Difference in rectal dosimetry between pre-plan and post-implant analysis in transperineal interstitial brachytherapy for prostate cancer

Background and PurposeTo investigate differences in rectal dosimetry between pre-plan ultrasonography (US) and post-implant computed tomography (CT).Patients and MethodsSubjects comprised 49 patients who underwent prostate brachytherapy using 125I seed implants. Prescribed dose was 145 Gy to the periphery of the prostate. Differences in rectal dosimetry between pre-plan US and post-implant CT analysis were evaluated. In addition, patients were divided into two groups according to timing of pre-planning (pre-plan group, n=28; intraoperative pre-plan group, n=21), and differences in rectal dosimetry between groups were assessed.ResultsThe average of volume differences between pre-plan and post-implant analysis (pre-plan minus post-implant analysis) for all patients were follows: −0.08 cm3 in V60 (volume of rectal wall receiving 60% of prescribed dose); −0.05 cm3 in V70; −0.16 cm3 in V80; −0.38 cm3 in V90; −0.40 cm3 in V100; −0.32 cm3 in V110; −0.22 cm3 in V120; −0.15 cm3 in V130; −0.10 cm3 in V140; −0.07 cm3 in V150; and −0.05 cm3 in V160. Apparent differences between pre-plan US and post-implant CT in rectal dosimetry were small. However, considering the steep curve of the relationship between tolerable volume and dose, a large actual difference should be assumed. No advantage was identified for the intraoperative pre-plan group. Safe volume to avoid proctitis tended to be smaller on ultrasonography than on CT at 1 month.ConclusionsThe present work shows that direct comparison of CT analysis and pre-plan US is unfavorable due to large differences in these modalities and overestimation of tolerable volume. However, by comprehending the degree of difference, comparison of data from CT analysis with a US pre-plan may be feasible and useful for providing feedback between these modalities.