BED Calculator (Biological Effective Dose)

The BED is derived from the Linear-Quadratic model of cell kill, which posits that ionizing radiation causes lethal DNA damage via two mechanisms: single-hit (α component) and two-hit (β component) events. Proposed by Fowler in 1989, BED provides a tissue-specific common currency for comparing the biological potency of radiation delivered under any combination of total dose, dose per fraction, and number of fractions. It is indispensable for planning dose escalation, hypofractionation (SBRT/SABR), brachytherapy conversions, managing treatment interruptions, and estimating normal tissue complication probability (NTCP) for late-responding organs at risk.

Before BED, oncologists used Ellis's Nominal Standard Dose (NSD, 1969) and the Orton-Ellis Time-Dose Factor (TDF, 1973) to compare fractionation schedules. These empirical formulae were later shown to be inadequate, particularly for hypo- and hyperfractionated regimens. In 1989, Fowler proposed the BED formulation in the British Journal of Radiology, deriving it from the LQ model and explicitly incorporating an overall time factor for tumour repopulation. Twenty-one years later, Fowler reviewed its widespread adoption across EBRT, brachytherapy, SRS, SBRT, particle therapy, and radionuclide treatment.