Real-time planning technique using intraoperative TRUS is adopted as an effective method to reflect real-time prostate shape and volume. In this technique, preplan is done just before seed insertion using real-time intraoperative TRUS image at operating room and according to preplan, seeds are inserted to prostate [9
]. But it is necessary to predict the intraoperative volume of prostate approximately before preparing 125
I seeds for brachytherapy to reduce number of wasted seeds.
Fogh et al. [9
] reported that the mean difference between preplan TRUS-based volume and intraoperative TRUS-based volume was 3.59 mL and the correlation coefficient was 0.84. The mean difference between preplan CT-based volume and intraoperative TRUS-based volume was 5.2 mL and the correlation coefficient was 0.82. Tanaka et al. [13
] reported that preplan CT-based prostate volume is 1.17 times larger than preplan TRUS-based prostate volume and preplan MRI underestimated prostate volume 0.73 mL in comparison to TRUS-based volume. Smith et al. [14
] demonstrated the average ratio for CT/MR volumes, TRUS/MRI volumes, CT/TRUS volumes were 1.16, 0.90 and 1.30, respectively.
The previously stated studies reported that CT overestimated prostate volume than TRUS and MRI underestimated prostate volume than TRUS or demonstrated similar volume with TRUS. But in this study, both CT and MRI overestimated prostate volume in comparison to TRUS.
A large part of studies have been reported that the CT-based prostate volume was larger than the MR-based prostate volume. Roach et al. [15
] reported that the volume of prostate from 10 patients who had both CT scan and MRI scan was compared and CT-based volume was 1.32 times (range, 1.05 to 1.63 times) larger than MRI-based volume. A study by Rasch et al. [16
] compared CT-based volume and MRI-based volume from 18 patients delineated by 3 radiation oncologists for all images. And CT volumes were larger than MRI volumes in 52 of 54 delineations. And CT volumes were 1.4 times larger than MRI volumes on average. Kagawa et al. [17
] reported the mean CT-based prostate volume and MRI-based prostate volume was 63.0 (+/- 25.8) cm3
and 50.9 (+/- 22.9) cm3
On the other hand, Parker et al. [18
] reported that the average ratio between the CT and MRI volumes was 1.02 and Prete et al. [19
] reported that MRI overestimated prostate volume by an average of 9.1% than CT. Parker et al. [18
] explained the similarity in prostate volume measured by CT and MRI with various reasons. The investigators knew that CT tended to overestimate volume in comparison to MRI and contouring was done considering sagittal and coronal reconstructed images. Prete et al. [19
] suggested that interobserver variation which was known to happen in CT-based contouring more than MRI-based contouring might be the reason for the opposed result. In this study, only 4 patients obtained both CT and MRI scan and the mean ratio of CT/MRI volume was 1.1. In majority of cases, we could not compare CT-based volume and MRI-based volume directly in the same patient, but the difference in ratio between CT/TRUS volume and MRI/TRUS volume was less than previous reports. We also contoured prostate on axial image and modified the contour according to coronal and saggital reconstruction images. And this contouring method might affect the result in our study as stated before by Parker et al. [18
The limitation of this study is that not all patients had both CT and MRI scan. So direct comparison between CT-based volume and MRI-based volume was not possible for all cases and we just evaluated the difference between preplan CT or preplan MRI-based volume and intraoperative TRUS-based volume.
In this study, 50 of 60 cases received NHT and it might affect the ratio of prostate volume between preplan image and intraoperative image. In addition, variability of duration and regimen of NHT might cause biased result. The duration of NHT after image scan (p = 0.055) and overall duration of NHT (p = 0.859) did not have statistically significant difference between preplan CT and preplan MRI group. But in each group, the duration of NHT was not consistent among cases. The duration of NHT after image scan was median 17 days (range, 2 to 35 days) for cases with preplan CT and 21 days (range, 2 to 46 days) for cases with preplan MRI. Langenhuijsen et al. [20
] reported that 31% reduction of prostate volume occurred during the first 3 months after initiation of NHT. During the next 3 months, 9% reduction of prostate volume was reported. And after that, no significant change of prostate volume was measured. To our knowledge, there is no report about the extent of prostate volume reduction for less than 3 months duration of NHT. So it is hard to make an accurate estimate of the extent of volume reduction by NHT for this study. Prospective study comparing the volume of prostate between CT and MRI in the same patient is needed under the control of NHT regimen and duration to confirm the volume discrepancy between image modalities and the effect of NHT on volume reduction. After that, we can make an accurate estimate of the number of seeds needed for dose coverage according to preplan image-based prostate volume.
Preplan CT volume and preplan MRI volume have a tendency to overestimate prostate volume in comparison to intraoperative TRUS volume. To reduce wasted seeds and cost of the brachytherapy, we should take the volume discrepancy into account when we estimate the number of 125I seeds for permanent brachytherapy.