Timing of postseed imaging influences rectal dose-volume parameters for cesium-131 prostate seed implants

Purpose: To study the influence of timing of postseed implant imaging on rectal dose-volume parameters for cesium-131 (¹³¹Cs) seed prostate implants. Methods and Materials: Fifteen patients were treated in our institution with combination ¹³¹Cs brachytherapy followed by pelvic external beam radiation therapy for intermediate to high-risk prostate cancers. For all patients, CT scans were scheduled at 7 days (CT₇) and again at 2 months for external beam radiation therapy simulation purpose (CT₆₀) postseed implantation. Comprehensive postseed implant dosimetry was performed for both CT₇ and CT₆₀ scans. In each case, dose-volume histogram parameters, rectal separation (the distance between the center of posterior most seed and most anterior rectal wall), and posterior row activity (the total activity implanted within 2-4mm anterior to the posterior wall of the prostate) data were collected. The absolute rectal volumes receiving 100% and 110% prescription dose were also collected. Results: Rectal dose correlated strongly with rectal separation (p<0.001). The mean change in rectal separation between CT₇ and CT₆₀ scans was 1.1 (±1.7) mm, and the corresponding change in 0.1-cc rectal dose was 18 (±26.5) Gy. Posterior row activity did not correlate with rectal dose (p=0.51). The mean volume of rectum that receives between 100% and 110% of the prescription dose (RV₁₀₀ and RV₁₁₀) increased twofold, between CT₇ and CT₆₀ evaluations (0.03 [±0.06] cc vs. 0.07 (±0.05) cc, respectively, p=0.06). Conclusions: Our study has demonstrated that rectal doses after ¹³¹Cs seed implants are influenced by the timing of postseed imaging. This may be a consequence of prostatic and periprostatic edema resolution.