THE SIEVERT INTEGRAL REVISITED - EVALUATION AND EXTENSION TO I-125, YB-169, AND IR-192 BRACHYTHERAPY SOURCES

Purpose: The goal of this study is to assess the accuracy of the Sieve rt integral dose calculation model for medium and low-energy brachythe rapy sources (photon energy: 25-500 keV), A simple modification of the basic model, the isotropic scattering correction, is proposed that si gnificantly improves its accuracy, Methods and Materials: Both the cla ssical model and revised Sievert algorithms were tested against 2D dos e distributions derived from Monte Carlo photon transport (MCPT) calcu lations for the following sources: a Yb-169 interstitial source, pulse d and high dose rate Ir-192 sources and the model 6702 I-125 source, T he Sievert model was implemented as a 3D numerical integral over the r adioactivity distribution and included photon attenuation and scatteri ng by the surrounding medium, The Sievert filtration coefficients were approximated by linear energy absorption coefficients, parameters of best fit, and curve fits to simulated open-air transmission measuremen ts, The revised model consists of using the Sievert integral only to c alculate the primary dose distribution using contact absorber filtrati on coefficients, The dose component due to photon scattering in the me dium is assumed to be isotropically distributed and is modeled by poin t-source scatter-to-primary dose ratios, Results: The classical Siever t integral produces maximum and RMS average dose-calculation errors ra nging from -53 to -20% and 3 to 19%, respectively, In contrast, the re vised model reproduces the MCPT dose distribution with maximum and RMS mean errors ranging from 5 to 13% and 1 to 6%, respectively, Conclusi ons: The classical Sievert model fails to accurately describe brachyth erapy dose distributions around heavily filtered sources emitting phot ons with average energies of 28 to 400 keV, The revised Sievert model accurately models single-source dose distributions for a wide range of sources, using well-defined filtration coefficients and scatter ratio s that can be measured or calculated without knowledge of the final do se distribution, The model is potentially useful as a single-source do se-array generator for clinical treatment planning in the low energy d omain. Copyright (C) 1996 Elsevier Science Inc.