Radiotherapy delivers radiation in multiple sessions called fractions. Different schedules — fewer large fractions vs. many small ones — produce different biological effects even at the same total dose. EQD2 and BED are two ways to compare them on a common scale:
The total dose that — if delivered as 2 Gray (Gy) per session, the historical clinical standard — would cause the same biological effect as the actual schedule. Gray is the unit of absorbed radiation dose (1 Gy = 1 J/kg). Because most outcome data in literature uses 2 Gy/fraction, EQD2 lets you compare any schedule to that reference.
Biologically Effective Dose — a theoretical measure of the total biological damage from a schedule, independent of fraction size. The basis for calculating EQD2.
| α/β | EQD2 (Gy) | BED (Gy) |
|---|---|---|
| α/β = | — | — |
| α/β = | — | — |
| α/β = | — | — |
| α/β = | — | — |
| α/β = | — | — |
Biologically Effective Dose:
\[ \text{BED} = D \left(1 + \frac{d}{\alpha/\beta}\right) \]
Equivalent Dose in 2 Gy fractions:
\[ \text{EQD}_2 = D \cdot \frac{d + \alpha/\beta}{2 + \alpha/\beta} \]
where \(D\) = total dose, \(d\) = dose per fraction, \(\alpha/\beta\) = tissue-specific radiosensitivity ratio.
| α/β (Gy) | Tissue / tumour type |
|---|---|
| 1–2 | Prostate cancer, breast cancer (late effects) |
| 3 | Late-responding normal tissues (spinal cord, lung) |
| 6 | Breast tumour (early-responding) |
| 10 | Most tumours, early-responding normal tissues |
This calculator is for educational and planning purposes only. Clinical treatment decisions must be made by qualified radiation oncologists. Values based on the linear-quadratic model (Fowler, 2006).