  Monte Carlo simulation with PENELOPE (v.~2003) is applied to calculate
Leksell Gamma Knife$^{\circledR}$ dose distributions for heterogeneous
phantoms. The usual spherical water phantom is modified with a spherical bone
shell simulating the skull and an air-filled cube simulating the frontal or
maxillary sinuses. Different simulations of the 201 source configuration of the
Gamma Knife have been carried out with a simplified model of the geometry of
the source channel of the Gamma Knife recently tested for both single source
and multisource configurations. The dose distributions determined for
heterogeneous phantoms including the bone- and/or air-tissue interfaces show
non negligible differences with respect to those calculated for a homogeneous
one, mainly when the Gamma Knife isocenter approaches the separation surfaces.
Our findings confirm an important underdosage ($\sim$10%) nearby the air-tissue
interface, in accordance with previous results obtained with PENELOPE code with
a procedure different to ours. On the other hand, the presence of the spherical
shell simulating the skull produces a few percent underdosage at the isocenter
wherever it is situated.
