MONTE CARLO-BASED DOSE-RATE TABLES FOR THE AMERSHAM CDCS.J AND 3M MODEL-6500 CS-137 TUBES

Purpose: (1) To present reference-quality dose-rate distributions for the Amersham CDCS.J-type Cs-137 intracavitary source thitherto unavail able in the literature) and updated tables for the 3M model 6500/6D6C source. (2) To assess the accuracy of the widely used ID pathlength (S ievert integral) algorithm for lightly filtered Cs-137 tube sources. M ethods and Materials: A Monte Carlo photon-transport code is used to c alculate the dose-rate distributions about the 3M source and the CDCS. J source based on radiographic examination of the sources and the vend ors' specifications. Dose-rate distributions are provided in the form of Cartesian ''away-and-along'' lookup tables. Using a general form of the Sievert integral, calculated dose-rate distributions were compare d to the Monte Carlo benchmark calculations treating the filtration co efficients as best-fit parameters as well as approximating them by lin ear energy absorption coefficients, In addition, the errors introduced by approximating the active source core by uniform cylinders or line sources was evaluated. Results: The Model CDCS.J dose distribution dif fers from that of the 3M model 6500 source by -5.9% to +14.4% (root-me an-square [RMS] average: 2.6%). The RMS accuracy of the Sievert algori thm is 2.4% to 2.8% terror range of -1.4% to 7.6%) when filtration coe fficients for steel and ceramic media are approximated by linear energ y absorption coefficients. If the filtration coefficients are treated as parameters of best fit, selected to minimize the discrepancies betw een ID pathlength and Monte Carlo calculations, the RMS error is reduc ed to 0.8% (error range of -1.8% to 4.1%), The optimal values of stain less steel and low-density ceramic or glass filtration coefficients ar e approximately independent of the source geometry. Conclusions: The w idely used Sievert integral algorithm accurately characterizes the dos e distribution around stainless-steel clad low-density matrix Cs-137 s ources, particularly if design-independent best-fit values of the filt ration coefficients are used. Although both families of source designs studied produce similar dose distributions, source-design specific do se distributions should be used for clinical treatment planning and do se-algorithm validation. (C) 1998 Elsevier Science Inc.