Fig 4. Schematic representation of the competing margin generation concepts to cope with residual deformations after a simulated IGRT correction.

Approach 1: The constant margin is applied by increasing the CTV (red) contours by 3 mm resulting in the internal target volume (green). Approach 2: The variable margin is applied locally by increasing the CTV (red) by a value calculated using measured landmark displacements of adjacent landmarks. Approach 3: The variable finite-element-model-based margin is created by statistical sampling of the propagated target volumes based on DVFs generated from landmark displacements by the patient-specific biomechanical model.