The characterization of small hypoattenuating renal masses on contrast-enhanced CT

Abstract

Purpose: To determine if small hypoattenuating renal masses can be characterized as simple cysts or renal cell carcinomas on contrast-enhanced computed tomography (CT).

Materials and methods: We retrospectively identified 20 small (<or=1.5 cm) hypoattenuating renal masses seen on contrast enhanced CT, consisting of 14 simple cysts and six renal cell carcinomas. Three independent readers recorded subjective visual impression (five-point scale from 1=definitely fluid to 5=definitely solid), CT attenuation, border (well circumscribed or ill defined), and shape (ovoid or irregular) for each lesion.

Results: The overall area under the receiver operator characteristic curves for subjective visual impression, CT attenuation, border, and shape were 0.97, 0.82, 0.59, and 0.55, respectively. Using dichotomized ratings (1-2=cyst and 3-5=carcinoma), subjective impression had a sensitivity and specificity of 100% and 79-100%, respectively, for the diagnosis of renal cell carcinoma. Using a threshold of 50 Hounsfield Units (HU) or more, CT attenuation had a sensitivity and specificity of 100% and 43-64%, respectively.

Conclusion: Small hypoattenuating renal masses can be characterized with reasonable accuracy by subjective impression and CT attenuation; lesions that appear solid on visual inspection or have an attenuation value of 50 HU or more are likely to be renal cell carcinoma.

Fig. 1

A 50-year-old man with simple renal cyst. A, Portal venous phase contrast-enhanced CT scan with 2.5 mm collimation demonstrates a 1.2 cm hypoattenuating lesion. B, Axial T2 weighted fast spin echo MR image shows the lesion is of uniformly high T2 signal. C, Axial T1-weighted spoiled gradient echo MR image after intravenous gadolinium shows the lesion is non-enhancing.

Fig. 2

A 58-year-old woman with renal cell carcinoma. A, Portal venous phase contrast-enhanced CT scan with 7 mm collimation demonstrates a 1.1 cm hypoattenuating lesion in the left kidney. Subsequent resection demonstrated a renal cell carcinoma.

Fig. 3

Plot of CT attenuation measurements (HU) for cyst and renal cell carcinomas by reader. The mean CT attenuation for the simple cysts was significantly less than that of the renal cell carcinomas (P<.01, P<.05, and P<.01 for Readers 1, 2, and 3 respectively). The horizontal line drawn at 50 HU denotes the level below which only cysts are found.

Fig. 4

Overall receiver operator characteristic curves for CT criteria and prediction of malignancy. Az indicates area under the receiver operator characteristic curve.