05 Biology
Equivalent dose in 2 Gy fractions
Convert a schedule to the equivalent dose in 2 Gy fractions.
Listen
Listen · English
Simulation
Equivalent dose in 2 Gy fractions — Change the numbers; the scene follows.
Where it works
Linear accelerator
Isocenter
In the treated volume — tumour and OARs at isocenter, after the dose has been delivered.
Open this machineFormula
Typical values
Variables
Results
EQD₂
Equivalent dose in 2 Gy fractions
60Gy
n
Number of fractions
30
Explanation
What it means
EQD₂ converts a schedule to the total dose in 2 Gy fractions that is isoeffective at a given α/β: EQD₂ = D (d+α/β)/(2+α/β). It is BED divided by (1 + 2/(α/β)). This is a working relation in Radiobiology.
Where it is used
Clinically it sits on the Linear accelerator — Isocenter. In the treated volume — tumour and OARs at isocenter, after the dose has been delivered. Radiobiology sits between the prescription and the organ-at-risk: LQ, BED, EQD2, and why 2 Gy is not 2 Gy if the fraction size changed. Use it to compare regimens, not to invent one.
Linear accelerator · Open this machineHow to use it
Quote EQD₂_10 for tumour and EQD₂_3 for late OAR. Example: 60 Gy in 20 fractions (3 Gy) → EQD₂_10 = 65 Gy, EQD₂_3 = 72 Gy. Change one input and watch the curve and the simulation follow.
Symbols
- DTotal dose60 Gy
- dDose per fraction2 Gy
- α/βAlpha/beta10 Gy
Worked example
A typical case from the default values: D = 60 Gy (Total dose); d = 2 Gy (Dose per fraction); α/β = 10 Gy (Alpha/beta). Substituting into the relation gives EQD₂ = 60 Gy; n = 30. These are teaching numbers — align them with your machine.
Typical values give
- EQD₂ = 60Gy
- n = 30
Where it comes from
The displayed formula is the working relation. Convert a schedule to the equivalent dose in 2 Gy fractions. Usual reference: Withers / Fowler. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Withers / Fowler
Assumptions & limits
Same LQ limits as BED. Heterogeneous dose (SBRT hotspots) needs a DVH-based EQD₂, not a single D.
Pitfalls
α/β is a model parameter, not a measured organ. BED from incomplete repair or a changed overall time is not the simple n·d·(1+d/(α/β)). Never EQD2 a stereotactic dose with an α/β you did not state. Same LQ limits as BED. Heterogeneous dose (SBRT hotspots) needs a DVH-based EQD₂, not a single D.
Keep this
Always write α/β and the fraction size next to a BED or EQD2. Same LQ limits as BED. Heterogeneous dose (SBRT hotspots) needs a DVH-based EQD₂, not a single D.
In this specialty