2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-02-1 MONTE CARLO SIMULATION OF AN Ir-192 BRACHYTHERAPY SOURCE SPECTRA, GEOMETRY AND ANYSOTROPY FACTORS USING GEANT4 CODE Sane S. O. F. Rodrigues 1 , Denison S. Santos 1 , Márcia Begalli 2 and Pedro P. Queiroz Filho 1 1 Instituto de Radioproteção e Dosimetria (IRD / CNEN - RJ) Av. Salvador Allende, s/n 22780-160 Rio de Janeiro, RJ sane@ird.gov.br santosd@ird.gov.br queiroz@ird.gov.br 2 Departamento de Física Nuclear e Altas Energias – Instituto de Física Universidade do Estado do Rio de Janeiro Rua São Francisco Xavier, 524 20550-900 Rio de Janeiro, RJ begalli@uerj.br ABSTRACT In brachytherapy cancer treatments, methods of calculation of dose delivered to tumours and organs have been continuously improved. The goal of calculations is the maximization of dose deposition in the target volume, while minimizing the deposition in the healthy tissues. The use of interstitial brachytherapy sources follows the recommendations of AAPM REPORT Nº 51. The present work is based on Monte Carlo simulations with GEANT4 code for the transport of subatomic particles in matter. This code incorporates a number of physical processes, presenting many important characteristics for medical applications, due to its precision in the treatment of geometries, materials, particle trajectories inside the volumes, particle fields, energy range, visualization of events, etc. Iridium–192 MicroSelectron HDR source was simulated and the geometry of dose deposition in media and primary spectra were obtained. Calculations also include photon-fluence distributions at two radial distances and accumulated air Kerma. A matrix of radial and angular coordinates was constructed, obtaining the Geometry Factor G(r,), the Anisotropy Function F(r,) and the Radial Dose Function g(r,) needed for dosimetry calculations, adopting the AAPM REPORT Nº 51 recommendations. 1. INTRODUCTION Recently, many works in brachytherapy have studied the dose distributions around of radioactive sources. Monte Carlo codes, like MCNP and EGSnrc, was specifically constructed for to simulate the charge particles transport and the radiation interaction with matter, in energy levels used in medical physics. The Geant4 Code [1], originally constructed for High Energy Physics experiments, has been used in many applications like Nuclear Physics, Medical Physics and Space Sciences, and