Definition and Dosimetric Evaluation of a Clinical Target Volume (CTV) in the Post Implant (CT/MR Fusion) Analysis of Low-Dose-Rate Brachytherapy for Prostate Cancer at Princess Margaret Hospital

Abstract

Purpose: The extent of extra prostatic extension in low and intermediate risk prostate cancer is mostly confined within a radial distance of 3mm which have implications in evaluating the post-implant dosimetry of LDR brachytherapy. There is a scarcity of data in evaluating the dosimetry around the prostate in the post implant setting. 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. Materials and 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. The margins used in the preplan ultrasound were 3mm anterolaterally 5mm inferiorly and 0mm posteriorly. On the post implant MRI, the CTV was created by adding 3mm uniform margins around the prostate as per guidelines of ESTRO and AAPM. We modified this CTV by editing it back from the bladder wall and rectal wall to respect the anatomic barriers of microscopic spread. We evaluated the dosimetry of prostate and CTV using the dosimetric parameters V80, V90, V100, V150, V200, D80, D90 and D100 after performing the CT/MR fusion. Implants were qualified as optimal if their V100 was >85% and D90 was >90% for the prostate in this analysis and the same indicators were applied for the CTV. Univariate analysis using Wilcoxon rank sum test for continuous variables and Fisher’s exact test for categorical variables was performed to evaluate associations of factors with V100 and D90 for the CTV. 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. The detailed analysis is given in the table.Tabled 1VariableProstate Mean (SD)CTVMean (SD)V80 (%)99 (1.5)97 (2.5)V90 (%)98 (2.9)94 (2.5)V100 (%)95.5 (4)90.6 (4.9)V150 (%)62 (11)57 (9.3)V200 (%)31 (8)28 (7)D80 (%)131 (11)121 (10)D90 (%)117 (10)103 (9.2)D100 (%)65 (10)55 (8) Open table in a new tab Six patients had V100 <85% and 7 patients had D90 <90% for the CTV. On univariate analysis, the factors which correlated with suboptimal implant for the CTV were edema and seed implantation technique. 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 implants with V100 <100% and D90 <90% (26%) compared to patients who had strands on the antero-lateral margins (56 patients; 3.5%). The prostate volume (ranging from 18 to 73cc) did not influence the dose coverage of CTV in our study. Conclusions: In this study, we defined the CTV as a 3mm expansion of the prostate to account for extra capsular extension in the post-implant setting for LDR brachytherapy and evaluated its dosimetry. We were able to achieve adequate dose coverage of CTV in most patients with current technique but implants with optimal dosimetry to prostate still may have suboptimal D90 and V100 for the CTV especially in the presence of post-implant edema. A consensus is needed for definition and evaluation of CTV in post-implant setting for LDR prostate brachytherapy.