Displacement of periurethral stranded seeds and its dosimetric consequences in prostate brachytherapy

Purpose: The use of stranded seeds for prostate brachytherapy has raised concern that displacement of strands, particularly in the periurethral region, may result in inadequate coverage of the prostate. We sought here to evaluate the displacement of periurethral stranded seeds after a prostate brachytherapy implant (Day 0) and its dosimetric consequences 1 month later (Day 30). Methods and Materials: Subjects were 10 consecutive patients who underwent implantation with ¹²⁵I stranded seeds via a peripheral-loading technique. Computed tomography scanning was done on Days 0 and 30. Seeds were located and dose distributions calculated with a Variseed 7.2 treatment planning system (Varian Medical Systems). Images were registered by two methods, one using the penile bulb as reference and the other using the pubic bones for verification. Only seeds within the periurethral strands were analyzed. Results: The mean displacement of periurethral stranded seeds relative to the prostate did not exceed 1mm in any direction. Calculated displacements were not affected by the registration method used. The mean dose covering 90% of the prostate volume (D ₉₀) and prostate volume receiving 100% of the prescribed dose (V ₁₀₀) were 169Gy and 97% on Day 0 and 186.5Gy and 98.7% on Day 30 (p<0.001 for D ₉₀). Conclusions: Displacement of periurethral stranded seeds 30 days after implantation was minimal and did not compromise dosimetric coverage of the prostate.