Attenuation values may help differentiate renal neoplasm from simple cyst on contrast-enhanced CT

1. In a retrospective review of 125 patients with primary renal cell neoplasm that underwent computed tomography (CT), simple renal cysts were found to have a mean attenuation value up to 30 Hounsfield Units (HU) on contrast-enhanced CT, while the lowest attenuation value of homogenous renal cell neoplasms was 42 HU.

2. No overlap was found in the attenuation characteristics of simple renal cysts and renal cell neoplasms.

Evidence Rating Level: 3 (Average)

Study Rundown: Cystic renal lesions are common incidental findings on CT and most commonly represent benign simple cysts. The diagnosis can be made on non-enhanced CT scans on cysts that display homogenous attenuation, antennation value between -10 and 20 HU, thin or imperceptible wall, and lack of other concerning feature (i.e. septa, nodules, calcifications). However, homogenous renal cysts with mean attenuation value between 20 and 30 HU on contrast-enhanced CT represent a challenge for radiologists to entirely exclude neoplasm, as concurrent non-contrast examinations are often not performed to evaluate for enhancement characteristics. The purpose of this study was to determine whether simple renal cysts and renal neoplasms can be differentiated on contrast-enhanced CT using attenuation and heterogeneity.

The study retrospectively reviewed the contrast-enhanced CT images of 125 patients with primary renal neoplasms. At the conclusion of the study, simple renal cysts were shown to have a mean attenuation of up to 30 HU.  Renal neoplasms were demonstrated to have a mean attenuation of at least 42 HU or higher, with no overlap between the cysts and neoplasm groups. The result of this trial suggests that homogenous cystic lesions on contrast-enhanced CT may not require further work-up if they have a mean attenuation of up to 30 HU. The study is limited by the small sample size. Other limitations include the lack of pathologic confirmation of renal cysts, and the small number of papillary RCC in the study (hypovascular neoplasms that may have attenuation overlap with simple cysts).

Click to read the study in AJR

Relevant Reading: The current radiological approach to renal cysts

In-Depth [retrospective cohort]: The article retrospectively reviewed a pathology database from 2008 to 2014 for all cases of primary renal cell neoplasm. This was cross-referenced with data in PACS to obtain patients who underwent either CT of the abdomen with and without contrast (n=94) or only with contrast (n=31). Masses were subjectively determined as homogenous and heterogeneous by two experienced abdominal radiologists and differences were settled by consensus using the Cohen kappa test for inter-reader agreement. Attenuation was measured using as much of the region of interest (ROI) as possible for homogenous masses and the highest and lowest attenuation values were obtained in heterogeneous masses.

Overall, 116 heterogeneous renal cell neoplasms (99 clear cell, four papillary, four oncocytic, seven chromophobe, and two unclassified renal cell carcinomas), 13 homogenous renal cell neoplasms (10 papillary, two oncocytic, and one chromophobe), and 24 simple cysts were evaluated. All renal neoplasms were identically categorized as homogenous or heterogeneous (p=1.0, p<0.001). The lowest mean attenuation for homogenous renal cell neoplasms was 42 HU on contrast-enhanced CT and 32 HU on non-contrast CT. All heterogeneous renal cell neoplasms had a mean attenuation difference of >20 HU, with the exception of one with a difference of 19 HU but a high value of 105 HU. All simple renal cysts had a mean attenuation value of less than 30 HU.

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