  We study the inverse problem in Optical Tomography of determining the optical
properties of a medium $\Omega\subset\mathbb{R}^n$, with $n\geq 3$, under the
so-called diffusion approximation. We consider the time-harmonic case where
$\Omega$ is probed with an input field that is modulated with a fixed harmonic
frequency $\omega=\frac{k}{c}$, where $c$ is the speed of light and $k$ is the
wave number. We prove a result of Lipschitz stability of the absorption
coefficient $\mu_a$ at the boundary $\partial\Omega$ in terms of the
measurements in the case when the scattering coefficient $\mu_s$ is assumed to
be known and $k$ belongs to certain intervals depending on some a-priori bounds
on $\mu_a$, $\mu_s$.
