Dosimetric Characteristics of 103 Pd (Theragenices, Model 200) Brachytherapy Source

Abstract

Accurately determination of dosimetric parameters for low-energy ¹⁰³ Pd source is crucial to use by treatment planning systems. The dosimetric parameters such as the air kerma strength, Sk, dose rate constant, L, radial dose function, g(r) and anisotropy function, F(r, q) for the lowenergy ¹⁰³ Pd source were investigated. MCNPX (2.6.0) simulation code was used to calculate all dosimetry parameters. Full details of Theragenices-¹⁰³ Pd source model 200 (dimentiones, materials and energy spectrum) were implementd in MC simulations. The ¹⁰³ Pd source was centered in infinite water phantom and the radial dose function calculation the dose was scored in concentric rings with different thickness bonded by 88° and 92° conics. Due to symmetry of source geometry and in order to reducing of errors, the 0.5 mm thickness rings were bounded with two oblique cones (with interval angel of 1°) from 0° to 90° in order to calculate Anisotropy function. The air kerma strength, Sk and dose rate constant, L were found equal to 0.646 cGycm2h⁻¹mCi⁻¹ and 0.798 cGyh⁻¹U⁻¹, respectively. For radial dose function, g(r), using an equation as g(r) = a₀+a₁r+a₂r²+a₃r²lnr+ a₄/r², Where a₀=1.6440, a₁=-0.8338, a₂=0.1917, a₃=-0.0539, a₄=-0.0062 with R²=0.9995 have been yielded a better fitting than the fifth-order polynomial equation recommended by AAPM, TG-43U1. The calculated anisotropy values included the distances of clinical interest for this type of lowenergy photon source, up to 2 cm, and especially for smaller angles, q 0.9935. The calculated results and extracted equations are in good agreement with the corresponding values reported by AAPM, TG-43U1 and can be used in Monte Carlo based treatment planning system.