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

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 dynamic ¹²⁵I brachytherapy procedure.

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.

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.

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.