What Gy₂ Means in Radiation Oncology
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. 1
Purpose and Clinical Application
Gy₂ allows direct comparison of different radiation regimens that use varying fraction sizes and total doses by accounting for the biological effects of fractionation 1
This normalization is essential when evaluating cumulative radiation exposure in reirradiation scenarios, where patients have received multiple courses of radiation therapy over time 2, 3
The conversion is based on the linear-quadratic (LQ) model of radiobiology, which accounts for how different fraction sizes affect both tumor control and normal tissue toxicity 4
Calculation Basis
The biological effective dose (BED) forms the foundation for calculating Gy₂, with the formula incorporating the α/β ratio specific to the tissue being evaluated 4
Different tissues have different α/β ratios: tumors typically use α/β = 10, while late-responding normal tissues use α/β = 3 4
The standard formula converts any dose/fractionation scheme to its 2 Gy per fraction equivalent, allowing clinicians to sum doses from multiple treatment courses 1
Clinical Context in Head and Neck Reirradiation
In the reirradiation setting, cumulative Gy₂ doses exceeding 100-180 Gy have been reported in carefully selected patients with recurrent head and neck cancer using modern techniques like stereotactic radiosurgery 2, 3
One case series documented a patient receiving cumulative EQD2 of 127.70 Gy across multiple reirradiation courses without significant toxicity at 1-year follow-up 3
Another report described cumulative doses >180 Gy (EQD2max) delivered safely using robotic radiosurgery for second local recurrences 2
Important Limitations
The Gy₂ concept has significant limitations when applied to normal tissue dose constraints, as it does not accurately represent the actual total dose for normal tissues in the same way it does for tumor targets 4
The number of fractions derived from EQD2 for tumor dose differs from that for normal tissues, making direct application problematic 4
Alternative metrics like fractionation-specific biological equivalent dose (FEQD) may be more appropriate for determining normal tissue constraints across different fractionation schemes 4
Practical Implications
When reviewing prior radiation records, converting previous doses to Gy₂ helps estimate cumulative normal tissue exposure and guides safe reirradiation planning 1, 2
Standard fractionation in head and neck cancer uses 2 Gy per fraction to 66-70 Gy, which equals 66-70 Gy₂ by definition 1
Hyperfractionated regimens (e.g., 1.2 Gy twice daily to 81.6 Gy total) convert to approximately 70 Gy₂ 1