3D voluminometry of urolithiasis as a predictor of success in ureterorenoscopic (URS) treatment

Kidney stones are well known for their acute colic pain and is frequently paired with fever and/or renal function deterioration. The lifetime incidence of symptomatic urolithiasis is estimated to be between 2% and 15% in the Western population [1, 2]. Up to 80% of the patients with urolithiasis pass their stone spontaneously with or without the use of analgesics or medical expulsive therapy (MET) when stones are less than 4 mm [3]. All other patients are more likely to receive active treatment. The size, position, shape of the stone and patient characteristics at presentation are factors that influence treatment decisions [4, 5].

Unfortunately, not all stones can be removed during one surgical procedure. A well-known important factor that plays a role in the success (complete removal of all stones) after active treatment is the size of the stones or so-called stone burden. In daily practice, size is measured by several two-dimensional (2D) methods such as the longest stone axis, mean stone axis or stone surface area (length × width) on non-contrast enhanced CT-scan or abdominal X‑ray [2, 6].

Since urolithiasis are irregular three-dimensional (3D) structures that often have different geometric shapes, 2D measurements may inaccurately predict stone volume and not reflect the actual total stone burden [5, 7]. Some studies have investigated 3D measurements of urolithiasis to predict spontaneous passage and outcomes of treatment after Shockwave Lithotripsy (SWL). They showed 3D measurements to be a clinically significant independent predictor of success [8‐11]. Regarding ureterorenoscopy (URS), reports have been published that indicate the potential use of 3D measurements [10, 12]. So far, for percutaneous treatment no advantage has been found of 3D measurements on treatment outcome [13].

This study was designed to investigate if automated 3D volume measurements have a higher predictive value of stone-free rates (SFR) after URS compared to conventional 2D measurements. Secondary outcomes include complications and need for secondary procedure. Our goal is to introduce a potential pretreatment tool for counseling urolithiasis patients about their chance of stone-free status after URS and the potential need for a secondary procedure or the occurrence of complications.

For this retrospective study, we identified patients who were treated by ureterorenoscopy (URS) for urolithiasis from April 2008 to October 2019 at the Canisius Wilhelmina Hospital Nijmegen, the Netherlands. Criteria for inclusion were: age > 18 years and single (clinically relevant) stone diagnosed by CT-scan. Patients with multiple stones, obscure stones and no available pre-operative CT-scans were excluded from the study. Our institutional review board approved this study and the need for signed informed consent was waived (ref. 035-2020).

Preoperative, perioperative and postoperative characteristics of all patients were recorded and saved in a database (CastorEDC). Preoperative characteristics included age, sex, stone position, stone side, stone density, renal function and mean size of the stone. Stone position was classified as ureteral (proximal or distal) or renal (upper-, mid-, lower pole, pelvis). Ureteral position was based on stone position above (proximal) or below (distal) the iliac vessels.

Perioperative characteristics involved the use of a semi-rigid or flexible ureterorenoscope. For proximal and renal stones a reusable Olympus Flex URF-V3 or a disposable PUSEN ureterorenoscope was used and when needed, an ureteral access sheath was applied. An Olympus semi-rigid ureterorenoscope was used for distal/proximal ureteral stones [12]. Postoperative characteristics were stone-free status, complications, mean stone density and need for second surgery. Stone-free status was considered if no identifiable stone was present after the URS procedure determined by the operating urologist or confirmed by CT-scan 4 weeks postoperatively (mainly assessed by urologist).

A total of 227 patients met the inclusion criteria (Tab. 1). The mean age was 50.6 years and 63.9% were men. Stones were overall located in the kidney (26%) or ureter (74%). 170 patients (74.9%) were classified as stone free. Type of URS, semi rigid or flexible, were equally used for the procedures. Stone size measurements (2D and 3D), stone position, and type of URS procedure appeared to be significantly different in the SF-group compared to the group that had residual stones. Complications such as pain, infection, discomfort by JJ catheter, hematuria and urine retention were only found in 12.3% of the cases. A secondary procedure was needed in 20.1% of all patients.

