Discussion
The current European Association of Urology (EAU) and American
Association of Urology (AUA) guidelines recommend PCNL for the treatment
of renal stones sized >2 cm.19,20However, due to the potential advantages of fURS (e.g., not causing
renal parenchymal damage and severe bleeding, applicability in patients
with bleeding diathesis or those receiving anticoagulant therapy, short
length of hospitalization, and daily work routine not being restricted)
and its ability to access almost all calyceal stones as a result of
improvements in deflection, fURS has become a preferred method for the
treatment of both proximal ureteral and renal
stones.2,21 In a recent meta-analysis, the final SFR
was reported to be 89.4% in an average of 1.4 procedures performed in
2-3 cm stones, and this rate was stated to be comparable to
PCNL.22 Although complication rates of up to 16% have
been reported in previous studies, most were classified as minor. In
addition to the development in fURS technology, increasing surgical
experience has reduced the rate of major complication from 5.01%
between 1990 and 2011 to 1.48% between 2011 and 2016 and increased the
success of treatment.23 While semi-rigid URS is
sufficient in most cases in the treatment of ureteral stones, performing
a procedure without fURS in the treatment of middle and upper ureteral
stones creates problems in terms of medicolegal aspects. In a study
published by the Clinical Research Office of the Endourological Society
ureteroscopy study group in 2014, it was reported that middle and
proximal stones were both larger and difficult to reach compared to
distal stones. In the same study, it was emphasized that the risk of
perforation due to impaction was high in middle ureteral stones and
low-caliber URS should be used.24 fURS, which has a
wide range of treatment options in terms of both localization and stone
burden and is even preferred in much larger stones to avoid PCNL-related
complications, requires further investigation in terms of how to predict
its outcomes. Almost all the scoring systems used in fURS have been
developed for renal stones, and STONE, which is an option for ureteral
stones in which fURS will be performed11,12,14,15, is
based on low stone burden and use of old devices. Hori et al. identified
this gap in the literature and defined a scoring system, which they
named T.O.HO., including the stone diameter, localization and density
parameters and covering patients with larger stones that are planned to
undergo fURS.16 The authors reported that T.O.HO. had
better predictive value than STONE (AUC = 0.833 and 0.633,
respectively).
In this study, the external validation of the T.O.HO. and STONE scores
was performed, and SFS-related parameters after fURS were evaluated. The
overall SFR was found to be 79.9%, and SFR in renal and ureteral stones
was 77.6% and 90.5%, respectively. Our SFR was similar to the rate
reported in the original study. In addition, in our study, the SFS
prediction accuracy (AUC value) was calculated as 0.758 for T.O.HO. and
0.634 for STONE. T.O.HO. The cut-off value of T.O.HO. was determined as
8, at which it had 71.4% sensitivity and 68.8% specificity in
predicting SFS. STONE was able to predict SFS with 57.5% sensitivity
and 63.2% specificity at a cut-off value of 11. When compared to the
original T.O.HO. study, the modified T.O.HO. score had lower specificity
but similar sensitivity (AUC = 0.758). This difference may be due to the
large stone sizes in our study and the lower number of patients with
ureteral stones compared to the original TOHO study. The modified
T.O.HO. scoring system was observed to have better predictive value than
the original version (AUC = 0.821 and 0.758, respectively). At a cut-off
value of 7, the modified T.O.HO. scoring system was able to predict SFS
with 71.2% sensitivity and 80.8% specificity.
Many studies in the literature have shown that stone burden is the most
important parameter affecting SFS after
fURS.11,12,14,15 In the T.O.HO. scoring system, stone
burden was reported to be the most important predictive
value.16 We also determined that stone burden was
associated with SFS (p < 0.001). The effect size of stone
burden was clearly demonstrated by the constructed nomogram (Figure 1).
Stone diameter is widely used in clinical practice since it is simple
and easy to obtain in the assessment of stone
burden.25 The EAU and AUA guidelines also include
stone diameter in their recommendations concerning decision-making with
regard to the treatment of urinary system stones.19,20Hori et al. used stone diameter while evaluating stone burden and
categorized it over 5 points based on the effect size obtained from the
nomogram. They reported that treatment success decreased by
<30% in patients with 5 points.16 However,
since stone diameter does not reflect the width and depth of the stone,
it will naturally have certain limitations in predicting the results of
the operation compared to stone volume. Ito et al., evaluating patients
who underwent fURS, emphasized that stone diameter was able to
accurately predict stone volume in <2 cm stones but it was
necessary to directly calculate stone volume in stones larger than 2
cm.25 Considering that stone volume increases
exponentially as stone diameter increases, this result is expected.
