The purpose of the present study was to identify preimplant factors affecting postimplant CT-determined prostate volume and the CT/TRUS volume ratio in prostate cancer patients treated with transperineal interstitial prostate brachytherapy with 125I free seeds. The results showed that preimplant prostate volume by TRUS and NHT are significant independent factors affecting both postimplant prostate volume by CT and the CT/TRUS volume ratio. From these results, we have developed regression equations to predict postimplant prostate volume by CT and the CT/TRUS volume ratio.
NHT was revealed to be a significant preimplant factor affecting both postimplant prostate volume by CT and the increase in prostate volume after implantation. The present results indicate that a patient with NHT has a higher CT/TRUS volume ratio than a patient without. These results are in agreement with a previous report [13
]. Ash and coworkers reported that the mean CT/TRUS volume ratio was 1.17 for patients with NHT vs. 0.98 for those without (p
< 0.001) [13
]. This means that patients with NHT have a greater increase in prostate volume after implantation. The reason why NHT is positively associated with the increase in prostate volume after implantation is unclear. It has been reported that NHT is associated with an increased risk of acute urinary morbidity after implantation [14
]. The association of NHT with the increase in prostate volume would provide an explanation for this clinical observation, however, it does not explain the pathophysiology of the increased tendency to prostate swellings. This issue needs to be explored in further studies.
The present results, however, may seem to contradict those of some other studies [18
]. Badiozamani and colleagues reported that NHT had no consistent effect on postimplant volume changes [18
]. Tanaka and colleagues reported that no predictive factors for edema were found, including NHT [20
]. Potters and colleagues reported that the CT/TRUS volume ratio was significantly lower for patients treated with NHT [19
]. The discrepancies could be attributed to differences in the timing of postimplant CT scans among these studies. In the studies by Badiozamani and colleagues and Tanaka and colleagues, the postimplant CT scans were obtained the day after implantation, when prostatic swelling was the greatest. In the study by Potters and colleagues, they were obtained 1.6-6.5 weeks (median, 3.1 weeks) after implantation, which seems somewhat early for all of the prostatic edema to resolve, whereas, in the study by Ash and coworkers, they were obtained 6-8 weeks after implantation, when the swelling had resolved [8
]. These findings suggest that the impact of NHT on postimplant prostate volume changes differs depending on the timing of the postimplant CT scans. In the present study, the postimplant CT scans were obtained at a mean of 7.6 weeks after implantation, which is similar to the study of Ash and coworkers. Consequently, it is considered that the results of the present study are consistent with those by Ash and coworkers, showing a positive association between the CT/TRUS volume ratio and NHT. However, further study will be needed to assess the potential impact of NHT on the increase in prostate volume after implantation.
The next factor affecting postimplant prostate volume by CT and the increase in prostate volume after implantation is preimplant prostate volume by TRUS. In univariate and multivariate analyses, preimplant prostate volume by TRUS was associated significantly with postimplant prostate volume by CT and the CT/TRUS volume ratio. The present results indicate that a patient with a smaller gland has a higher CT/TRUS volume ratio. This is consistent with a previous report [21
]. Pinkawa and colleagues reported that preimplant prostate volume was correlated with the extent of postimplant edema both on day 1 and day 30, indicating that smaller prostates developed greater edema [21
]. However, some previous reports are inconsistent with the present study [6
]. Badiozamani and colleagues reported that no single parameter, including preimplant prostate volume, could accurately predict the degree of swelling on day 1 [18
]. Moreover, Taussky and colleagues reported in their study consisting of only 20 patients that, although preimplant prostate volume was associated with the CT/TRUS volume ratio on Day 1, it was not associated with the CT/TRUS volume ratio on Day 30 [6
]. These discrepancies are due to the different timings of postimplant CT scans and the small numbers of patients in their studies. Further study will be needed to assess the potential impact of preimplant prostate volume on the volume increases after implantation.
Although the CT/TRUS volume ratio is important because it affects postimplant dosimetric results, it has not been fully determined what the optimal cut-off value of the CT/TRUS volume ratio should be for predicting suboptimal dosimetry. Few investigators have determined a cut-off value of the CT/TRUS volume ratio for predicting suboptimal dosimetry [19
]. Potters and colleagues reported that a CT/TRUS volume ratio >1.5 was an independent predictor of poor D90 dose [19
]. It is, however, difficult to compare their results directly to ours. The reasons are as follows. First, the results of Potters and colleagues showed a relatively higher CT/TRUS volume ratio (mean, 1.43), due to the early timing of postimplant CT scans. On the contrary, the results of the present study showed the mean CT/TRUS volume ratio of 1.16, which is in agreement with those of many other studies [6
]. Second, in the Cox regression analysis of Potters and colleagues, which was performed to identify independent factors predictive of poor D90 dose, patients with NHT were excluded [19
]. Therefore, the cut-off value presented by Potters and colleagues could not be applied directly to the present data. An optimal cut-off value of the CT/TRUS volume ratio to predict suboptimal dosimetry will need to be explored in further studies.
The results of the present study show that in patients who had a smaller prostate gland and/or who underwent NHT, a greater increase in prostate volume is predicted after brachytherapy, which may affect postimplant dosimetric results. For these patients, to achieve optimal dose coverage of the prostate, it is thought to be useful to implant more seeds than expected. However, this speculation should be validated in future investigations.