Renal Lesion Assessment at Portal Venous Phase Photon Counting Computed Tomography: Iodine Concentration and Attenuation Analysis.

Objective: To determine the optimal iodine concentration (IC) to differentiate enhancing from nonenhancing renal lesions at venous phase photon-counting CT (PCCT).

Materials and methods: Sixty-seven patients [41 males, mean (SD) age: 68 (13) y] who underwent contrast-enhanced venous phase abdominal PCCT and contrast-enhanced MRI were retrospectively identified. MRI characterization of renal lesions as simple cysts, hemorrhagic/proteinaceous cysts or masses was the reference standard. Two radiologists, blinded to MRI characterization, independently drew region-of-interests (ROI) within each renal lesion and the aorta to determine IC, iodine concentration normalized to aortic enhancement (NIC), postcontrast and virtual noncontrast attenuation. These variables were compared using nonparametric tests, and intraclass correlation coefficient was assessed. ROC analysis for IC, NIC, and attenuation difference was performed using mean measurements.

Results: Median (IQR) IC was 0.015 (0.00 to 0.20) mg/mL for cysts (n = 41) and 3.00 (1.59 to 4.10) mg/mL for masses (n = 26; P < 0.001). NIC was 2 (-1 to 4)% for cysts and 58 (33 to 81)% for masses (P < 0.001). For differentiating cysts from masses, 0.6 mg/mL IC had 100% sensitivity and 95% specificity [AUC = 1.00 (0.99, 1.00)] and 14.1%. NIC had 100% sensitivity and 98% specificity [AUC = 1.00 (0.99, 1.00)]. For differentiating cysts from enhancing masses, a 20 HU attenuation change from VNC to postcontrast images demonstrated a sensitivity of 96% and specificity of 95% [AUC = 1.00 (0.99, 1.00)]. IC was 0.13 (-0.11 to 0.20) mg/mL for hemorrhagic cysts (n = 16) and 1.63 (1.31 to 1.89) mg/mL for papillary neoplasms (n = 8; P < 0.001). NIC was 2 (-1 to 5)% and 32 (22 to 40)%, respectively (P < 0.001). For differentiating hemorrhagic cysts from papillary neoplasms, 0.53 mg/mL IC had 100% sensitivity and 88% specificity [AUC = 0.98 (0.93, 1.00)] and 14.1% NIC had 100% sensitivity and 94% specificity [AUC = 0.98 (0.95, 1.00)]. A 20 HU change had 88% sensitivity and specificity. For differentiating hemorrhagic cysts from papillary neoplasms, a 20 HU attenuation change had 88% sensitivity and 88% specificity [AUC = 0.98 (0.93, 1.00)].

Conclusions: PCCT iodine concentration showed outstanding diagnostic performance to differentiate renal cysts from neoplasms. A 20 HU change in attenuation from VNC to postcontrast imaging on PCCT remains a reliable threshold for enhancing lesion, similar to conventional CT. Iodine concentration demonstrated superior sensitivity and specificity compared with 20 HU change when identifying renal cysts versus masses.