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TUL is a method for treating ureteral stones transurethrally by removing them through an endoscope inserted in a retrograde fashion into the ureter. Together with ESWL, TUL is now an established method for the treatment of ureteral stones.[1-6] It is currently recommended in the EAU guidelines as an appropriate treatment for cases of ureteral stones based on EBM criteria. Selection of the procedure for active ureteral stone removal depends on stone location and size. For stones in the proximal ureter, ESWL is recommended as the first-choice treatment for stones <10 mm, and TUL (anterograde or retrograde) or ESWL is recommended for stones ≥10 mm. For stones in the distal ureter, ESWL or TUL is recommended as first-choice treatment for stones <10 mm, and TUL is recommended as the first choice for stones ≥10 mm in this location.
Depending on individual cases, however, it might be difficult to determine whether ESWL or TUL is the more appropriate choice of treatment. In such cases, pretreatment prediction of the stone-free rate for each type of treatment might enable selection of the optimal treatment for ureteral stones in individual patients. With respect to urological tumors, nomograms have been developed as prognostic and predictive tools for predicting the cancer detection rate of prostate biopsy in prostate cancer, as well as pathological staging after radical prostatectomy.[7-16]
With respect to ureteral stones, however, although some models for predicting the outcome of ESWL have been previously reported,[17-19] there has not been a nomogram for predicting the outcome of TUL. In the present study, a nomogram for predicting the stone-free rate in TUL treatment of ureteral stones was developed and internally validated.
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It is generally preferable to develop nomograms using cohorts with little bias toward characteristics in order to establish their broad utility. With respect to TUL nomograms, it is thought to be particularly important to avoid extreme bias in treatment outcomes. In order to verify the validity of the cohort in this study, the current treatment outcomes were compared with those for ureteral stone treatment published in the 2011 EAU Guidelines. According to the 2011 EAU Guidelines, the median stone-free rates for TUL were 82%, 87% and 93% for the proximal, mid and distal ureter, respectively. The corresponding stone-free rates for the present cohort were 75%, 88% and 89%, respectively, showing a somewhat lower rate for the proximal ureter. The reason for this might be that the Guidelines were based on a meta-analysis including cases in which flexible ureteroscopes were used, whereas the present cohort included only cases treated using semi-rigid ureteroscopes. In recent years, the availability of flexible ureteroscopes has led to better treatment outcomes compared with those for rigid ureteroscopes, particularly for proximal ureteral stones. Thus, the treatment outcomes of the present cohort were almost equivalent to those of the Guidelines, and therefore constituted a valid cohort for the production of a universally applicable nomogram.
During the development of the nomogram, stone size, number of stones, stone location and the presence or absence of pyuria were shown to be independent factors determining the stone-free rate after TUL treatment. There are several reports about the factors related to the stone-free rate in TUL treatment. Yencilek et al. reported that the success rate of semi-rigid TUL was relatively low in the proximal ureter (71.7%) when compared with the mid (94.8%) and distal ureter (98.9%; P = 0.021). El-Nahas et al. reported that significant factors for unfavorable TUL results were proximal ureteral stones, ureteroscopy carried out by surgeons other than experienced endourologists, stone impaction and stone width. Leijte et al. reported that surgeon experience is a predictive factor for complications and success for TUL.
Abe et al. suggested that pyuria is an independent factor predicting the stone-free rate after ESWL treatment. In the present cohort, as for ESWL, pyuria was a predictive factor for the stone-free rate after TUL, and multivariate analysis also showed a significant difference. Although the reason why the stone-free rate becomes worse in the case of positive pyuria is unknown, it is assumed that in such cases with hydronephrosis by the impacted stone, hydronephrosis might cause pyuria, and it might decrease the stone-free rate by increasing the risk of push up by hydronephrosis and the difficulty of TUL because of the stone impaction.
It has also been reported that the longer length of the urethra in men affects mechanical operations during the procedure, meaning that the occurrence of complications is influenced by sex, whereas other studies have reported that sex has no effect on the stone-free rate. In the present cohort, the stone-free rates for men and women were 79.8% and 83.1%, respectively; the difference was not significant.
As aforementioned, a range of nomograms has been produced as prognostic and predictive models for treatment efficacy with respect to malignant urological tumors, but in terms of predictive models for ureteral stone treatment, there have been only a few previous reports regarding ESWL.[17-19] There has been no previous report of the development of a nomogram for predicting the efficacy of TUL treatment, and this is the first nomogram for TUL. In this nomogram, AUC were 0.7432 for the nomogram itself, 0.5641 for stone size, 0.5908 for the number of stones, 0.6594 for stone location and 0.6076 for pyuria. Validation using 20% of the data also achieved a reasonable predictive accuracy (AUC = 0.682), indicating that the nomogram had a reasonable accuracy using simple factors alone.
Although there are still a number of impediments, this nomogram might benefit us in choosing optimal treatment options by comparing predicted treatment outcomes of ESWL and TUL nomograms in the future.
However, this nomogram has some limitations. The first is that, although it is useful in practice for providing information to patients, prediction of treatment efficacy does not always lead to choice of actual treatment method. The second is that this nomogram's high predictive accuracy might not be maintained when it is used in other institutions because of differences in the operator's skill and in treatment policies among institutions. Because a high level of skill is required for ureteroscopic operations, it has been suggested that treatment outcomes are influenced by the operator's skill, and the high predictive accuracy of this nomogram might not be maintained in other institutions. For this reason, external validation of this nomogram is required.
In addition, the introduction of f-TUL in recent years has also improved treatment outcomes for mid and upper ureteral stones (especially in men), and f-TUL has now been adopted by numerous institutions. However, from the global standpoint, there seems to be many institutions that still use only semi-rigid ureteroscopes. As the f-TUL nomogram cannot be applied to institutions using semi-rigid ureteroscopes, both semi-rigid TUL nomogram and f-TUL nomogram will be required for universal use. In the present study, we developed a nomogram for predicting treatment outcomes of TUL using only semi-rigid ureteroscopes. Production of a nomogram for predicting treatment outcomes of f-TUL will also be required in the future.
In terms of the future potential of nomograms for ureteral stones, it will be of value to establish nomograms to predict treatment efficacy for individual treatments, such as f-TUL and percutaneous nephrolithotripsy. By comparing predicted values between different treatment methods, we might objectively choose the most effective treatment based on individual patient characteristics. Although further studies are required, it is our hope that this nomogram will pioneer the development of new strategies for treatment choice.
The first nomogram for predicting the stone-free rate after TUL was developed. It has a reasonable predictive accuracy, and in combination with ESWL nomograms, it might be useful for deciding treatment methods in the future.