STONE DISEASE

Stone attenuation and skin-to-stone distance on computed tomography predicts for stone fragmentation by shock wave lithotripsy
Perks AE, Schuler TD, Lee J, Ghiculete D, Chung DG, D’A Honey RJ, Pace KT
Division of Urology, Department of Surgery, and the Department of Radiology, St. Michael’s Hospital and University of Toronto, Toronto, Ontario, Canada
Urology. 2008; 72: 765–769

• Objectives: To determine whether stone attenuation and the skin-to-stone distance (SSD) can predict for stone fragmentation by SWL independently. Identifying the factors predictive of shock wave lithotripsy (SWL) outcome would help streamline the care of patients with stones.
• Methods: A retrospective review was performed of 111 patients undergoing initial SWL for a solitary, 5-20 mm, renal calculus. Stone size, location, attenuation value, and SSD were determined on pretreatment noncontrast computed tomography. The outcome was categorized as stone free, complete fragmentation <5 mm, and incomplete fragmentation >/=5 mm or unchanged at 2 weeks on kidney/ureter/bladder radiography.
• Results: After SWL, 44 (40%) were stone free, 27 (24%) had complete fragmentation, and 40 (36%) of 111 patients had incomplete fragmentation. The stone attenuation of the successfully treated patients (stone free and complete fragmentation groups) was 837 +/- 277 Hounsfield units (HU) vs 1092 +/- 254 HU for those with treatment failure (incomplete fragmentation; P < .01). The mean SSD also differed: 9.6 cm +/- 2.0 vs 11.1 cm +/- 2.5 for the successful treatment group vs the treatment failure group, respectively (P = .01). On multivariate analysis, the factors that independently predicted the outcome were stone attenuation, SSD, and stone composition. When patients were stratified into 4 risk groups (stone <900 HU and SSD <9.0 cm, stone <900 HU and SSD >/=9.0 cm, stone >/=900 HU and SSD <9.0 cm, and stone >/=900 HU and SSD >/=9.0 cm), the SWL success rate was 91%, 79%, 58%, and 41%, respectively (odds ratio 7.1, 95% confidence interval 1.6-32 for <900 HU and SSD <9.0 cm group vs other 3 risk groups; P = .01).
• Conclusions: The results of our study have shown that a stone attenuation of <900 HU, SSD of <9 cm, and stone composition predict for SWL success, independent of stone size, location, and body mass index. These factors will be considered important in the prospective design of a SWL treatment nomogram at our center.
  
  
• Editorial Comment
  This study helps establish parameters to guide the counseling of patients undergoing SWL. It is important to consider that the predictive stone attenuation and skin-to-stone distance will be dependent on the peak pressures at F2 and focal area of the lithotripter respectively. As such, this may require the establishment of criteria for each individual lithotripter.
  Outcomes were defined by KUB at two weeks - one would expect that the sensitivity of KUB (at best 70%) would be higher for stones with higher stone attenuation and for thinner patients (smaller skin-to-stone distances). As such, the primary conclusions of the study may be skewed by the outcome measure selected - CT scan imaging would have provided a more critical evaluation for this study.
  The authors note that collimation widths > 3 mm can impact stone attenuation measurements, smaller stones will have lower stone attenuation levels due to volume averaging with surrounding soft tissue, and indeed in this study stone size correlated with stone attenuation. However, concerns regarding radiation exposure warrant the continued use of 5-mm collimation widths.

Dr. Manoj Monga
Professor, Department of Urology
University of Minnesota
Edina, Minnesota, USA
E-mail: endourol@yahoo.com