Cystine stones are often hard . are Hounsfield Units correspondingly high?
BAUS ePoster online library. Thomas K. Jun 24, 2019; 259462; P1-3
Kay Thomas
Kay Thomas
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Abstract
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Cystine stones have traditionally been considered hard and difficult to treat. Hounsfield units (HU) are often used in other stone types to judge 'hardness' and treatments based on that finding.

A prospective database of cystinuria patients from a single centre was analysed. Demographic, genetic and follow up data was collected. Available CT imaging prior to stone intervention was assessed by two independent radiologists and HU measured by drawing a circular region of interest at the centre of the stone at its largest diameter. Inter-rater reliability was assessed using Cronbach's alpha reliability coefficent.

169 adult cystinuric patients(55% male) were reviewed; 112 had CT scans available; median follow up 31 months(IQR 10–62). Median age 41 years(range 18–79). Inter-rater reliability of measurements was good (Cronbach's alpha =0.88). Mean HU 618(range 118-1453). 93% of values had HU<800 with only 2 patients having HU>1200. Genetic analysis available for 102/112(91%) patients with calculi on CT imaging demonstrated 70(69%) SLC3A1 mutation and 32(31%) SLC7A9 mutation(20 homozygous, 12 heterozygous). There was no difference in mean HU between SLC3A1 and SLC7A9(p=0.77) or between SLC7A9 homo and heterozygous mutations(p=0.98). There was no difference in HU from one independent stone episode to another(p=0.97), and no association between mean HU and total laser energy required to break up stone at ureteroscopy.

Due to the variability of HU and the relatively low values, they are not useful in judging the relative hardness of cystine stones and should not be used to base treatment decisions.
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