EQD2

Equivalent dose in 2 Gy fractions (EQD2) is a radiobiological metric used to compare different radiotherapy fractionation schedules. It represents the dose delivered in 2 Gy fractions that would produce the same biological effect as a given treatment regimen, based on the linear-quadratic (LQ) model of cell kill. EQD2 depends on the total dose, the dose per fraction, and the tissue’s alpha/beta ratio, which reflects its sensitivity to fraction size. The approach assumes a consistent fractionation effect across the treatment and is most applicable when the LQ model holds.

Under the linear-quadratic model, for a regimen delivering n fractions of size d Gy (total dose D = n*d), EQD2 = D × (d + α/β) / (2 + α/β). The α/β value is tissue-specific: early-responding tissues and many tumors are often assigned α/β ≈ 10 Gy, whereas late-responding tissues have α/β ≈ 3 Gy. For a given regimen, this formula yields the dose in 2 Gy fractions that would have the same biological effect on that tissue.

EQD2 is used to compare regimens, convert doses to a common reference for treatment planning, evaluate cumulative doses in re-irradiation, and facilitate protocol comparisons across institutions. It is also extended to BED (biologically effective dose) concepts, where BED = D × (1 + d/(α/β)).

Limitations include reliance on the LQ model, particularly for large fractions or very high doses per fraction (hypofractionation). α/β values are estimates and tissue-dependent; not all tissues behave as ideal LQ. Uncertainties in repair kinetics, patient-specific factors, and differences in dose rate or modality may limit applicability.