  A systematic study of energy spectra throughout the rare-earth region
(even-even nuclei from $_{58}$Ce to $_{74}$W) is carried out in the framework
of the interacting boson model (IBM), leading to an accurate description of the
spherical-to-deformed shape transition in the different isotopic chains. The
resulting IBM Hamiltonians are then used for the calculation of nuclear charge
radii (including isotope and isomer shifts) and electric monopole transitions
with consistent operators for the two observables. The main conclusion of this
study is that an IBM description of charge radii and electric monopole
transitions is possible for most of the nuclei considered but that it breaks
down in the tungsten isotopes. It is suggested that this failure is related to
hexadecapole deformation.
