Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom

PEREGRINE is a three-dimensional Monte Carlo dose calculation system writte n specifically for radiotherapy. This paper describes the implementation an d overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geome tries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerato r head, resulting in the identification of primary and scattered photon sou rces and an electron contaminant source. The magnitude of the electron sour ce is increased to improve agreement with measurements in the buildup regio n in the largest fields. Published measurements provide an estimate of back scatter on monitor chamber response. Commissioning consists of selecting th e electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C pho ton beams. All calculations are reported as dose per monitor unit. Aside fr om backscatter estimates, no additional, field-specific normalization is in cluded in comparisons with measurements. Maximum discrepancies were less th an either 2% of the maximum dose or 1.2 mm in isodose position for all fiel d sizes and beam modifiers. (C) 2001 American Association of Physicists in Medicine.