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Dose absorption

The absorbed dose represents the quantity of energy absorbed per unit of matter. It is measured in Grays: 1 Gy represents 1 Joule deposited in 1 kg of matter.

A dose of 5 Gy in a single fraction corresponds to the lethal dose in 50% of subjects (DL50),

The absorbed dose is different from the emitted energy due to interactions between radiation and biological tissue. Each particle, either photons or electrons, will transfer its energy to the electrons of the crossed tissue, thud creating new secondary electrons or photons which in turn react with neighbouring molecules.

Thus, the absorbed energy will be higher, not at the surface but at a depth, which depends on the incident energy (the higher the energy, the deeper the depth): this is known as the increasing depth-dose or build-up.

Thus, clinically, very severe epidermitis will be observed with low energetic beams (photons 1.25 Mev of ⁶⁰Cobalt) and absent with high energy from accelerators (photons of 25 MeV).

Dose variation at the entry and in depth in relation to the beam photon energy.

Absorption of electrons

Electrons are very rapidly absorbed which allows superficial irradiation.

Dose distribution in water according to depth by an electron beam.