BED Calculation for 20 Gy in 5 Fractions
The biologically effective dose (BED) for 20 Gy delivered in 5 fractions is 28 Gy (assuming α/β = 10 Gy) or 36 Gy (assuming α/β = 3 Gy), depending on the tissue type being evaluated.
Calculation Method
The linear-quadratic (L-Q) model formula for calculating BED is: BED = D [1 + d/(α/β)] 1
Where:
- D = total dose (20 Gy)
- d = dose per fraction (20 Gy ÷ 5 fractions = 4 Gy)
- α/β = tissue-specific ratio
Tissue-Specific BED Values
For Tumor Tissue (α/β = 10 Gy):
- BED = 20 [1 + 4/10]
- BED = 20 [1 + 0.4]
- BED = 28 Gy₁₀
This is the standard assumption for most tumors and acute-reacting tissues 1.
For Late-Reacting Normal Tissue (α/β = 3 Gy):
- BED = 20 [1 + 4/3]
- BED = 20 [1 + 1.33]
- BED = 36 Gy₃
This applies to late-responding tissues like spinal cord, brain, and other organs at risk.
Clinical Context
The 20 Gy in 5 fractions regimen is specifically mentioned in guidelines for:
Neuropathic pain from bone metastases, where this dose schedule (BED₁₀ = 28 Gy) is more effective than single 8 Gy fractions for pain control 2
Prophylactic cranial irradiation in extensive-stage small cell lung cancer, where 20 Gy in 5 fractions may be considered 3
Palliative radiotherapy for soft tissue sarcomas, where this is one of several acceptable palliative fractionation schemes 4
Important Caveats
The α/β ratio of 10 Gy is commonly used across various tumors as a standardized empirical rule 1, though actual values may vary by tumor type. The L-Q model becomes less accurate for doses exceeding 13 Gy per fraction 1, but remains valid for the 4 Gy per fraction used in this regimen.
When comparing treatment regimens, always calculate BED for both tumor (α/β = 10) and late-responding tissues (α/β = 2-3) to assess the therapeutic ratio 5.