Electron Extended SSD Calculator

Calculate electron beam output corrections for extended source-to-surface distances (SSD) using virtual source position and inverse square law. Essential for electron treatments with non-standard setups, irregular surfaces, and bolus applications.

Calculation Method

Beam Parameters

Calibration SSD
Actual treatment distance

Virtual Source Data

Enter measured data to determine virtual source position or use typical values

SSD (cm) Output (nC or cGy) Relative Output
100 1.000
105 0.952
110 0.907
115 0.865
120 0.826

Virtual Source Visualization

Calculation Results

Virtual Source Distance
85.5cm
Effective SSD (f_eff)
195.5cm
Output Correction Factor
0.907
Percentage Change
-9.3%

Inverse Square Law Correction:

OF(SSD) = [(SSD₀ + d_vs) / (SSD + d_vs)]²
OF(110) = [(100 + 85.5) / (110 + 85.5)]² = 0.907

Virtual Source Position from Measurements:

Least-squares fit (Khan): 1/√O − 1 = (SSD − SSD₀) / (SSD₀ + d_vs)
Fit pending…
dvs is fit to every measured output above (slope = Σxy/Σx²), not to a single pair. A small RMS residual means the inverse-square model fits the data well.

Typical Virtual Source Positions

Energy (MeV) 10×10 cm² (cm) 15×15 cm² (cm) 20×20 cm² (cm) 25×25 cm² (cm)
6 95.0 92.5 90.0 88.0
9 92.0 89.5 87.0 85.0
12 88.0 85.5 83.0 81.0
16 84.0 81.5 79.0 77.0
20 80.0 77.5 75.0 73.0

Clinical Applications

Important Considerations