What is the difference between absorbed dose and effective dose?
The difference between absorbed dose and effective dose lies in what they measure and how they are used.
Absorbed dose is the physical amount of radiation energy deposited per unit mass of tissue and is measured in gray (Gy). It describes energy transfer only.
Effective dose is a calculated quantity that estimates overall stochastic risk (primarily cancer induction) by accounting for both the absorbed dose to individual organs and the relative biological sensitivity of those organs. It is measured in sievert (Sv).
Absorbed dose measures physical energy deposition in tissue (Gy), while effective dose estimates overall stochastic risk by weighting organ doses according to biological sensitivity (Sv).
Absorbed dose is a physical quantity; effective dose is a risk-weighted protection quantity.
Understanding the physics
When ionising radiation interacts with tissue, it transfers energy through ionisation and excitation. The total energy deposited per kilogram defines the absorbed dose:
D = Energy deposited / Mass
This is a measurable physical quantity and forms the foundation of radiation dosimetry.
However, radiation risk does not depend solely on how much energy is deposited. It also depends on:
The type of radiation
The organ receiving the dose
The biological sensitivity of that organ
To account for radiation type, absorbed dose is first converted to equivalent dose by applying a radiation weighting factor. For diagnostic nuclear medicine, this factor is usually 1 because emissions are predominantly low-LET beta particles and photons.
To account for organ sensitivity, equivalent doses to different tissues are multiplied by tissue weighting factors, which reflect each organ’s relative contribution to overall cancer risk. These weighting factors are derived from epidemiological data and international radiation protection models.
The effective dose is the sum of these weighted contributions.
Absorbed dose therefore tells us how much energy a specific organ receives.
Effective dose estimates the overall stochastic risk to the whole person.
They serve fundamentally different purposes.
Why this distinction matters
Absorbed dose is used when:
Assessing organ toxicity
Planning radionuclide therapy
Evaluating deterministic effects
Effective dose is used when:
Comparing different imaging procedures
Estimating population-level cancer risk
Informing justification of diagnostic studies
For example, a bone scan may deliver a relatively low effective dose, but the absorbed dose to the bladder wall may be higher than to other organs. These two quantities answer different clinical questions.
Importantly, effective dose should not be used to predict individual cancer risk precisely. It is a standardised protection tool based on reference populations.