Three-dimensional visualization and dosimetry of stranded source migration following prostate seed implant

Computed tomographic (CT) imaging was used to determine the displacement of stranded seeds between day 0 and day 30 following prostate seed implants. Post-plan dosimetry was used to assess the dosimetric consequence of strand displacement.

Between March 2006 and December 2009, 86 prostate seed implant patients had day 0 and day 30 post-plan CT imaging. Migrated strands were first identified by inspection of day 0 and day 30 scans. The exact distance of displacement was measured using 3-dimensional fusion software. Post-plan dosimetric analysis was performed using CMS software.

Of the 1550 strands placed, 23 strands exhibited substantial movement and these displacements occurred in 21 of the 86 cases. The measured distance of strand movement ranged from 0.31 cm to 3.44 cm, with mean displacement of 2.75 cm. Of the displaced strands, 15 strands moved away from the implant center while 8 strands moved toward the implant center. A comparison of changes in dosimetric parameters between day 0 and day 30 revealed expected increases in D90 related to resolution of prostate edema. When cases of strand displacement were compared with cases without displacement, there were no differences in D90 or V100 at day 0. At day 30, however, statistically significant decreases in D90 (96.8% vs 89.5%; P = .0061) and V100 (85.9% vs 82.2%; P = .046) were noted for cases with strand displacement. When the data were analyzed by looking at the change in dosimetric parameters over time for each individual case (eg, comparing the difference in D90 from day 0 to day 30 per patient) there was a trend toward decrease in D90 with displacement of the strands but this did not achieve statistical significance (P = .09).

Stranded seeds show unexpected spatial instability in the craniocaudal dimension. Strand displacement may occur in approximately 1.5% of stranded sources placed and about 25% of cases. One may expect mean decreases in D90 and V100 of about 7% and 3%, respectively, among cases exhibiting strand migration in the first month following seed implant. Resolution of prostate edema during the same time period accounts for an approximately 30% increase in D90 and V100.