The use of cylindrical coordinates for treatment planning parameters of anelongated Ir-192 source

Purpose: The doses given to the intima, media, and adventitia are very cruc ial quantities in intravascular brachytherapy. To facilitate accurate compu terized treatment planning calculations, we have determined dose distributi ons in away-and-along table format around an Ir-192 wire source and develop ed pertinent dosimetric parameters in cylindrical coordinates. Methods and Materials: The Monte Carlo method (MCNP4C code) was used to cal culate the dose distributions for the AngioRad Ir-192 wire source (model SL -77HS, Interventional Therapies). The calculations were carried out for pho ton, beta, and electron (conversion and Auger) contributions for radial dis tances from 0.03 to 2.0 cm with 0.01-cm increments, and up to 2.24 cm from the source center in the longitudinal direction with 0.04-cm resolution. Do se rate values are determined in away-and-along format (cylindrical coordin ates) and then converted to spherical coordinate format. Dosimetric paramet ers, such as the geometry factor, G(r, theta), and anisotropy function, F(r , theta), are generated in both cylindrical (R, Z, phi) and spherical (r, t heta, phi) coordinates. The use of a cylindrical coordinate system for trea tment planning parameters is proposed as a more suitable approach for accur ate calculations. Results: The photon contribution to dose varies nearly inversely with radia l distance (from the source center) along the perpendicular bisector with 0 .199 X 10(-3) cGy U-1 s(-1) (0.802 cGy Ci(-1) s(-1)) at I cm. The beta and electron contributions start at very high values of about 35.5 X 10(-3) cGy U-1 s(-1) and 11.0 x 10(-3) cGy U-1 s(-1), respectively, at 0.03 cm and fa ll off exponentially to negligible amount near 0.2 cm. The total dose rate at 0.2 cm is 1.428 X 10(-3) cGy U-1 s(-1) (5.754 cGy Ci(-1) s(-1)). The rad ial dose function, g(R), is nearly unity between 0.2 cm and 2 cm. Due to th e beta and electron dose contributions, g(R) increases steeply to 5.5 as ra dial distance decreases from 0.2 cm down to 0.03 cm. The F(R, Z) values are close to unity for the majority of the region of interest. In contrast, F( r, theta) experiences a steep rise as shallow angles are approached (closer to the source), related to the beta dose contributions. Accurate treatment planning calculations would be possible with linear interpolation of F(R, Z), but difficult with F(r, theta) in the spherical coordinate system and t he original normalization point as recommended in the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) formalism. Conclusion: The AngioRad Ir-192 wire source, model SL-77HS, was completely characterized dosimetrically using Monte Carlo methods. The use of cylindri cal coordinates and a modified anisotropy function normalization point for dosimetric parameters of an elongated Ir-192 source is more suitable for ac curate computerized treatment planning calculations in intravascular brachy therapy. (C) 2001 Elsevier Science Inc.