  In this paper, we presented the zero- and first-order radiative corrections
to the Casimir energy for a massive scalar field confined with Dirichlet
boundary condition in an open-ended rectangular waveguide. In the calculation
procedure, we applied a systematic renormalization program that allows all
influences imposed by dominant boundary conditions in a problem be
automatically reflected in the counterterms, leading the counterterms to be
obtained in a position-dependent manner. To remove the appeared divergences in
the computation task, the Box Subtraction Scheme as a regularization technique
was used. In this regularization technique, usually, two similar configurations
were introduced. Then, to find the Casimir energy, the zero point energies of
these two configurations were subtracted from each other via defining
appropriate limits. In the present work, first, the leading-order Casimir
energy for the massive scalar field in a waveguide is briefly presented. Next,
by applying this renormalization and regularization procedures, the first-order
radiative correction to the Casimir energy in the waveguide is calculated.
Finally, all the necessary limits of the obtained answers for massive and
massless cases are computed and the consistency of the obtained results are
discussed.
