Prostate size, source configuration, and dosimetry dynamics of stranded ¹²⁵I seed implants

Area of Science:

• Radiation Oncology
• Medical Imaging
• Urology

Background:

• Transperineal implantation of stranded ¹²⁵I seeds is a common treatment for prostate cancer.
• Accurate dosimetry is crucial for treatment efficacy and minimizing side effects.
• Understanding prostate size changes and seed migration post-implantation is essential for dose assessment.

Purpose of the Study:

• To quantify prostate size changes and seed movement following ¹²⁵I brachytherapy.
• To assess the impact of these changes on prostate dosimetry over time.
• To evaluate the reliability of seed movement for inferring prostate dimensional changes.

Main Methods:

• CT and MR imaging (T2, balanced steady-state free precession) were performed on 20 patients at days 0, 3, 10, and 30 post-implantation.
• Automated image registration was used to align sequential scans.
• Prostate contours and seed positions were analyzed to calculate volume, dimensions, seed movement, and dose metrics (V200, V150, V100, D90).

Main Results:

• Prostate dimensions in the anterior-posterior and left-right directions showed modest reductions consistent with contour measurements.
• Prostate volume and superior-inferior dimension changes were overestimated when inferred from seed movements.
• Average seed movement was 4.8 ± 3.0 mm, primarily attributed to prostate edema resolution.
• By day 30, cohort-averaged dose metrics showed ratios relative to day 0 of 1.67 for V200, 1.33 for V150, 1.02 for V100, and 1.08 for D90.

Conclusions:

• Prostate size reduction in the implant direction cannot be reliably determined from stranded seed movements.
• Residual seed movements, excluding edema resolution, are largely random.
• Prostate V100 and D90 show modest changes by day 30, while V150 and V200 experience substantial increases.