Diagnostic Accuracy of Unenhanced CT Analysis to Differentiate Low-Grade From High-Grade Chromophobe Renal Cell Carcinoma

OBJECTIVE. The objective of our study was to evaluate tumor attenuation and texture on unenhanced CT for potential differentiation of low-grade from high-grade chromophobe renal cell carcinoma (RCC). MATERIALS AND METHODS. A retrospective study of 37 consecutive patients with chromophobe RCC (high-grade, n = 13; low-grade, n = 24) who underwent preoperative unenhanced CT between 2011 and 2016 was performed. Two radiologists (readers 1 and 2) blinded to the histologic grade of the tumor and outcome of the patients subjectively evaluated tumor homogeneity (3-point scale: completely homogeneous, mildly heterogeneous, or mostly heterogeneous). A third radiologist, also blinded to tumor grade and patient outcome, measured attenuation and contoured tumors for quantitative texture analysis. Comparisons were performed between high-grade and low-grade tumors using the chi-square test for subjective variables and sex, independent t tests for patient age and tumor attenuation, and Mann-Whitney U tests for texture analysis. Logistic regression models and ROC curves were computed. RESULTS. There were no differences in age or sex between the groups (p = 0.652 and 0.076). High-grade tumors were larger (mean +/- SD, 62.6 +/- 34.9 mm [range, 17.0-141.0 mm] vs 39.0 +/- 17.9 mm [16.0-72.3 mm]; p = 0.009) and had higher attenuation (mean +/- SD, 45.5 +/- 8.2 HU [range, 29.0-55.0 HU] vs 35.3 +/- 8.5 HU [14.0-51.0 HU]; p = 0.001) than low- grade tumors. CT size and attenuation achieved good accuracy to diagnose high-grade chromophobe RCC: The AUC +/- standard error was 0.85 +/- 0.08 (p < 0.0001) with a sensitivity of 69.0% and a specificity of 100%. Subjectively, high-grade tumors were more heterogeneous (mildly or markedly heterogeneous: 69.2% [9/13] for reader 1 and 76.9% [10/13] for reader 2; reader 1, p = 0.024; reader 2, p = 0.001) with moderate agreement (kappa = 0.57). Combined texture features diagnosed high-grade tumors with a maximal AUC of 0.84 +/- 0.06 (p < 0.0001). CONCLUSION. Tumor attenuation and heterogeneity assessed on unenhanced CT are associated with high-grade chromophobe RCC and correlate well with the histopathologic chromophobe tumor grading system.