Dose build-up

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Depth dose curves of different radiation qualities in a tissue equivalent phantom

Among dosage structure (also dose build effect called) means the phenomenon that the radiation dose high energy photon radiation (511 keV >>) upon entry into an absorber, such as fabric , initially rises with increasing depth. The increase in dose is explained by an imbalance between KERMA , the kinetic energy released in the course of ionizations, and the radiation dose, the energy deposited in the tissue.

Explanation

Photon radiation

With increasing photon energy, the probability of forward scattering of the electrons released during ionization increases due to the properties of the Klein-Nishina cross-section , that is, they move predominantly in the direction of the incident radiation field. The electrons are released directly on the surface of the skin, but they only release their energy deep down. At the point at which the same number of electrons are released as they stop, there is secondary electron equilibrium . Here is the point of greatest dose deposition (see fig.). As a result of the reduction in the photon fluence , the deposited dose drops exponentially at a further depth.

The build-up effect is useful because it protects the skin, which is sensitive to radiation. Since the point with the highest dose moves further back with higher kinetic initial energy of the electrons, the photon energies have been gradually increased in the course of development from 1 MeV for cobalt-60 decay radiation ( cobalt cannon ) to 20 MV acceleration voltage for modern linear accelerators (as of 2015).

In materials with a low atomic number, the range of electron beams is a few centimeters.

In addition, in the case of irradiation with broad radiation fields, there is an additional dose contribution from multiple scattered photons.

The law of absorption , which describes a purely exponential decrease in beam intensity, is supplemented by the build-up factor B when the dose build-up effect is taken into account

The build-up factor depends on the thickness and material of the absorber, the photon energy and the beam cross-section.

Particle radiation (corpuscular radiation)

Since particle radiation mainly scatters secondary electrons laterally, the dose build-up effect is of no importance in hadron therapy . On the contrary, the skin dose can even be higher compared to X-ray irradiation (note the normalization to the maximum dose in the figure). The dose build-up effect must be distinguished from the Bragg peak .

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  • Krieger / Petzold: Radiation Physics, Dosimetry and Radiation Protection, Volume 1 Basics; BG Teubner Stuttgart 1992
  • Krieger, Hanno: Fundamentals of radiation physics and radiation protection , BG Teubner, Stuttgart 2004