Jf. Williamson, THE SIEVERT INTEGRAL REVISITED - EVALUATION AND EXTENSION TO I-125, YB-169, AND IR-192 BRACHYTHERAPY SOURCES, International journal of radiation oncology, biology, physics, 36(5), 1996, pp. 1239-1250
Citations number
34
Categorie Soggetti
Oncology,"Radiology,Nuclear Medicine & Medical Imaging
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.