The Future of Urolithiasis Measurement: Determining Stone Volume (2024)

Measurement is the first step that leads to control and to improvement.

If you can’t measure something, you can’t understand it.

If you can’t understand it, you can’t control it.

If you can’t control it, you can’t improve it.

H. James Harrington

Maximum linear stone measurements continue to be the standard of care for stone burden characterization according to the AUA and European Association of Urology’s guidelines. Previous studies have highlighted the inherent limitations of linear measurements among the growing number of stone patients globally.^1,2 Patel et al noted that when comparing the linear measurements of the same stone across 3 different board-certified radiologists, the average interobserver error was 26.3%.³ This discrepancy is concerning because the efficacy of maximum linear measurements in predicting actual stone volume diminishes significantly as stone size increases.⁴ Specifically, for stones < 10 mm, the maximum stone diameter predicts 76% of the actual stone volume, whereas for > 20-mm stones, the volumetric predictive capacity of maximum diameter drops to only 10%.⁴

Clearly, kidney stones are 3D structures, and the 2D kidney, ureter, and bladder x-ray measurements of the past, when applied to CT scans, do not accurately reflect the true stone burden (Figure 1). To address the challenge of accurately quantifying stone volume, Finch et al proposed the utilization of “best-fit” ellipsoid formulas.⁵ These formulas incorporate 3 linear measurements of the stone.⁵ They found that smaller stones (<9 mm) were more suitably characterized by the prolate ellipsoid formula, while medium-sized stones (9-15 mm) correlated with an oblate formula, and larger stones (>15 mm) aligned best with the scalene formula.⁵ Due to the complex, irregular shape of renal calculi, especially as they become larger, the various ellipsoid formulas become less accurate as the stone’s size increases (determined through water displacement or gas pycnometry).^5-7 Indeed, even using the best-fit ellipsoid formula, the actual stone volume is overestimated by 27% for stones < 9 mm and by 89% for stones 20 mm or larger.⁴

To overcome these inaccuracies, we developed a 3D stone volume artificial intelligence (AI) algorithm.⁴ The 16-layer contracting-expanding convolutional neural network technology facilitates 1- to 2-minute volume compilation while ensuring accuracy (R Pearson correlation coefficient = 0.99) and precision (Dice 3D overlap score = 0.88) when compared to the manually calculated 3D characterization of stone burden using the 3D slicer program.⁴ The AI algorithm obviates the need for manual measurements, negates interobserver variability, eliminates the inaccuracies of the ellipsoid formulas, and provides a rapid, accurate volume assessment.

From this work, several important questions have arisen. First: How does/should stone volume impact the choice of surgical management? Although current guidelines recommend percutaneous nephrolithotomy as the first-line option for the management of stones > 2 cm, is clearing a 20- × 7- × 2-mm stone percutaneously reasonable when one would use a ureteroscopic approach for a 15- × 10- × 8-mm stone, given the fact that the latter has a 4-fold larger volume? Further investigation is warranted to elucidate whether differences in volumetric stone burden among subgroups with equivalent 1D linear sizes have discernable effects on surgical outcomes and patient management.

Second: Is volumetric stone clearance a reliable metric of successful surgery?Although volumetric stone clearance (cubic millimeters of stone per minute of surgery) allows for a more standardized means of reporting operative outcomes, it is essential to exercise caution when relying solely on volume reduction when assessing outcomes. For example, a 95% reduction in stone burden, although commendable, could trivialize the presence of a residual 3- to 4-mm stone fragment (Figure 2). According to the CT-based grading scale proposed by the Journal of Endourology,⁸ a fragment of this size would correspond to a relative stone-free Grade C (2.1- to 4-mm fragments). These fragments are not “clinically insignificant” as previously thought.^9,10 Indeed, at a median postoperative follow-up of only 7 months, fragments 4 mm or smaller carry a considerable risk of reintervention (16%) and complications (11%). In fact, any residual stone fragment, irrespective of its size, has the potential to serve as a nidus for stone growth, eventually leading to recurrence and necessitating further intervention. Clearly, achieving absolute stone-free status (Journal of Endourology Grade A, no fragments present on a 2-to 3-mm noncontrast CT scan) is the goal in order for our patients to have the very best outcome from their stone surgery.

Third: What are the implications of volumetric stone burden follow-up for patients who have undergone a metabolic evaluation and are on medical management for their stone disease? To date, surveillance of urolithiasis patients primarily relies on correlating 24-hour urine parameters with the linear size growth of stones. Yet, as underlined by Eisner et al, the average interobserver variability when comparing linear stone measurements ranges between 1.2 and 1.9 mm.¹¹ With such low reproducibility, the reliance on linear stone size growth is problematic given that a 1- to 2-mm size change can be attributed to various factors: measurement error, change in the stone’s orientation within the collecting system, or true stone growth. This uncertainty significantly impacts the medical management of nephrolithiasis, as the detection of true stone growth usually prompts further patient evaluation and modification in both diet and medical therapy. This becomes even more important when dealing with patients with multiple stones, such as individuals with nephrocalcinosis due to medullary sponge kidney disease.

In summary, it is our belief that integrating volumetric stone burden assessment into routine clinical practice would be helpful with regard to nephrolithiasis surveillance and management, with implications extending to both surgical treatment planning as well as long-term follow-up care. Incorporation of preoperative and postoperative standardized volumetric stone burden outcomes into current clinical urolithiasis research would help to further optimize guidelines-based treatment options.