Urolithiasis is a common health problem among children with an estimated incidence of approximately 5 to 36 per 100,000 children. The risk of recurrence of urolithiasis in children is high (up to 50% within 3 years of the first episode) and many have a predisposing metabolic risk factor, such as hypercalciuria (1). The recurrence of kidney stones in adults is very common, with up to 50% of adult incident stone formers experiencing a second occurrence within 5-10 years (2). However, the reported rates of recurrence among pediatric series are more variable, ranging from 16 to 67% at a median interval of 1-5 years between episodes (3-5).
Shock wave lithotripsy (SWL) is one of the first-line treatments in children for kidney stones smaller than 2 cm (6). There is wide disparity in the stone free rates documented in the literature for SWL in children ranging from 33 to 95%. The results are incomparable not just for lack of information but also because of variability of the confounding factors like patient-related factors including age, gender, patient tolerability, symptom severity, anatomy, and skin to stone distance and stone related factors including stone burden, location, composition, beside the type of lithotripter used, and the number of shockwaves in each session. All these factors impact SWL outcome in children. One such stone factor is stone density, diagnosed using non-contrast computer tomography (NCCT) scans (7, 8).
There are some factors that affect the success of SWL and stone composition is one of them. Hounsfield unit is known to be an independent predictor of ESWL success in adults (9). El-Assmy et al. showed that ≤ 600 HU is an independent predictor for ESWL success in children (10), McAdams et al. divided the patients into two groups (\< 1000 and ≥ 1000 HU) and concluded that \< 1000 HU is an important predictor of ESWL success (11).
Numerous studies have suggested an inverse association between stone density and SWL success with cut-offs are ranging from as low as 482 HU up to 1000 HU. However, these studies have shown variation in the assessment, HU markers, and utility of HU measurements. Researchers have also looked into adjunct factors to help maximize predictability of success (8) .
The majority specified the method of measuring mean stone density (MSD), creating single elliptical region of interest (ROI), a squared 10 × 10-pixel map, or a freehand ROI drawn along the stone edge to take into account abnormal shapes. These studies used 3 ROI within the image, either over-lapping or non-overlapping, and taking the mean of the three results or the peak HU (12-14), some measured the stones on a single axial plane that displayed the stone at its maximal diameter (15). A single study defined MSD as the average of the minimum and maximum HU readings (14).
Stone heterogeneity index (SHI) was identified to be a better indicator of success in patients with larger stones. SHI designated as the standard deviation of HU was found to be a truer reflection of SWL success in patients with larger stones, with higher heterogeneity likely to result in clearance. This new ratio was seen to have a higher negative predictive value than stone attenuation alone in predicting stone composition
