Inferring Postimplant Dosimetry and Determining Low Radiation Dose Volumes Using Images Obtained at the Time of Recurrence After Permanent Prostate Implant Brachytherapy: A Pragmatic Study

Post-implant (PI) V100 & D90 were calculated for each sector and compared with whole prostate as well as intraoperative (IO) parameters. Optimal values were O90% for D90 & O85% for V100. Summary statistics (mean, SD, median, IQR, range) were reported for V100 and D90. The Wilcoxon Rank Sum test was used to compare the percentage drop from intra-op to post-implant, and to compare differences between individual sectors/groups of sectors and the whole prostate. Spearman’s rank correlation was used to examine the association between intra-op and post-implant values. Results: 71 seed implants were performed, mean PI prostate V100 and D90 was 95.5% and 117.1%. 70 had optimal D90 and V100. Only 1 sector (anterior base) had suboptimal PI V100 and D90 (70.3% and 85.9%) which were significantly and clinically lower than corresponding IO values (p! 0.001). This affected PI V100 for the prostate base (88%) and anterior quadrant (86.9%), but not D90 (108% & 106%). Inherent characteristics with the modified peripheral loading approach became evident as the mean IO D90 for the apical sectors (126%) was consistently lower than the base (131%) and midgland (132%), and lateral sectors (134%) had higher D90 than the anterior (123%) and posterior (126%) sectors. There was consistent decrease in the D90 from the IO to the PI plan in the anterior sectors (123% vs 106%, p! 0.001) and base sectors (132% vs 108%, p! 0.001) while an increase was noted in the apical sectors (126% vs 133%, p ! 0.001) as well as posterior sectors (126% vs 131%, p 5 0.006). The lateral sectors and mid sectors had minimal change from the IO plan. Conclusions:Overall dosimetry results to date are very encouraging with 11/12 sectors showing optimal dosimetry in most patients. Sector analysis provides insight into the planning aspects which may have impact in the PI dosimetry and provides useful data for further refinement in practice, further study correlating sector analysis with clinical outcomes is required.