The effect of interobserver differences in post-implant prostate CT image interpretation on dosimetric parameters

Area of Science:

• Radiology
• Medical Physics
• Oncology

Background:

• Prostate brachytherapy utilizes CT scans for treatment planning and dosimetry.
• Interobserver variability in CT interpretation can affect the accuracy of dosimetric calculations.
• Standardized protocols are needed to minimize discrepancies in image interpretation.

Purpose of the Study:

• To assess interobserver variability in CT scan interpretation for prostate brachytherapy.
• To correlate interpretation differences with clinically relevant dosimetric parameters.
• To evaluate the suitability of dosimetric parameters as interinstitutional quality indices.

Main Methods:

• Twenty patients undergoing I-125 or Pd-103 brachytherapy were retrospectively analyzed.
• Four physician investigators independently outlined the prostate on post-implant CT scans.
• Dosimetric parameters including V100 and D90 were calculated and compared.

Main Results:

• Moderate interobserver variability was observed in CT volume determination (median SD: 17%).
• Variability was highest in prostate length, width, and thickness, impacting treatment margins.
• A significant percentage of implants were deemed inadequate based on D90 (24%) versus V100 (14%).

Conclusions:

• CT-based dosimetry provides useful, reproducible information despite interpretation variability.
• V100 and treatment margins are less susceptible to interobserver variability than D90.
• V100 and treatment margins may serve as more reliable interinstitutional quality indices for prostate brachytherapy.