TY - JOUR

T1 - Dosimetric characteristics of the DRAXIMAGE model LS-1 I-125 interstitial brachytherapy source design

T2 - A Monte Carlo investigation

AU - Williamson, Jeffrey F.

PY - 2002

Y1 - 2002

N2 - We have used Monte Carlo photon transport simulations to evaluate dosimetric characteristics of a new 125I seed, the DRAXIMAGE BrachySeed™ (model LS-1) source for interstitial brachytherapy and to calculate the dosimetric parameters recommended by the AAPM Task Group 43 (TG-43). To eliminate thick end welds, preformed closed-ended Ti shells are welded together near the transverse plane. The radioactivity is localized near the seed ends in order to maximize isotropy at typical prescription distances. Transmission radiography reveals that its internal components can move by as much as 200μm under the influence of gravitational forces affecting the dose-rate constant and relative dose distribution by as much as 5% and 10%, respectively. However, the clinical dose distribution can be well represented by a weighted average of dose distributions corresponding to three different orientation-dependent internal geometry configurations. This average dose distribution agrees closely (within 1% at 1 cm) with that derived from the geometry specified by the vendor model (seed components uniformly spaced), from which the recommended relative dosimetry parameters were derived. An average dose-rate constant of 0.935 cGy h-1 U-1 is recommended for clinical application and values of 0.919, 0.963, and 0.920 for vertical orientation, horizontal orientation, and the vendor specified geometries, respectively. The NIST wide-angle free-air chamber realization of the air-kerma strength primary standard, SK,N99, was explicitly simulated. We show that the output of Ag characteristic x rays is about half that of the model 6711 source resulting in a relative dose distribution between that of the model 6711 source and pure 125I x-ray emitters. Finally, it is demonstrated that the TG-43 point-source formalism more accurately estimates solid-angle weighted dose at small distances if one uses a radial dose function data prepared using a point-source, rather than line-source, geometry factor.

AB - We have used Monte Carlo photon transport simulations to evaluate dosimetric characteristics of a new 125I seed, the DRAXIMAGE BrachySeed™ (model LS-1) source for interstitial brachytherapy and to calculate the dosimetric parameters recommended by the AAPM Task Group 43 (TG-43). To eliminate thick end welds, preformed closed-ended Ti shells are welded together near the transverse plane. The radioactivity is localized near the seed ends in order to maximize isotropy at typical prescription distances. Transmission radiography reveals that its internal components can move by as much as 200μm under the influence of gravitational forces affecting the dose-rate constant and relative dose distribution by as much as 5% and 10%, respectively. However, the clinical dose distribution can be well represented by a weighted average of dose distributions corresponding to three different orientation-dependent internal geometry configurations. This average dose distribution agrees closely (within 1% at 1 cm) with that derived from the geometry specified by the vendor model (seed components uniformly spaced), from which the recommended relative dosimetry parameters were derived. An average dose-rate constant of 0.935 cGy h-1 U-1 is recommended for clinical application and values of 0.919, 0.963, and 0.920 for vertical orientation, horizontal orientation, and the vendor specified geometries, respectively. The NIST wide-angle free-air chamber realization of the air-kerma strength primary standard, SK,N99, was explicitly simulated. We show that the output of Ag characteristic x rays is about half that of the model 6711 source resulting in a relative dose distribution between that of the model 6711 source and pure 125I x-ray emitters. Finally, it is demonstrated that the TG-43 point-source formalism more accurately estimates solid-angle weighted dose at small distances if one uses a radial dose function data prepared using a point-source, rather than line-source, geometry factor.

KW - Dosimetry

KW - I-125

KW - Monte Carlo simulation

KW - Permanent brachytherapy

KW - Task Group 43

UR - http://www.scopus.com/inward/record.url?scp=0036118006&partnerID=8YFLogxK

U2 - 10.1118/1.1452733

DO - 10.1118/1.1452733

M3 - Article

C2 - 11991122

AN - SCOPUS:0036118006

SN - 0094-2405

VL - 29

SP - 509

EP - 521

JO - Medical physics

JF - Medical physics

IS - 4

ER -