A radiation therapy system includes a diagnostic image scanner (12) which acquires a multidimensional dataset of a subject that is reconstructed into at least one image representation of an object of interest. An image processing apparatus (72), of radiation therapy system, includes a segmentation unit (74) which identifies a surface contour of the object of interest, or other critical structures. A masking unit (82) determines a non-constant margin, based on the identified surface contour and appends the determined non-constant margin to the identified surface contour. The non-constant margin is based on at least one of anisotropic motion, surface morphology, positional uncertainty, proximity to other organs, and probability of dose distribution. A planning processor (70) generates a radiation therapy plan which limits the delivery of therapeutic radiation to anatomy associated with the surface contour and appended non-constant margin. A radiation delivery system (40) delivers therapeutic radiation according to the generated plan.
