Differentiation of clear from non-clear cell renal cell carcinoma using CT washout formula

Introduction: To further elucidate potential patterns of contrast enhancement for renal neoplasm subtypes, we investigated utility of contrast washout formula to differentiate renal tumor histology after multiphase computerized tomography (CT). Materials and methods: Single center retrospective cohort study of 163 patients with multiphase CT for renal masses obtained October 2007 to July 2012. Pathology confirmed clear cell (CC-RCC; n = 92), papillary (Pa-RCC; n = 43), chromophobe (Ch-RCC; n = 6), oncocytoma (OC; n = 11), or angiomyolipoma (AML; n = 11) histology. Two radiologists in consensus and blinded to histology recorded tumor size, morphology, and attenuation measurements in Hounsfield Units (HU). Data were analyzed between subgroups based on histology. Enhancement washout of the tumor was calculated by the formula (Mass nephrographic HU-Mass delayed HU)/(Mass nephrographic HU-Mass non-contrast HU) and used to calculate sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). Results: Tumor size was largest among CC-RCC (p < 0.001). Homogeneous composition was more common among Pa-RCC and Ch-RCC (p < 0.001). Median washout for Ch-RCC (0.27) was significantly different from that of OC (0.54, p = 0.05). Overall 25 (15.3%) of tumors had washout < 0. Tumors with washout value < 0 were Pa-RCC 24/43 (56%), and Ch-RCC 1/6 (14%). Washout value < 0 had a specificity of 99.2% for Pa-RCC and 100% for non-CCRCC. Washout value ≥ 0 had a sensitivity and NPV of 100% for CC-RCC, OC, and AML. Washout value ≥ 0 had a specificity of 35.2% and a PPV of 66.7% for CC-RCC. Conclusions: Enhancement washout value < 0 is highly specific for Pa-RCC and non-CC-RCC. Washout value ≥ 0 is highly sensitive for CC-RCC, OC, and AML while there was a significant difference in median washout between OC and Ch-RCC. Further prospective investigation is requisite to confirm these findings.