Dosimetric impact of contouring and image registration variability on dynamic 125 I prostate brachytherapy.

PURPOSE: The quality of permanent prostate brachytherapy can be increased by addition of imaging modalities in the intraoperative procedure. This addition involves image registration, which inherently has inter- and intraobserver variabilities. We sought to quantify the inter- and intraobserver variabilities in geometry and dosimetry for contouring and image registration and analyze the results for our dynamic125 I brachytherapy procedure.

METHODS AND MATERIALS: Five observers contoured 11 transrectal ultrasound (TRUS) data sets three times and 11 CT data sets one time. The observers registered 11 TRUS and MRI data sets to cone beam CT (CBCT) using fiducial gold markers. Geometrical and dosimetrical inter- and intraobserver variabilities were assessed. For the contouring study, structures were subdivided into three parts along the craniocaudal axis.

RESULTS: We analyzed 165 observations. Interobserver geometrical variability for prostate was 1.1 mm, resulting in a dosimetric variability of 1.6% for V100 and 9.3% for D90 . The geometric intraobserver variability was 0.6 mm with a V100 of 0.7% and D90 of 1.1%. TRUS-CBCT registration showed an interobserver variability in V100 of 2.0% and D90 of 3.1%. Intraobserver variabilities were 0.9% and 1.6%, respectively. For MRI-CBCT registration, V100 and D90 were 1.3% and 2.1%. Intraobserver variabilities were 0.7% and 1.1% for the same.

CONCLUSIONS: Prostate dosimetry is affected by interobserver contouring and registration variability. The observed variability is smaller than underdosages that are adapted during our dynamic brachytherapy procedure.