Lithiasis size estimation variability depending on image technical methodology

Abstract

The lithiasic size is a determining factor in selecting the most suitable treatment, surgical or medical. However, the method for obtaining a reliable lithiasic size is not standardized. Our objetives are to determine the differences between the estimated lithiasic sizes shown by plain radiography test and by computerized axial tomography (CT) scan (using different techniques) in relation to the actual size, and to establish which is the ideal type of imaging for this purpose. We present an in vitro model with lithiasis obtained in cooperation with four centers. Inclusion criteria: lithiasis >0.5 cm, intact, and visible via simple radiography. A sample of 245 lithiases was obtained, with 87 rejected as they did not fulfill the inclusion criteria. Initially the three main actual diameters of each lithiasis were measured with a calibrator, then a plain X-ray and a CT scan were taken of the samples to determine the surface size in cm² for simple radiography; surface size and volume in cm³ for CT scan, in bone window and soft tissue (Toshiba Aquillion 64, sections of 0.5 mm, 120 Kv, 250 mA). The tomographic area was calculated by employing the formula recommended by the European Association of Urology and scanner software. The actual, radiographic and tomographic measurements were taken by three different researchers who were unaware of the results obtained by the each other. The statistics program IBM SPSS Statistics^® 19 was used. Differences were analyzed using the Wilcoxon sign test. The bone window CT scan slightly overestimated the actual lithiasic size (0.12 vs. 0.17 cm³), while in soft tissue window the actual volume was practically doubled (0.12 vs. 0.21 cm³) (p < 0.05). We did not find statistically significant differences in the comparison between actual surface size (0.39 cm²) and bone window CT scan size when using the EAU formula or scanner software (0.36/0.37 cm²). Resulting measurements in soft tissue window tended to significantly overestimate the surface size, although only slightly (0.42/0.44 cm²), whilst the plain radiography underestimated it slightly but significantly (0.37 cm²). CT scan, using the bone window, is the technical methodology with which the greatest in vitro accuracy in which actual lithiasis measurements can be estimated, although the craniocaudal diameter measurement will be overestimated. Using soft tissue window gives an overestimated size.