Medium modifications of parton fragmentation provide a novel tomographic tool for the study of the hot and dense matter created in ultrarelativistic nucleus-nucleus collisions. Their quantitative analysis, however, is complicated by the strong dynamical expansion of the collision region. Here, we establish for the multiple scattering induced gluon radiation spectrum a scaling law which relates medium effects in a collision of arbitrary dynamical expansion to that in an equivalent static

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... . Based on this scaling, we calculate for typical kinematical values of the RHIC and LHC heavy ion programs medium-modified parton fragmentation functions for heavy ion collisions with realistic dynamical expansion.