BED (biologically effective dose)

"In fracionated radiotherapy, the total dose that would be required in very small dose fractions to produce a particular effect" [1]. It is proportional to log cell kill, and it's more conceptually useful as a measure of biological damage than a physical dose, the effects of which vary with fraction size and dose rate. Formally, it is the radiation dose equivalent to an infinite number of infinitely small fractions or a very low dose-rate. It corresponds to the intrinsic radiosensitivity (α) of the target cells when all repairable radiation damage (β) has been given time to be repaired. In linear quadratic modeling, BED=total dose x relative effectiveness (RE), where RE=(1+d/α/β), with d=dose per fraction [2]. "Otherwise known as extrapolated total dose (ETD). BED values calculated from different α/β ratios are not directly comparable. For time-dose calculations, EQD2 is preferred" [1].

Bibliographic references:

[1] Joiner, M. and van der Kogel, A. ed., (2009). Glossary of terms in radiation biology. In: Basic Clinical Radiobiology, 4th ed. London, United Kingdom: Hodder Arnold, an Hachette UK Company, p.353.

[2] Fowler, J. (2006). Part I: Basic Concepts in Treatment Planning, 1. Practical Time-Dose Evaluations, or How to Stop Worrying and Learn to Love Linear Quadratics. In: S. Levitt, J. Purdy, C. Perez and S. Vijayakumar, ed., Technical Basis of Radiation Therapy, Practical Clinical Applications, 4th ed. Springer-Verlag Berlin Heidelberg, pp.3-31.