Is it possible to get adequate dose coverage of a clinical target volume (CTV) to account for the possible extra capsular extension in early-stage prostate cancer by low dose rate (LDR) brachytherapy?

Abstract

98 Background: There is no consensus for the definition and evaluation of CTV in post-implant setting for LDR prostate brachytherapy to account for extra capsular extension in clinically localized prostate cancer. In this study, we defined a CTV and evaluated its dosimetry in the post implant CT/MR scans done at 1 month after the LDR brachytherapy procedure. Methods: The initial consecutive 71 patients who underwent LDR brachytherapy under a single physician at Princess Margaret Hospital from June 2009 to July 2010 were included in this retrospective study. On the post implant MRI, the CTV was created by adding 3mm uniform margins around the prostate but respecting the anatomical boundaries like bone, bladder and rectum. Post implant dosimetry was based on CT/MR fusion using the dosimetric parameters V80, V90, V100, V150, V200, D80, D90 and D100. Implants were qualified as optimal if their V100 was >85% and D90 was >90% for both prostate and CTV. Univariate analysis was performed to evaluate associations of factors with V100 and D90 for the CTV using Wilcoxon rank sum test and Fisher's exact test. Results: The mean (SD) prostate V100 and D90 were 95.5% (4.2) and 117% (10) respectively with only 1 patient having sub optimal implant (V100 <85% and D90 <90%). The mean (SD) V100 and D90 for the CTV were also acceptable at 90.6% (4.9) and 103% (9), respectively. Six patients had V100 <85% and 7 patients had D90 <90% for the CTV. On univariate analysis, edema and seed implantation technique correlated with sub-optimal implant for the CTV. The mean (SD) edema for patients with V100 <85% was 18% (10) and with D90 <90% was 15% (12). The corresponding values for the optimal implants both in terms of V100 and D90 were 3% (13). Patients implanted with exclusively loose seeds (15 patients only) had higher incidence of sub-optimal implants (26%) compared to patients who had strands on the antero-lateral margins (56 patients; 3.5%). Conclusions: In this study, adequate dose coverage of CTV was achieved in most patients with current technique but implants with optimal dosimetry to prostate still may have sub-optimal D90 and V100 for the CTV. No significant financial relationships to disclose.