Understanding Biologically Effective Dose (BED)
Biologically effective dose (BED) is a radiobiology quantity from the linear-quadratic (LQ) model that expresses the biological impact of a radiotherapy course independent of how it is split into fractions. Two schedules with the same total physical dose can produce different biological effects because a larger dose per fraction carries more weight. BED captures that difference so schedules can be compared on equal footing.
The formula
For a course of n fractions delivering d gray (Gy) each, with a tissue-specific alpha/beta ratio α/β, the calculator reports three values:
- BED = n × d × (1 + d/(α/β)) — the total dose n×d multiplied by the relative effectiveness factor (1 + d/(α/β)).
- Total physical dose = n × d — the raw prescribed dose in Gy, before any biological weighting.
- EQD2 = BED / (1 + 2/(α/β)) — the equivalent dose in 2 Gy fractions, a common scale for comparing schedules.
Choosing the alpha/beta ratio
The α/β ratio describes how sensitive a tissue is to fraction size. Tumors and early-responding (acute) tissues are commonly modeled around 10 Gy, while late-responding normal tissues are commonly modeled around 3 Gy; prostate tumors are often taken near 1.5 Gy. Use the value appropriate to the tissue you are evaluating, and report BED with its ratio (for example, writing "BED10" when α/β = 10 Gy).
When to consult a professional
This tool performs the standard LQ-model arithmetic for education and cross-checking. Real radiotherapy decisions depend on the specific tissue, treatment volume, dose-rate effects, and individual patient factors that a generic formula cannot capture — always confirm any clinical dose calculation with a qualified radiation oncologist or medical physicist.