Predicting pubic arch interference in prostate brachytherapy on transrectal ultrasonography-computed tomography fusion images

摘要

This study was conducted on patients with localized prostate cancer who were scheduled for seed implantation therapy in 2007. The procedure was explained and informed consent was obtained from all the patients. The patient eligibility criteria for this therapy included clinical stage T1–T2 cancer, a Gleason score of ≤7 and serum prostate specific antigen (PSA) 20?ng/ml. We performed CT and constructed TRUS-LBCT fusion (TCF) images in 21 consecutive patients, whose characteristics are summarized in Table?1. Risk classification for prostate brachytherapy was conducted using the Seattle technique [9], and the clinical staging was performed in accordance with the 1992 American Joint Committee on Cancer (AJCC) staging system [10]. About 1?month before the seed implantation therapy, TRUS (Logiq; General Electric Healthcare Japan, K.K. Tokyo, Japan) was performed using a Micro-Touch stepper unit (CIVCO Medical Solutions, Kalona, IA, USA), for volume study and preplanning. For the TRUS, serial axial images of the prostate were obtained at 5-mm intervals from the base of the gland to the apex. The prostate contour was outlined at each level. The planning target volume included the prostate gland with a 3- to 5-mm margin all around. The treatment planning was performed using the radiation treatment planning (RTP) system (VariSeedTM, version 7.1, Varian Medical Systems, Inc., Palo Alto, CA, USA). Acquired CT images were transferred to the RTP system. We selected three anatomical points on axial slices of both CT and TRUS images; urethra and the ventral and dorsal aspects of the rectum (Fig.?2). The RTP recognized these points and automatically created the TCF images. We verified the accuracy of the fusion images by comparing the organ position on both images. We delineated the prostate, rectal anterior wall and urethra on the TRUS images and the pubic bone on the CT images. The TRUS image in which the prostate outline was the largest was overlaid on the LBCT section in which the pubic arch was the narrowest. The results of estimation of the prostate volume, angles of the pubic arch and PAI in the patients by TRUS are summarized in Table?2; they revealed the absence of significant PAI in 18 cases and the presence of significant PAI in the remaining three patients. The median prostate volume and angle of the pubic arch as evaluated by TRUS were 28.5?cm3 and 45° (range 35–60°). The results of evaluation of the PAI and of the angle of the pubic arch on the LBCT and TCF images are also shown in Table?2. This evaluation required less than 25?min for each patient. LBCT was successfully performed with the patients in the lithotomy position. The pubic arches were clearly visualized in all of the patients. Significant PAI was detected on the LBCT and TCF in six and five cases, respectively. The median prostate volume and angle of the pubic arch on the LBCT was 29.5?cm3 and 40° (range, 20–55°). We performed seed implantation in the 18 patients in whom no significant PAI was detected by TRUS. Among these patients, significant intra-operative PAI was observed in one patient (Case 7), which was predicted by TCF (Fig.?5). On the other hand, TCF also yielded one false-positive result (Case 18). LBCT showed two false-positive cases (Case 4 and Case 18). The differences in the angles between the LBCT and TRUS are shown in Table?2. The angle was determined to be larger by LBCT than by TRUS in three arches, was equal in both modalities for seven arches, and was judged to be smaller (by 15° in seven arches) according to TRUS in 32 arches.