EQD2 & BED Calculator
Estimate biologically equivalent dose using the linear-quadratic model. Enter the physical dose, fraction size, and the tissue-specific α/β ratio.
What is EQD2?
EQD2 stands for Equivalent Dose in 2 Gy fractions. In radiation oncology, treatments are delivered using many fractionation schedules: conventional (for example 2 Gy per fraction), hypofractionated (larger fractions), or stereotactic regimens (very large fractions). EQD2 is a way to compare these schedules on a common scale.
Instead of only looking at physical dose, EQD2 accounts for how fraction size changes biological effect. That makes it useful when comparing treatment plans, discussing dose constraints, and evaluating cumulative dose from multiple courses.
Core formulas used by this calculator
The calculator applies the linear-quadratic model in two steps:
- BED (Biologically Effective Dose):
BED = D × (1 + d / (α/β)) - EQD2:
EQD2 = BED / (1 + 2 / (α/β))
Equivalent compact form:
EQD2 = D × (d + α/β) / (2 + α/β)
What each variable means
- D: total physical dose in Gy
- d: dose per fraction in Gy
- α/β: tissue-specific parameter in Gy
How to use the calculator
Step-by-step
- Enter total dose (Gy).
- Enter dose per fraction (Gy).
- Enter or select an α/β value.
- Click Calculate EQD2.
You will get:
- Estimated number of fractions (
D / d) - BED (Gy)
- EQD2 (Gy)
Example interpretation
If a regimen is 54 Gy in 3 fractions (18 Gy each):
- Using α/β = 10 Gy, EQD2 is high and reflects a strong biological effect compared with conventional fractionation.
- Using α/β = 3 Gy, EQD2 rises even more, showing how late-responding tissues are sensitive to larger fraction sizes.
This is exactly why choosing an appropriate α/β is essential for meaningful comparisons.
Common α/β choices
- 10 Gy: many tumors and early-responding tissues
- 3 Gy: many late-responding normal tissues
- ~1.5 Gy: often used for prostate in some modeling contexts
These are broad conventions, not universal truths. Published values vary by disease site, endpoint, and patient population.
Important limitations
Model scope
EQD2 is an approximation. It can be very useful, but it does not capture everything that matters clinically.
- It does not directly include overall treatment time and repopulation effects.
- At very high doses per fraction, linear-quadratic assumptions may become less reliable.
- It does not replace physician judgment, full plan evaluation, or protocol-specific guidance.
Practical caution
Use EQD2 as a decision-support metric, not as a stand-alone treatment rule. Always cross-check with institutional constraints, contour quality, dose-volume data, and current literature.
Bottom line
An EQD2 calculator helps translate different fractionation schedules into a common reference framework. That makes communication clearer and comparisons faster. Used carefully, it is one of the most practical tools for understanding radiobiological dose intensity.