted line), photon (solid line), and proton (dotted line) energies. The electron has a high skin dose but falls off quickly. The photon “spares” the skin and has a relatively shallow maximum dose, but it has a long tail. The proton deposits a small amount of dose initially with a sharp increase at the end of its range.

Sources of radiation

Ionizing radiation can be obtained from either a radioactive source or a linear accelerator, where electrons are accelerated to high kinetic energies before hitting a target, releasing x-rays.

Radioisotope

  • Constant emission (radiation safety issue)
  • Energy, half-life, and type of radiation depend on isotope

Table 8.2. Characteristics of common isotopes

Isotope Cobalt60 Iodine-125 Iridium-192
Half-life 5 years 60 days 74 days
Energy 1.25 MV 0.027 MV 0.38 MV
Use External beam irradiation

Permanent implants

Temporary implants

Linear accelerator

  • Radiation is generated only when the machine is turned on.
  • Energy can be manipulated based on electron acceleration.
  • It can deliver photons and electrons.
  • Specialized accelerators can deliver protons and neutrons.

Radiation safety

Radiation exposure of individuals is measured by effective dose, which considers dose, type of radiation, and tissue radiosensitivity (see Box 8.1). The SI unit is the Sievert (Sv). For most clinical therapeutic radiotherapy, 1 Gy = 1 Sv.

Regulations are in place to protect workers with potential radiation exposure (e.g., medical staff, nuclear power plant workers, airline staff ). The annual occupational exposure limit is 50 mSv/year. The actual average effective dose received by a radiation worker is approximately 2 mSv/year.

Box 8.1. Annual effective dose in the U.S is 3.6 mSv, primarily due to radon exposure

Natural sources
Radon 2 mSv
Cosmic radiation 0.3 mSv
Terrestial radioactivity 0.3 mSv
Ingested radioactivity 0.4 mSv
Manmade sources
Medical imaging 0.5 mSv
Consumer products 0.1 mSv
Trans-atlantic flight 0.05 mSv
Diagnostic tests
Chest X-ray 0.03 mSv
Chest CT 8 mSv
PET scan 4 mSv

 

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