The prevalence of stone disease in the United States has been estimated at 8.8% with an annual cost of $3.79 billion.¹ A recent publication, assuming increases in population size and stone prevalence, estimated a continual annual increase in cost of $1.24 billion per year through 2030.¹ It is not surprising that technology surrounding the planning and execution of surgical treatment of nephrolithiasis is evolving to simplify and expedite stone removal to meet the increasing demands of stone disease.

Traditionally, the most common measured outcomes after nephrolithotripsy have included stone-free rates, postoperative pain scores, rates of sepsis and septic shock after surgery, operative time and, more recently, procedural costs.² Emerging now, however, as urologists are pushed to treat more disease, is a greater emphasis on surgical time stone clearance time, and lithotripsy potential is emerging as urologists are pushed to treat more stone disease.

Additionally, the current spectrum of surgical approaches has become expansive. Ureteroscopy alone can be done with or without a sheath, as a dusting or basketing procedure, with standard or pulse-modulated holmium or thulium fiber lasers. For percutaneous nephrolithotomy surgeons are adjusting patient positioning, sheath size, number of accesses, and most importantly lithotripters. Lastly, while external shockwave lithotripsy has had fewer advances, it is the workhorse for stone treatment and has an exciting contemporary in burst wave lithotripsy. Despite the explosion in surgical options, the guidelines are rigidly aligned with the greatest stone diameter in their recommendations for surgical approaches,³ having many downstream effects–one example being the contentious point of physician compensation discrepancies that exist for percutaneous nephrolithotomy (50081 vs 50080) and ureteroscopy (52356).⁴

Figure. Example of qSAS imaging software with tracings of kidneys in axial plain (A), validation of appropriate kidney tracings on software rendered coronal imaging (B), and representation of qSAS dashboard with stone characteristics and 3D rendered image of lower pole stone (C).

Quantifying stone burden is a critical element from a surgeon’s perspective⁵ and along with stone location is considered a major preoperative predictor for treatment success. The obvious pitfall with using stone diameter to estimate stone burden is highlighted in an editorial by De Coninck and Traxer.⁶ They illustrate that a “20 × 20 × 20 mm stone (4,189 mm³) is 16 times larger than a 20 × 5 × 5 mm stone
(262 mm³), which is equivalent to an 8 × 8 × 8 mm stone (268 mm³).”⁶ The glaring difference in stone burden between these 2 hypothetical 20 mm stones would likely be appreciated, radiographically, by the treating urologist and an appropriate surgical approach selected to render the patient stone-free. The academic interest in accurately quantifying stone burden is intensifying as increasingly more trials emerge comparing surgical approach and lithotripters. This is exemplified by a recent publication demonstrating that a difference in stone clearance rates between the Trilogy (1,220±1,670 mm³/minute) and Shock Pulse-SE (770±680 mm³/minute, p=0.054) systems appeared only after controlling for total stone volume calculated with novel imaging software.⁷ This study was a straightforward comparison of 2 ultrasonic intracorporeal lithotripters, but perhaps more interesting would be the comparison of the extremes such as dusting a 5,000 mm³ stone with ureteroscopy and pulse modulated laser technology versus standard percutaneous nephrolithotomy with a novel lithotripter. These types of studies answer interesting clinical questions that have real world application for the practicing urologist, but a fair comparison starts with total stone volume as the metric for stone burden.

There are several software applications currently available including, but not limited to: qSAS (Mayo Clinic, Rochester, Minnesota, https://ctcicblog.mayo.edu/hubcap/qsas-stone-toolkit/), 3DSlicer (Boston, Massachusetts, https://github.com/fredericpanthier/SlicerKidneyStoneCalculator) and MATLAB 9.1 (Natick, Massachusetts; see figure). 3DSlicer is a free application while qSAS has a licensing agreement specifying its use for academic purposes. There are limitations to these applications. They typically require axial Digital Imaging and Communications in Medicine (DICOM) images only with thickness ranging from 1–5 mm. They are semi-automated software applications that require human stone or kidney tracings to calculate stone number, volume and density (Hounsfield units). Additional limitations include the lack of validation trials for these advanced software packages and studies demonstrating equivalence to hand traced stone surface area for predicting stone free rates after surgery.⁸ Nevertheless, there is a growing interest in the application of these software adjuncts highlighted by Ventimiglia et al in a recent publication proposing 2 novel lithotripsy metrics: stone ablation (Joules/mm³) and ablation speed (mm³/second).⁹ Characterizing stone burden and complexity has been a primary focus for academic and clinical practice demonstrated with simple metrics like stone diameter to more comprehensive grading systems like the Guy’s stone score. The importance of reliably reporting stone burden has never been more apparent, and as stone volume software becomes fully automated and vetted with validation trials we should expect to see stone volume become the research standard for quantifying stone disease.