Monte Carlo simulation with PENELOPE (v. 2003) is applied to calculate Leksell Gamma Knife^ 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

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... th 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 (∼10 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.