  We extend the four-dimensional unsubtraction method, which is based on the
loop-tree duality (LTD), to deal with processes involving heavy particles. The
method allows to perform the summation over degenerate IR configurations
directly at integrand level in such a way that NLO corrections can be
implemented directly in four space-time dimensions. We define a general
momentum mapping between the real and virtual kinematics that accounts properly
for the quasi-collinear configurations, and leads to an smooth massless limit.
We illustrate the method first with an scalar toy example, and then analyse the
case of the decay of a scalar or vector boson into a pair of massive quarks.
The results presented in this paper are suitable for the application of the
method to any multipartonic process.
