Radiation Oncology Dose Equivalence

Introduction to Gy₂

Gy₂ (also written as EQD2) is a normalized radiation dose unit that converts any radiation fractionation scheme into the biologically equivalent dose if that same treatment were delivered using the standard 2 Gy per fraction schedule, according to the American Association for Neurological Surgeons 1
Gy₂ allows direct comparison of different radiation regimens that use varying fraction sizes and total doses by accounting for the biological effects of fractionation, as recommended by the American Association for Neurological Surgeons 1

Standard fractionation in head and neck cancer uses 2 Gy per fraction to 66-70 Gy, which equals 66-70 Gy₂ by definition, as stated by the National Comprehensive Cancer Network 2, 3, 4
Hyperfractionated regimens (e.g., 1.2 Gy twice daily to 81.6 Gy total) convert to approximately 70 Gy₂, according to the National Comprehensive Cancer Network 4, 5

Converting previous doses to Gy₂ helps estimate cumulative normal tissue exposure and guides safe reirradiation planning, as suggested by the American Association for Neurological Surgeons 1