Solid Renal Cell Carcinoma Measuring Water Attenuation (-10 to 20 HU) on Unenhanced CT

OBJECTIVE. The objectives of this study were to evaluate the incidence of solid renal cell carcinoma (RCC) with attenuation similar to that of water (-10 to 20 HU) on unenhanced CT and to examine imaging features that can allow RCC to be differentiated from simple cysts. MATERIALS AND METHODS. We performed an enriched quantitative and qualitative retrospective analysis of consecutive patients who underwent unenhanced CT before resection of solid (noncystic and nonnecrotic) renal masses measuring < 5 cm from 2008 to 2013. In all, 93 patients with 96 tumors (55 clear cell, 27 papillary, and 14 chromophobe) were evaluated with unenhanced CT. Attenuation was measured at three standardized levels and compared using the Kruskal-Wallis test. Two radiologists independently assessed calcification, margin (smooth or irregular), and heterogeneity (homogeneous or heterogeneous). Results were compared using the chi-square test with Bonferroni correction. RESULTS. Mean +/- SD attenuation values were 31.8 +/- 9.6 HU (range, 11-63 HU) overall, 29.9 +/- 8.8 HU (range, 11.0-49.0 HU) for clear cell tumors, 34.6 +/- 10.3 HU (range, 20.3-63.0 HU) for papillary tumors, and 35.5 +/- 9.2 HU (range, 20.7-47 HU) for chromophobe tumors (p = 0.06). Eight clear cell RCCs had attenuation similar to that of water (15.7 +/- 2.4 HU; range, 11-18.7 HU). There was no significant difference in calcification or margin among different types of tumors (p = 0.91 and p = 0.55, respectively). Chromophobe tumors were more likely to be homogeneous (p < 0.001). Interobserver agreement was moderate to very good (kappa = 0.91 for calcification, kappa = 0.55 for margin, and kappa= 0.44 for heterogeneity). All eight clear cell RCCs with attenuation similar to that of water were considered heterogeneous by both readers. Irregular margins were identified in three of these eight tumors by reader 1 and four of eight tumors by reader 2. CONCLUSION. A minority of solid RCCs have attenuation similar to that of water on unenhanced CT. In this study, all such tumors were of the clear cell subtype and qualitatively heterogeneous, suggesting they can be distinguished from simple cysts on unenhanced CT.