Rectal Dose-Volume Dependence on Timing of Post-Seed Imaging for 131Cs Prostate Seed Implants

Abstract

Purpose: To investigate if rectal dose-volume values vary with timing of post seed implantation imaging for 131Cs seed prostate implants. Materials and Methods: Fifteen patients were treated in our institution with 131Cs (t1/259.7 days) combination brachytherapy followed by pelvic EBRT for high-risk prostate cancers. Intraoperative TRUS volume study, planning and implant activity optimization were performed in all cases. For all patients, post-seed CT scans were scheduled approximately at 7e10 days (CT7 - median 7 days, range 6e15) and again for EBRT simulation purpose at 2 months post seed implantation (CT60 -median 57 days, range 40e85). For rectum, outer wall including contents was contoured. All contouring on CT-images was performed by the same radiation oncologist. Comprehensive post-implant dosimetry was performed for both CT7 and CT60 scans. The total number of most posterior row seeds and corresponding activity, rectal separation (defined as the distance in mm between the center of posterior most seed and most anterior rectal wall) were recorded in each case. From the rectal DVHs minimal doses to highest irradiated 0.1, 1, and 2 cc volumes of rectum were individually recorded in each case (designated as RD0.1cc, RD1cc and RD2cc respectively). The absolute rectal volume (cc) receiving 100 and 110% prescription dose (VR100, VR110) were also collected. Results: Intraoperative TRUS-based mean prostate volume was 24.3 ((plus or minus)5.4) cc and post-implant CT7 and CT60 based prostate volumes were 27.7 ((plus or minus)3.8) cc and 25.8 ((plus or minus)4.1) cc, respectively. Median implanted activity was 113 mCi (range 86e134.5). In general, rectal dose correlated strongly with rectal separation (rs5 -0.785, P<0.001). Changes in rectal separation between CT7 and CT60 scans strongly influenced the change in rectal dose (rs5-0.75, P 5 0.005) (Figure 1). The mean decrease in rectal separation between CT7 and CT60 scans was 1.1 ((plus or minus)1.7) mm and the corresponding increase in rectal dose was 18 ((plus or minus)26.5) Gy. The median numbers and actvity of posterior row seeds were 12 and 29.5 mCi respectively (range 6e16 #; 14.8 e 41.4 mCi). Posterior row activity (#seeds) did not correlate with rectal dose (rs5 0.123, P 5 0.51). The mean ratio of rectal doses for 0.1 cc and 2 cc rectal volumes at scans CT60 to CT7 (expressed as ratios[RD0.1ccCT60/RD0.1ccCT7] and [RD2ccCT60/RD2ccCT7] respectively) was approximately 1.27. The mean volume of rectum that receives between 100 to 110% of reference dose (VR100- VR110) increased two-fold based between CT7 and CT60 evaluations (0.03 cc ((plus or minus)0.06) Vs 0.07 ((plus or minus)0.05) cc respectively, p50.06). Conclusions: Our study, albeit small sample size, has demonstrated that dose evaluation based on imaging at 7e10 days post 131Cs seed implant underestimates actual rectal dose compared to at 2 months post implant. This may be a consequence of prostatic edema resolution and reduction of prostate volume. While there is no universally accepted consensus on the optimal timing of post seed imaging for 131Cs implants, we strongly believe that more than 1 time post-seed imaging such as at 7e10 days and again at 60 days post seed may very well be necessary to get an accurate rectal dose and toxicity assessment for short-lived 131Cs seed implants.