NIAGARA FALLS, ONTARIO—Canadian researchers have validated a nomogram they say accurately predicts the success of treatment for renal or ureteral stones with shockwave lithotripsy (SWL).

Andrea Lantz, MD, a kidney stone and endourology fellow at the University of Toronto, described the nomogram at the Canadian Urological Association’s 68th Annual Meeting. The nomogram was developed based on a study of patients who underwent treatment with an electromagnetic lithotripter. Dr. Lantz conducted the study with lead investigator Kenneth Pace, MD, and R. John Honey, MD, both of the University of Toronto.

Drs. Pace and Honey, together with other collaborators, had created a nomogram to predict the success of a single treatment with SWL (J Urol 2011;186:556-562). It was based on patients treated with the Lithotron (Philips), an electrohydraulic lithotripter, from 2004 to 2009. The nomogram was based on patient- and stone-related factors. For renal stones, it incorporated three variables for predicting the single-treatment success: age, skin-to-stone distance (SSD), and stone area. For ureteral stones, the nomogram used two factors for predicting treatment success: body mass index (BMI) and stone area.

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Dr. Pace’s team wanted to determine whether the nomogram is valid and generalizable to SWL with an electromagnetic lithotripter in a more recent patient population. They reviewed data from patients treated at their lithotripsy unit from June 2010 to June 2013 using a Modulith SLX-F2 lithotripter (Storz). Their analysis included 147 patients with renal stones and 139 with ureteral stones. All had a solitary treated radio-opaque stone not more than 2 cm in diameter and had a computed tomography (CT) scan taken within four weeks of the SWL treatment. Using the CT images, the investigators determined the skin-to-stone distance and the mean stone density.

The procedure began with fluoroscopic localization of the stone, followed by SWL with dose escalation up to a power setting of six for renal and upper ureteral stones and nine for stones in the mid- and distal ureter, all at a rate of 120 shocks per minute. The treatment was stopped when either good fragmentation had taken place or the patient had received 3,000 shocks, whichever came first. A three-month follow-up radiograph was used to determine whether patients with renal stones were stone free or had only insignificant stone fragments and those with ureteral stones were stone free.

The single-treatment success rates for renal and ureteral stones were 74.8% and 59.7%, respectively. For renal stones, significant predictors of successful SWL were stone area, stone density, and SSD; for ureteral stones, significant predictors were BMI, stone area, stone density, and SSD.

There were more men than women in the study population. The average patient age was 53 years and the average stone area was 65.9 mm2 for renal stones and 51.5 mm2 for ureteral stones. The stone length averages were 9.0 and 8.6 mm, respectively, the mean stone densities were 921.8 and 1000.8 Hounsfield units, and the mean SSD were 105.8 and 134.3 mm.