Today, with the developments in technology and increase in experience,
it is possible to apply fURS treatment to larger stones; therefore, it
would not be realistic to expect stone diameter alone to predict
success. Supporting this, in our study, the rate of treatment success
was 42% in the patients with 5 points in stone diameter (≥30 mm)
according to the original T.O.HO. score while it was only 21% for those
with 5 points (>480 mm2) according to the
modified T.O.HO. score, in which stone area rather than diameter was
evaluated. Hori et al. also stated that the STONE scoring system, which
has different cut-off values, does not have predictive value for stone
size classification.16 Consistently, we found that the
patients scoring 3 points (>10 mm) in the stone diameter of
the STONE scoring system had a treatment success rate of 79%. This
indicated that the stone size classification of the STONE scoring system
was far from differentiating SFS.
Another component of the T.O.HO. scoring system is stone localization.
Studies have shown that stone localization is an independent marker in
the treatment of ureteral and renal stones, and especially lower pole
stones are associated with fURS treatment
failure.11,12,14,15,19,26 For practical use, T.O.HO.
classified renal stone localizations as upper, middle and lower pole and
ureteral stones as proximal, middle, and distal. In our study, it was
observed that the rate of SFS was 71.2% in lower pole stones and 66.7%
in multi-calyceal stones, while it was 89.5%, 91.2% and 87.9% for
middle ureteral, proximal ureteral and pelvic stones, respectively.
However, the authors that developed T.O.HO. did not specify how
multi-calyceal were graded in this scoring system. In order to continue
the validation process, we scored multi-calyceal stones containing those
with middle and upper pole localizations and similar SFR as the upper
and middle pole group, and multi-calyceal stones with low SFR located in
the lower calyx as the lower pole group. According to the T.O.HO. score
based on stone localization, the worst SFR was in the lower pole, and
this was at a statistically significant level (p < 0.001). In
the multivariate analysis, it was determined that the middle ureter and
middle pole stones provided an increase of 76.2% and 50.4% in the
operation success, respectively, compared to the lower pole stones. In
the original T.O.HO. study, lower SFR (51.6%) was reported in the upper
pole stones than in the lower calyceal group, whereas in our study,
higher SFR (82.4%) was found in the upper pole stones similar to the
middle pole stones. This difference was attributed to the small number
of patients with upper pole stones in both studies and presumably
different stone sizes.
As an important parameter in the treatment of urinary system stones,
stone density is also a component of the T.O.HO. scoring system. The
relationship of stone density with SFS has been shown in many
studies.12,27 Hussain et al.28 used
the cut-off value of T.O.HO. stone density as 1100 HU and graded the
cases over 3 points. In our study, it was observed that stone density
was an independent marker for SFS in the multivariate analysis, and the
cut-off value was calculated as 1125 HU. A 100 HU increase in stone
density increased treatment failure by 1.1 times. The STONE scoring
system, which has a different cut-off value for stone density, was also
found to have similar predictive value for SFR (AUC = 0.570 for STONE
and T.O.HO for and 0.581). In our study, according to the stone density
score, SFR was determined as 83.7%, 84.0%, and 72.2% for 1, 2 and 3
points, respectively. There was no difference between 1 and 2 points in
terms of SFS (p > 0.05). We consider that the HU value can
be reduced to 2 points for a practical scoring system. However, in the
current study, we left the HU prediction values as in the original
system since it would not further increase the predictive value of the
modified system.
In addition to the three main parameters explained above, many other
parameters have been defined in the literature to be associated with SFS
after fURS, such as the number of stones, preoperative stenting,
presence of hydronephrosis, and operator
experience.11,12,14,15 The STONE scoring system uses
preoperative stent application, number of stones, and presence of
hydronephrosis as predictive factors.12 In the
original T.O.HO. study, Hori et al. reported both parameters to be
associated with SFS but found no independent marker in the multivariate
analysis.16 Similarly, in our study, the presence of
multiple stones was statistically significantly associated with SFS, but
it was observed that there was no independent marker in the multivariate
analysis. Since stone burden is directly related to the number of
stones, the latter loses its importance. We did not determine
preoperative stenting to be associated with SFS. There are publications
in the literature stating that preoperative stenting increases the
success of ureteral access sheath and is not associated with
SFS.29,30 Another parameter included in the STONE
scoring system is the presence of hydronephrosis. Hori et al. did not
evaluate the presence of hydronephrosis. In our study, although the
presence of hydronephrosis was high in patients with residual stones, it
was not found to be a statistically significant parameter. In the
nomogram developed by Ito et al., the presence of hydronephrosis had
very low power but it was not used as a marker in other scoring
systems.15 The same authors also used operator
experience as a marker in their nomogram. Since all surgical procedures
in our study were performed by experienced endourologists, similar to
the original T.O.HO. study, this marker is not discussed further.
Our study has certain limitations. The main limitations are
retrospective design, relatively small number of patients, and lack of
data on second-session attempts in patients with residual stones and
final success rates. Another important limitation is the lack of stone
composition that may affect SFS.