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

Purpose: To quantify changes in prostate size and seed movement over time after transperineal implantation of stranded I seeds, and to determine their impact on prostate dosimetry.

Methods: CT and MR (T2, balanced steady-state free precession) image triplets were acquired on days 0, 3, 10, and 30 for a cohort of 20 patients and registered automatically. Prostate contours were drawn on MR-T2 images; seeds were found and matched in successive CT images. Prostate volume and dimensions, seed movements, and prostate dose metrics V200, V150, V100 and D90 were calculated, and their dynamic behaviors quantified in an operationally defined prostate coordinate system.

Results: Cohort-averaged reductions in prostate A-P dimension (∼8%) and L-R dimension (∼5%) inferred from seed movements agreed with those obtained from contour measurements, whereas prostate volume and S-I dimension (implant direction) reductions inferred from seed movements were overestimated by about 30%. Average overall seed movement was 4.8 ± 3.0 mm, of which the only identifiable systematic component was resolution of prostate edema. Cohort-averaged ratios of prostate V200, V150, V100, and D90 on day 30 relative to day 0 were 1.67, 1.33, 1.02, and 1.08, respectively.

Conclusions: Postimplant prostate size reduction in the SI (implant) direction cannot reliably be inferred from stranded seed movements. Apart from large-scale migration, residual seed movements relative to the prostate after accounting for edema resolution appear to be random. Prostate V100 and D90 changes 30 days post implant are modest, whereas those for V150 and V200 are substantial.