Renal Cell Carcinoma: Attenuation Values on Unenhanced CT

OBJECTIVE. The purpose of this study is to analyze the attenuation values of pathologically proven renal cell carcinomas (RCCs) on unenhanced CT and to determine the range of values wherein malignancy should be considered. MATERIALS AND METHODS. A retrospective review was performed of 189 consecutive patients with 193 pathologically proven RCCs 1 cm or larger on unenhanced CT. For each RCC, attenuation values were assessed throughout the tumor by continuous sampling with a 25-100-mm(2) region of interest (ROI), avoiding foci of calcification and peritumoral volume averaging. The lowest and highest ROI attenuation values per lesion were recorded. Each tumor was categorized as either homogeneous or heterogeneous on the basis of visual inspection with soft-tissue window settings. RESULTS. The 193 malignant tumors ranged in size from 1.1 to 20.1 cm (mean [+/- SD], 5.1 +/- 3.4 cm). Eighteen RCCs (9.3%) were homogeneous in appearance on unenhanced CT. The minimum and maximum ROI attenuation values obtained by sampling throughout each tumor were 27.5 +/- 10.4 HU (range, 4-67 HU) and 39.7 +/- 10.6 HU (range, 21-80 HU), respectively. Regional areas of minimum attenuation less than 20 HU and maximum attenuation greater than 70 HU were seen in 24.9% (48/193) and 2.1% (4/193) of RCCs, respectively. However, all 193 RCCs (100%) were predominantly composed of noncalcific regions within 20-70 HU; 72.5% (140/193) fell entirely within this 20-70 HU "danger zone," including all 18 homogeneous lesions. CONCLUSION. All proven RCCs in this series contained substantial noncalcified regions that measured 20-70 HU in ROI attenuation on unenhanced CT. Indeterminate renal lesions on unenhanced CT measuring within this 20-70-HU danger zone warrant further workup, whereas lesions that fall entirely outside this range may be considered benign.