A contouring module (22, 24) iteratively adjusts contours delineating a radiation target region and risk regions in a planning image. An intensity modulation optimization module (30) generates a radiation therapy plan conforming with dosage or dosage constraints (26) for the radiation target region and the risk regions delineated by the contours. A differential analysis module (40) is configured to invoke the intensity modulation optimization module (30) to estimate partial derivatives of an output of the intensity modulation optimization respective to the contours. The contouring module (22, 24) is configured to invoke the differential analysis module (40) after each iterative contour adjustment to estimate the partial derivatives respective to the contour segments and to render the contour segments on a display of the planning image with the contour segments coded based on the estimated partial derivatives to indicate impact of the contour segments on the intensity modulation optimization.
