Dosimetric impact of inter-observer catheter reconstruction variability in ultrasound-based high-dose-rate prostate brachytherapy.

Purpose: To investigate the dosimetric impact of interobserver catheter reconstruction variability in transrectal ultrasound-guided prostate high-dose-rate (HDR) brachytherapy.

Methods And Materials: Twenty consecutive patients with intermediate- or high-risk prostate cancer were treated with a single, 15-Gy HDR brachytherapy boost as part of this study. The treated plan was used as the study reference plan (P). Three expert treatment planners (observers) manually reconstructed the catheter paths on the static three-dimensional transrectal ultrasound images, and new plans were generated from the updated positions (P); subsequently, the dwell time and positions from the P plans were superimposed on the P catheter paths to evaluate the dosimetric effect of the interobserver variations (P). Plans from each group were stratified by observer and by number of catheters (12 or 16) and then compared using a one-way Kruskal-Wallis H test with post hoc Mann-Whitney U tests reserved for significant variations (α = 0.05).

Results: Greater than 98.9% of catheter reconstruction variations were <3 mm. When stratified by observer, there was a significant decrease (p << 0.05) in planning target volume (PTV) V and increases in the urethral D between the P plans propagated to the P catheter paths and dosimetry evaluated and P plans only. Stratification of plans by catheter number showed nonclinically significant decreases in PTV V, and D and increases in urethral D for the 12-catheter plans.

Conclusions: Limiting interobserver variability, and its effects on prostate HDR brachytherapy plan quality, is critical to achieving good dosimetric outcomes; small variations in catheter reconstruction may translate to inadequate PTV coverage, excessive urethral dose, or both.