We give the modification of formulas for $p_{perp}$-broadening and energy loss which are necessary to calculate parton interactions in a medium with flow. Arguments are presented leading to the conclusion that for large $p_{perp}$-spectra observed in heavy ion collisions at RHIC, the influence of transverse flow on the determination of the quenching power of the produced medium is small. This leaves open the question of the interpretation of data in a consistent perturbative framework.
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