How is radiation dose calculated in nuclear medicine (MIRD schema)?

Radiation dose in nuclear medicine is calculated using the MIRD schema (Medical Internal Radiation Dose), which estimates absorbed dose to organs based on the total number of radioactive decays occurring in source tissues and the energy deposited in target tissues.

The MIRD schema calculates organ dose by multiplying the total number of decays in a source organ by a radionuclide-specific energy deposition factor (S value).

In simple terms, the MIRD approach states:

Absorbed dose = cumulated activity × S value

Where cumulated activity represents the total number of decays in a source organ over time, and the S value represents the mean absorbed dose delivered to a target organ per unit cumulated activity.

This framework allows organ-specific dose estimation for both diagnostic and therapeutic nuclear medicine.

Understanding the physics

Unlike external beam radiation, nuclear medicine involves radioactive decay occurring inside the body over time. Dose therefore depends on how long activity remains in tissues and how emitted radiation deposits energy locally.

The MIRD scema separates the problem into two components.

First, we calculate cumulated activity (Ã) in a source organ. This represents the total number of nuclear transformations occurring over time. It is determined by integrating activity in that organ over the time the radionuclide is present. Cumulated activity depends on:

Administered activity
Biodistribution
Biological clearance
Physical half-life

The longer the activity remains, the greater the cumulated activity.

Second, we apply the S value, which represents the absorbed dose to a target organ per unit cumulated activity in a source organ. The S value incorporates:

Energy emitted per decay
Type of radiation
Fraction of energy absorbed
Geometry and mass of organs

Mathematically:

D_target = Ã_source ⋅ S_{source→target}

This equation reflects that dose to a target organ depends not only on activity within that organ, but also on radiation emitted from neighbouring organs.

For diagnostic tracers, S values are derived from standardised anthropomorphic phantoms. For radionuclide therapy, patient-specific dosimetry may be performed using imaging-based activity measurements.

The power of the MIRD schema is that it separates biological kinetics (cumulated activity) from physical radiation transport (S value).

Where this matters clinically

The MIRD schema underpins:

Organ dose estimates in diagnostic nuclear medicine
Treatment planning in radionuclide therapy
Regulatory dose reporting
Risk assessment models

In diagnostic studies, doses are usually estimated using standard reference models. In therapy, more detailed patient-specific dosimetry may be required to avoid toxicity, particularly for organs such as bone marrow and kidneys.

Understanding MIRD allows trainees to link effective half-life, biodistribution, and radionuclide physics directly to absorbed dose.