On the dosimetric influences of air-Kerma strength calibration geometry and internal source structure for ¹⁰³ and ¹²⁵I brachytherapy sources

Monte Carlo photon-transport simulation techniques are used to study the influence of the new wide-angle free-air chamber (WAFAC) on low energy interstitial source dosimetric parameter. For sources containing radio opaque components with sharp corners, or low density components coated with high density materials, oblique filtration unexpectedly induces primary photon flux anisotropy near the transverse plane at typical calibration distances. Thus, the dose-rate constant may vary as much as 11% with the solid angle subtended by the S_K measurement instrument. For the Model 200 ¹⁰³Pd source, and by extension, other "thin radio opaque layer" seeds, the WAFAC renders the absolute dose-rate distribution near the seed independent of this dense layer and its thickness.