MONTE CARLO-AIDED DOSIMETRY OF A NEW HIGH-DOSE-RATE BRACHYTHERAPY SOURCE

In this article we introduce a new high-intensity Ir-192 source design for use in a recently reengineered microSelectron-HDR remote afterloa ding device for high dose-rate (HDR) brachytherapy. The maximum rigid length and outer diameter of the new source are reduced to 4.95 and 0. 90 mm, respectively, compared to 5.50 and 1.10 mm for the previous sou rce design introduced in 1991. In addition, a smaller diameter and mor e flexible steel cable are used, allowing the source cable to negotiat e smaller diameter catheters or more tortuously curved catheters. Usin g Monte Carlo photon transport simulation, the complete two-dimensiona l (2-D) dose-rate distribution is calculated over the 0.1-7 cm distanc e range and are presented both as conventional 2-D Cartesian lookup ta bles and in the formalism recommended by the American Association of P hysicists in Medicine Task Group 43 (TG-43) Report. The dose distribut ion of this source is very similar to that of its predecessor, except near the source tip and in the shadow of the cable assembly, where dif ferences of 5%-8% are apparent. The accuracy of various methods for ex trapolating beyond the tabulated anisotropy functions to short distanc es is evaluated. It is demonstrated that linear extrapolation from the anisotropy functions defined by TG-43 accurately (+/- 2%) estimates d ose rate at short and long distances lying outside the radial distance range of the original measured data from which the anisotropy and rad ial dose functions were derived. In contrast, the algorithm used on th e vendor's planning system results in large calculation errors at dist ances less than 5 mm. (C) 1998 American Association of Physicists in M edicine. [S0094-2405(98)00511-2].