Decline of dose coverage between intraoperative planning and post implant dosimetry for I-125 permanent prostate brachytherapy: comparison between loose and stranded seed implants.
We have read with interest the comments of Al-Qaisieh et al. on our study concerning the decline of dose coverage between intraoperative planning and post implant dosimetry for I-125 permanent prostate brachytherapy . Their first remark concerns reporting only D90. However, the results section also gives the values of the parameters VT,100, VT,150 and VT,200 and explains why we chose to perform the analysis on D90 only. The second remark concerns patient selection and number of patients in different groups. As mentioned in the material and methods section the patients were arbitrarily selected for either a stranded seed implant or a loose seed implant, which makes this study a retrospective study and not a randomized one. Therefore, a statistical analysis was performed taking possibly influencing factors like prostate volume or physician preference into account (see also Table 2). Differences in group size are taken into account in standard statistical testing procedures. The third remark concerns experience and robustness of technique. As described in the discussion and conclusion section, we expect other differences in technique, apart from different seed types, not to be confounders in this comparison study. We have a large experience in using both implant techniques (loose and stranded). Both techniques are the same (template, US guidance, planning system, dose criteria), except for type of seeds (loose or stranded) and insertion (manually for stranded seeds or automatically with seedloader for loose selectSeeds). In both techniques needle position and seed delivery is verified on the live US image. We see no reason why one technique is more robust or reproducible than the other. And if experience plays a role, the strand technique is used since 1996, while the loose seed technique with the seedloader is routinely used since 2000. The 4th remark concerns the use of the 1D dose calculation formalism in this study. We agree that the use of a 3D dose calculation algorithm would give more accurate dosimetry results. However, for this study the use of the 1D dose formalism as described in the AAPM TG43 update report  is not a limiting factor to make judgments on implant quality for three reasons. Firstly, in current practice most institutes use the 1D formalism and reported results are generally based on the 1D calculations. Therefore the D90 values in ourmanuscript can be comparedwith reported values from literature. Secondly, due to the large amount of seeds of an implant D90 differences between the 1D and 2D calculations are small. Thirdly, the decline in D90 between post implant dosimetry and intraoperative plan stems from superior/inferior shifts of the seeds/strands irrespective of dose calculation formalism. The 5th remark concerns the impact of different prostate volumes resulting in different number of needles and sources when 386 Letters to the Editor / Radiotherap