Urolithiasis are 3D structures that can adopt complex geometric shapes. Stone size is typically assessed by measuring the longest stone axis, mean stone axis or surface area (which are considered as 2D measurements) on a plain abdominal CT-scan. Measuring one axis or a surface area fails to provide exact volumetric information since it is limited by the inability to measure the second or third dimension [9, 15]. It is known that success of surgical treatment (stone free) for urolithiasis depends on the actual size of the stone. A 3D measurement could possibly provide a better representation about the actual stone size (stone burden). This study was designed to investigate and validate if automated software-based 3D voluminometry of urolithiasis has a better predictive value for stone-free status after URS compared to conventional 2D measurements.

We found that automated 3D voluminometry predicts complications and SFS better than 2D measurements. Stone position, procedure type and mean stone density did not influence the results regarding complications. Our data showed that small stones (< 400 mm³) indicate a higher chance of success compared to large stone (> 400 mm³). Although the correlation of 3D measurements with SFS and complications, the predictive accuracy between the four methods was not significantly different (Fig. 1). Based on the logistic regression models and ROC curves, an actual clinical benefit for automated 3D volume measurement, could not be determined. Only one study has explored the predictive value of automated 3D volume on SFS after percutaneous nephrolithotomy (PCNL) and found similar results [13].

To our knowledge, our cohort is the largest used to explore the value of 3D voluminometry as a predictor of success after URS treatment of urolithiasis. Nevertheless, it is a retrospective study that included a small heterogeneous group of patients from one institute with renal and ureteral single stones. Another bias in our study is that stone treatment during URS was performed by individual surgeons preferences and patient characteristics. In our institute we used a low power Holmium laser, but settings and technique (dusting or fragmenting) differed. All these settings could affect the effectiveness of the treatment [16]. Unfortunately, because of our study design, this data could not be retrieved. A larger prospective multi-institutional trial could reveal more reliable correlations of 3D measurements with the outcomes after URS [9]. Furthermore, in our clinic, we evaluate SFS on plain CT-abdomen four weeks postoperatively or by visual control during surgery. Unfortunately this latter clinical observation does not correlate with actual SFS when assessed by radiographic studies [17]. Other studies have shown that residual stone fragments could also pass spontaneously in many patients even after four weeks. A postoperative CT after 12 weeks could provide a higher chance of SFS [10]. In this study, only patients with a single stone were included. Although a majority of patients that present themselves in our clinic have a single stone, a large group has multiple stones. For now it remains unclear if there is a clinical benefit of pre-operative 3D voluminometry in this group of patients.

For this study we applied 3D reconstruction software from the Philips Intellispace Portal. This software was used because it was already available in our hospital and has been applied in several clinical domains. So far, no studies have examined the use in analyzing urolithiasis [18]. Future software updates, more specifically adjusted to evaluation of urolithiasis, could improve the efficiency and accuracy of stone volume measurement [19]. Unfortunately, medical centers use different types of software for reviewing radiological images. Therefore, it is difficult to extrapolate our results to other hospitals. Ideally, a tool should be developed that can be applied on all different types of software.

We assume 3D stone voluminometry to be an easily obtainable measurement that could represent actual stone burden for all urolithiasis patients. A trained observer (radiologist or urologist) requires less than two minutes to assess the stone volume in the Intellispace Portal in case of a single stone [9]. Nevertheless, there are some significant shortcomings for 3D stone volume assessment. In case of multiple stones, which is related to lower SFR, radiologist will need more time to assess total stone volume. In daily practice, urologists can determine the clinical relevance of multiple stones better than a radiologist. Therefore, the clinical implication for urologists could possibly not be as high as described in previous studies [20, 21].

When the measurement of stone size is essential in treatment decision and success rates, the reproducibility of stone size measurements (interobserver variability) are also important [19]. The results in this study, demonstrated low variabilities and a high reproducibility values (ICC 0.891 vs 0.782) between observers. According to our findings, we can conclude that the automated volume measurement is reliable and reproducible [19].

Although 3D-stone voluminometry correlates significantly better to SFS and the occurrence of complications, it harbors no predictive value. Automated volume assessment of urolithiasis is easily performed and highly reproducible. More studies need to be performed to fully elucidate the potential of 3D-measurements regarding success rates after URS and other surgical treatments for urolithiasis.