BERKELEY, CA (UroToday Inc.) - Using a Medstone electrohydraulic shock wave lithotriptor, the authors reviewed their results with 100 patients undergoing shock wave lithotripsy (SWL) for a renal or proximal ureteral stone in the 5-10 mm size range. BMI and stone Hounsfield units (HU) independently correlated with stone free success, judged by a KUB 6 weeks following therapy.

In sum, stone free patients had a BMI of 26.9 +/-0.5 and Hounsfield units of 578 +/- 182 while patients with residual stones had a BMI of 30.8 +/- 0.9 and Hounsfield units of 910 +/- 190. These findings are in keeping with earlier works by Joseph (J Urol 2002;167: 1968) and Dretler (unpublished data - 2003), both showing that SWL induced stone fragmentation at 500 HU is excellent while at 1000 HU or greater, it is a rare event. The only questions I would raise are 1.) whether a subanalysis of these patients would reveal better success in the obese patient with an upper pole stone, as the distance from the patient's back to this portion of the kidney is the shortest and 2.) whether proximal ureteral stones, given their "enclosed" environment with little fluid interface did more poorly than renal calculi existing in their more expansive fluid filled environment.

Nonetheless, this is now additional and more extensive clinical data pointing to the important predictive nature of stone HU determined on a noncontrast CT scan: if it's 500 or less, SWL is the way to go and if it's 1000 or more, SWL is not going to be effective. The gray zone exists between 500 and 1000; in this area, a variety of other factors may come into play, including BMI, stone location and renal anatomy.

Urol. 2005;(Jan)65: 33-36