Winston-Lutz Analysis Calculator

The Winston-Lutz test is the gold standard for verifying radiation isocenter accuracy. This test involves imaging a small radio-opaque sphere at multiple gantry, collimator, and couch angles to determine the mechanical and radiation isocenter coincidence.

Input Measurements

For each image, enter the displacement of the field (radiation) center relative to the BB in the imager plane: lateral (cross-plane, left–right) and longitudinal (in-plane, sup–inf), in mm.

Gantry 0° (lat, long):

Gantry 90° (lat, long):

Gantry 180° (lat, long):

Gantry 270° (lat, long):

Collimator 90° (lat, long):

Collimator 270° (lat, long):

Couch 90° (lat, long):

Couch 270° (lat, long):

Isocenter Visualization

Understanding Winston-Lutz Analysis

Test Principle: A small metallic sphere (typically 3-5 mm) is positioned at the nominal isocenter using room lasers. Images are acquired at various gantry, collimator, and couch angles with a small radiation field.

Key Metrics:

2D Deviation: Radial distance √(lat² + long²) between the radiation field center and the BB in each imaging plane
Per-axis Isocenter Size: Diameter of the smallest circle enclosing the displacement points for that subsystem (gantry, collimator, or couch)
Radiation Isocenter Diameter: Diameter of the minimum enclosing circle of all displacement points — the headline radiation-isocenter metric

Note: this analysis works in the 2D imager plane. A full 3D isocenter requires combining lateral offsets across gantry angles to reconstruct the BB position in room coordinates.

Treatment Type	Recommended Tolerance	Action Level
Conventional RT	≤ 2.0 mm	> 3.0 mm
IMRT/VMAT	≤ 1.0 mm	> 1.5 mm
SRS/SBRT	≤ 0.5 mm	> 1.0 mm

Input Measurements

Isocenter Visualization

Analysis Results

Understanding Winston-Lutz Analysis

Key Metrics: