IMAGING

Can High-Attenuation Renal Cysts be Differentiated from Renal Cell Carcinoma at Unenhanced CT?
Jonisch AI, Rubinowitz AN, Mutalik PG, Israel GM
Department of Diagnostic Radiology, Yale University School of Medicine, New Haven, CT, USA
Radiology. 2007; 243: 445-50

• Purpose: To retrospectively determine if renal cell carcinoma can be differentiated from high-attenuation renal cysts at unenhanced computed tomography (CT) based on Hounsfield unit measurements and heterogeneity.
• Materials and Methods: The Human Investigation Committee at our institution approved this study with waiver of informed consent. This study was compliant with the HIPAA. Fifty-four pathologically proved renal cell carcinomas in 54 patients (36 men and 18 women; average age, 53 years; range, 23-90 years) and 56 high-attenuation renal cysts in 51 patients (30 men and 21 women; average age, 63 years; range, 28-86 years) were retrospectively evaluated at unenhanced CT. Two independent readers reviewed randomized unenhanced CT images and obtained Hounsfield unit readings of each mass. A subjective determination of lesion heterogeneity was also performed by using a four-point scale (1: homogeneous, 2: mildly heterogeneous, 3: moderately heterogeneous, 4: markedly heterogeneous). Statistical analysis was performed by using Bland-Altman regression tree, classification and regression tree, and Shapiro-Wilk normality test.
• Results: The average attenuation of cysts for reader 1 was 53.4 HU (range, 23-113 HU) and for reader 2 was 53.8 HU (range, 21-108 HU). The average attenuation of neoplasms for reader 1 was 34.7 HU (range, 21-60 HU) and for reader 2 was 38.4 HU (range, 22-60 HU). For cyst heterogeneity, a score of 1 was given in 55 of 56 (98%) cysts for reader 1 and in 53 of 56 (95%) cysts for reader 2. For neoplasm heterogeneity, a score of 1 was given in 35 of 54 (65%) neoplasms for reader 1 and in 36 of 54 (67%) for reader 2. Given the distribution of cyst and tumor attenuation values and lesion heterogeneity, a homogeneous mass measuring 70 HU or greater at unenhanced CT has a greater than 99.9% chance of representing a high-attenuation renal cyst.
• Conclusion: The findings from this study may help differentiate high-attenuation renal cysts from renal cell carcinomas at unenhanced CT and may suggest the next appropriate imaging study for definitive characterization.
  
  
• Editorial Comment
  A hyperdense cyst refers to a cyst that demonstrates high attenuation on nonenhanced CT scans. Hemorrhage or proteinaceous debris is the most common cause, but renal cell carcinoma may eventually demonstrate similar findings. A hyperdense renal cyst can be considered benign if it is sharply marginated or homogeneous or demonstrates a hematocrit effect on nonenhanced and contrast-enhanced scan and demonstrates no significant enhancement on post-contrast scans. Because internal structures within a hyperdense renal cyst cannot be well evaluated by nonenhanced CT, US or MR imaging can be used for the differentiation. When sonography is performed, the mass is usually cystic but occasionally do not present all the sonographic criteria for a simple cyst. Actually internal septations and absence of posterior wall trough-transmission are frequently found.
  The authors present an interesting observation, which should be useful for adequate characterization of hyperdense renal lesion found on nonenhanced CT scans particularly in those patients submitted to a non-contrast CT scans for the detection of urolithiasis. They found that the attenuation of a renal mass and its degree of heterogeneity are useful findings in distinguishing a high-attenuation renal cyst from renal cell carcinoma on unenhanced CT images. If the density of the mass is greater than 70 HU and the mass is homogeneous, there is a chance of almost 100% (99.9%) that the mass is benign hyperdense renal cyst. They concluded that in this situation there is no need for contrast enhanced CT scan and high-resolution US studies or MR imaging can be used as complimentary test.

Dr. Adilson Prando
Chief, Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil
E-mail: aprando@mpc.com.br