When a radionuclide decays inside the body, it emits radiation in the form of beta particles, positrons, alpha particles, or gamma photons. These emissions interact with surrounding tissues and transfer energy through ionisation and excitation of atoms.

The absorbed dose depends on three key factors:

First, the number of radioactive decays occurring in the tissue. This is determined by the administered activity and how long the radionuclide remains in that tissue (effective half-life).

Second, the energy released per decay. Different radionuclides emit particles with different energies. Higher-energy emissions deposit more energy per interaction.

Third, the fraction of emitted energy absorbed locally. Particulate emissions such as beta particles and alpha particles deposit energy over short distances and therefore contribute significantly to absorbed dose in the emitting tissue. Gamma photons are more penetrating and may escape the body or deposit energy elsewhere.

Absorbed dose can therefore be conceptualised as:

Total energy deposited in tissue ÷ mass of that tissue.

Importantly, absorbed dose describes only the physical energy deposition. It does not reflect how damaging that energy may be biologically. Two tissues receiving the same absorbed dose may have very different biological responses.