INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 47 (2002) 1995–2011 PII: S0031-9155(02)32817-3 Monte Carlo simulation of the Leksell Gamma Knife  R : I. Source modelling and calculations in homogeneous media Vadim Moskvin, Colleen DesRosiers, Lech Papiez, Robert Timmerman, Marcus Randall and Paul DesRosiers Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, 535 Barnhill Dr, RT041, IN 46202-5289, USA E-mail: vmoskvin@iupui.edu Received 17 January 2002, in final form 18 April 2002 Published 7 June 2002 Online at stacks.iop.org/PMB/47/1995 Abstract The Monte Carlo code PENELOPE has been used to simulate photon flux from the Leksell Gamma Knife R  , a precision method for treating intracranial lesions. Radiation from a single 60 Co assembly traversing the collimator system was simulated, and phase space distributions at the output surface of the helmet for photons and electrons were calculated. The characteristics describing the emitted final beam were used to build a two-stage Monte Carlo simulation of irradiation of a target. A dose field inside a standard spherical polystyrene phantom, usually used for Gamma Knife R  dosimetry, has been computed and compared with experimental results, with calculations performed by other authors with the use of the EGS4 Monte Carlo code, and data provided by the treatment planning system Gamma Plan R  . Good agreement was found between these data and results of simulations in homogeneous media. Owing to this established accuracy, PENELOPE is suitable for simulating problems relevant to stereotactic radiosurgery. (Some figures in this article are in colour only in the electronic version) 1. Introduction A high precision instrument used to treat intracranial lesions, the Gamma Knife R  was developed in 1968 by Professor Lars Leksell of the Karolinska Institute in Stockholm, Sweden and Professor Borge Larsson of the Gustaf Werner Institute at the University of Uppsala, Sweden. The Gamma Knife R  concentrates radiation dose from 201 60 Co sources distributed on the surface of the partial sphere, focusing photon beams on the target area. The beams exit the primary and final collimator system creating beam diameters of approximately 0031-9155/02/121995+17$30.00 © 2002 IOP Publishing Ltd Printed in the UK 1995