Dose volume assessment of high dose rate Ir-192 endobronchial implants

Purpose: To study the dose distributions of high dose rate (HDR) endobronchial implants using the dose nonuniformity ratio (DNR) and three volumetric irradiation indices. Methods and Materials: Multiple implants were configured by allowing a single HDR Ir-192 Source to step through a length of 6 cm along an endobronchial catheter. Dwell times were computed to deliver a dose of 5 Gy to points 1 cm away from the catheter axis. Five sets of source configurations, each with different dwell position spacings from 0.5 to 3.0 cm, were evaluated, Three-dimensional (3D) dose distributions were then generated for each source configuration. Differential and cumulative dose-volume curves were generated to quantify the degree of target volume coverage, dose nonuniformity within the target volume, and irradiation of tissues outside the target volume, Evaluation of the implants were made using the DNR and three volumetric irradiation indices. Results: The observed isodose distributions were not able to satisfy all the dose constraints, The ability to optimally satisfy the dose constraints depended on the choice of dwell position spacing and the specification of the dose constraint points, The DNR and irradiation indices suggest that small dwell position spacing does not result in a more homogeneous dose distribution for the implant, This study supports the existence of a relationship between the dwell position spacing acid the distance from the catheter aids to the reference dose or dose constraint points, Better dose homogeneity for an implant can be obtained if the spacing of the dwell positions are about twice the distance from the catheter aids to the reference dose or dose constraint points.