What determines radiation dose in nuclear medicine?

Radiation dose in nuclear medicine is determined by the amount of administered activity, the physical half-life of the radionuclide, the biological clearance of the radiopharmaceutical, the type and energy of emitted radiation, and the distribution of activity within the body.

Radiation dose in nuclear medicine depends on administered activity, emission energy, radiation type, and how long the radionuclide remains within tissues.

Unlike external beam imaging, where dose is delivered instantaneously, nuclear medicine dose arises from radioactive decay occurring inside the patient over time. The total absorbed dose depends on how long the radionuclide remains in tissues and how much energy is deposited locally.

Dose is therefore governed by both nuclear physics and physiology.

Understanding the physics

When a radiopharmaceutical is administered, radioactive decay begins immediately. Each decay event releases energy in the form of beta particles, positrons, gamma photons, or alpha particles, depending on the radionuclide.

The absorbed dose to a tissue depends on:

Cumulated activity: the total number of radioactive decays occurring in that tissue over time. This depends on both physical half-life and biological clearance. The longer activity remains, the greater the total dose.
Type of radiation emitted: Particulate emissions (beta particles, positrons, alpha particles) deposit energy locally and contribute significantly to absorbed dose. Gamma photons are more penetrating and may deposit energy at a distance or escape the body entirely.
Energy per decay: Higher-energy emissions deposit more energy per disintegration.
Distribution of activity: If activity is concentrated in a specific organ, dose to that organ increases.

Mathematically, absorbed dose is proportional to:

Cumulated activity
Mean energy emitted per decay
Fraction of energy absorbed in the target tissue

In diagnostic nuclear medicine, most administered activity results in relatively low absorbed doses because gamma photons escape and are detected externally. In therapeutic applications, radionuclides are selected specifically to maximise local energy deposition.

Effective half-life plays a key role because it determines how long activity remains in tissue. Rapid biological clearance reduces dose, even if the physical half-life is long.

Where this matters clinically

Understanding what determines dose explains:

Why administered activity is standardised
Why renal impairment may increase dose
Why therapeutic radionuclides deliver much higher organ doses
Why PET tracers typically result in modest effective doses

Dose estimation in nuclear medicine is commonly performed using models such as the MIRD schema, which calculate absorbed dose based on cumulated activity and radionuclide properties.