Time course of prostatic edema post permanent seed implant determined by magnetic resonance imaging

Abstract

Purpose: To quantify the time course of postimplant prostatic edema magnitude and spatial isotropy using serial magnetic resonance imaging (MRI).

Methods and materials: Forty patients with histologic diagnosis of prostate cancer received an iodine-125 seed implant (Day 0) and consented to 1.5-T MRI on Days -1, 0, 14, and 28. Seeds of strength 0.39mCi were placed in a modified peripheral loading pattern to deliver 145Gy to the target volume. MR images consisted of 3-4mm thick axial slices with no gap. The image sets were anonymized and randomized to minimize contouring bias, then contoured by a single radiation oncologist. Contours were reoriented about their center of mass to align the prostate long axis with the superior-inferior (S-I) direction; prostate volumes and dimensions in the left-right (L-R), anterior-posterior (A-P), and S-I directions through the center of mass were calculated.

Results: The average relative edema volume was 1.18±0.14 (1standard deviation) on Day 0 and 1.01±0.15 on Day 30. Between Days 0 and 30, the edema resolved linearly with time on average. Average relative edema dimensions on Day 0 in the L-R, A-P, and S-I directions were 1.01±0.07, 1.11±0.09, and 1.08±0.13, respectively.

Conclusions: As measured using MRI, the average edema magnitude for our study population was ∼20% on Day 0 and resolved linearly with time to ∼0% on Day 30. The edema exhibited spatial anisotropy, the prostate expanding on Day 0 by ∼10% in each of the A-P and S-I directions and by ∼0% in the L-R direction.