Dose uncertainty due to needle-tip localization error in prostate seed implantation

Purpose
This study quantified the dosimetric uncertainty caused by needle-tip detection errors in ultrasound images due to bevel-tip orientation differences, with respect to the location on template grid.

Material and methods
Trans-rectal ultrasound (TRUS) system with physical template grid and 18-gauge bevel-tip brachytherapy needles were used. TRUS was set at 6.5 MHz in water phantom, and measurements were taken with 50% and 100% B-mode TRUS gains. Needle-tip localization errors were then retrospectively applied back to 45 prostate seed implant plans to evaluate the important planning parameters for the prostate (D₉0, V₁₀₀, V₁₅₀, and V₂₀₀), urethra (D10 and D30), and rectum (V₁₀₀, D_2cc, and D_0.1cc), following the ABS and AAPM TG-137 guidelines.

Results
The needle-tip detection errors for 50% and 100% TRUS gains were 3.7 mm (max) and 5.2 mm (max), respectively. The observed significant decrease in prostate coverage (mean D₉₀ lower by 12.8%, and V100 lower by 3.9% for smaller prostates) after seed placements were corrected by compensating the needle-tip detection errors. Apex of the prostate was hotter, and the base was cooler. Dosimetric difference for urethral and rectal parameters were not statistically significant.

Conclusions
This study revealed that the beveled needle-tip orientation could considerably impact the needle tips detection accuracy, based on which the seeds might be delivered. These errors can lead to significant dosimetric uncertainty in prostate seed implantation.