  Objective: $^{177}$Lu is one of the most employed isotopes in targeted
radionuclide therapies and theranostics, and 3D internal dosimetry for such
procedures has great importance. Voxel S-Values (VSVs) approach is widely used
for this purpose, but VSVs are available for a limited number of voxel
dimensions. The aim of this work is to develop an analytic model for the
calculation of $^{177}$Lu-VSVs in any cubic voxelized geometry of practical
interest. Approach: Monte Carlo (MC) simulations were implemented with the
toolkit GAMOS to evaluate VSVs in voxelized geometries of soft tissue from a
source of $^{177}$Lu homogeneously distributed in the central voxel. Nine
geometric setups, containing 15x15x15 cubic voxels of sides l ranging from 2 mm
to 6 mm, in steps of 0.5 mm, were considered. For each l, the VSVs computed as
a function of the "normalized radius", Rn = R/l (with R = distance from the
center of the source voxel), were fitted with a parametric function. The
dependencies of the parameters as a function of l were then fitted with
appropriate functions, in order to implement the model for deducing
$^{177}$Lu-VSVs for any l within the aforementioned range. Main results: The
MC-derived VSVs were satisfactorily compared with literature data for
validation, and the VSVs computed with the analytic model agree with the MC
ones within 2\% for Rn $\leq$ 2 and within 6\% for Rn > 2. Significance: The
proposed model enables the easy and fast calculation, with a simple
spreadsheet, of $^{177}$Lu-VSVs in any cubic voxelized geometry of practical
interest, avoiding the necessity of implementing ad-hoc MC simulations to
estimate VSVs for specific voxel dimensions not available in literature data.
