Load image
Take a photo or load a GammaPlan screenshot showing the treatment plan with isodose lines.

Calibrate scale
Tap two points of known distance on the image (e.g. ruler marks, grid), then enter the real distance in mm.

Detect centroid
Auto-detect the tumor centroid from the bright mass on contrast MRI, or skip to place it manually.

Mark OAR
Select the organ at risk type (cochlea, optic apparatus, brainstem) and tap its position on the image.

Measure isodoses
A ray extends from centroid through OAR. Tap where the ray crosses the 50% isodose, then the 25% isodose line.

Result
DGI = (r_25% / r_50%)³ is calculated and compared against the safety threshold.