BERKELEY, CA (UroToday.com) - The IVP is gone and with it all of the ancillary information one could use to try to determine whether a given stone, especially one in the lower pole, would be amenable to shock wave lithotripsy (SWL). The list of additional measurements included: lower pole infundibulopelvic angle, the degree of hydronephrosis, calyceal pelvic height, lower pole infundibular width, and lower pole infundibular length. Unfortunately, in this age of CT, these measurements are no longer readily available. However, there are certain data that the CT can provide that may be very helpful in determining whether a stone should or should not undergo SWL. The first of these observations had to do with stone density as calculated in Hounsfield units. The first author as well as others has noted that if the stone is less than 500 units, it will break up well, whereas if it is over 1000 units, it will not respond well to SWL, with the 500-1000 unit area being a bit of a grey zone.

Now, a second worthwhile observation has been made. Specifically, if the average of the skin to stone distance, is 10 cm. or greater, the chance of successful SWL becomes marginal. In this study, 64 patients with lower pole stones were analyzed, using three measurements from the stone to the skin on the CT scan. In the stone free group, the skin to stone distance was 8.1 cm. whereas in the stone residual group it was 11.5 cm. As with other studies, HU density played a role but the skin to stone distance was a more significant predictor. More knowledge and more data translate into better SWL results. To be sure, our patients are seeing us in the hopes that we will always choose for them the least invasive yet highly successful method for treating their renal stones ? balancing these two goals is one of the most important judgments that we perform for our patients. Hounsfield units and skin-to-stone distance are two factors that can be very helpful in this regard and they are both there on the initial CT, "free" for the measuring.

Urology 66: 941-944, November, 2005