  The renormalization-group properties of gauge-invariant transverse-momentum
dependent (TMD) parton distribution functions (PDF) in QCD are addressed. We
perform an analysis of their leading-order anomalous dimensions, which are
local quantities, making use of the renormalization properties of
contour-dependent composite operators in QCD. We argue that attaching
individual gauge links with transverse segments to quark fields in the
light-cone gauge, the associated gauge contours are joined at light-cone
infinity through a cusp-like junction point. We find that the renormalization
effect on the junction point creates an anomalous dimension which has to be
compensated in order to recover the results in a covariant gauge. To this end,
we include in the definition of the TMD PDF an additional soft counter term
(gauge link) along that cusped contour. We show that the eikonal factors
entering this counter term are peculiar to the Mandelstam field formalism and
are absent when one uses a direct gauge contour.
