Rationale and Objective
We aimed to determine the frequency and clinical significance of homogeneous renal masses measuring 21–39 Hounsfield units on contrast-enhanced computed tomography (CT).
Methods
Subjects 40–69 years old undergoing portal-venous-phase contrast-enhanced abdominal CT from January 1, 2006 to December 31, 2010 with slice thickness ≤5 mm and no prior CT or magnetic resonance imaging were identified ( n = 1387) for this institutional review board-approved retrospective cohort study. Images were manually reviewed by three radiologists in consensus to identify all circumscribed homogeneous renal masses (maximum of three per subject) ≥10 mm with a measured attenuation of 21–39 Hounsfield units. Exclusion criteria were known renal cancer or imaging performed for a renal indication. The primary outcome was retrospective characterization as a clinically significant mass, defined as a solid mass, a Bosniak IIF/III/IV mass, or extirpative therapy or metastatic renal cancer within 5 years’ follow-up.
Results
Eligible masses ( n = 74) were found in 5% (63/1387) of subjects. Of those with a reference standard ( n = 42), none (0% [95% CI: 0.0%–8.4%]) were determined to be clinically significant.
Conclusion
Incidental renal masses on contrast-enhanced CT that are homogeneous and display an attenuation of 21–39 Hounsfield units are uncommon in patients 40–69 years of age, unlikely to be clinically significant, and may not need further imaging evaluation. If these results can be replicated in an independent and larger population, the practical definition of a benign cyst on imaging may be able to be expanded.
Introduction
Incidental renal masses are common, with some estimates suggesting that at least half of all patients over the age of 50 have at least one renal mass that may be detected with cross-sectional imaging . When an incidental renal mass is identified at contrast-enhanced computed tomography (CT), a diagnosis of a benign simple cyst can be made if the mass is homogeneous (ie, no wall thickening, septa, calcifications, or visible heterogeneity) and has a measured attenuation between −10 and 20 Hounsfield units (HU) . However, if the mass is heterogeneous, or if the mass is homogeneous and measures greater than 20 HU, it is considered incompletely characterized and in general prompts a recommendation for further imaging .
However, these attenuation criteria may be overly aggressive. For example, it is not possible for a homogeneous renal mass that measures 21–39 HU on contrast-enhanced CT to both fulfill the definition of a nonsimple cyst on unenhanced CT (ie, >20 HU) and meet the threshold for definitive enhancement on contrast-enhanced CT (ie, an increase by 20 HU or more) . In addition, attenuation values of renal masses at contrast-enhanced CT are commonly spuriously high due to pseudoenhancement , an artificial increase in attenuation due to the CT scanner’s overcorrection for beam-hardening . Therefore, an incidentally detected homogeneous renal mass that measures 21–39 HU on contrast-enhanced CT is more likely to be a benign non–enhancing hyperattenuating cyst (ie, Bosniak II cyst), or a simple cyst (ie, Bosniak I) with pseudoenhancement, than it is to be malignant. However, the threshold for definitive enhancement is not always reached for some renal cancers; for example, papillary cancers are known to be hyperattenuating and some may fail to show measurable enhancement on CT .
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Methods
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Subjects
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Index CT Examinations
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Reference Standard
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Data Analysis
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Results
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Table 1
Study Population Details
Characteristic Data Subjects 63 Masses 74 Female 44% (28/63) Male 56% (35/63) Mean age (years, range) 58 (40–69) Indication for CT Infection 5% (3/63) Nonrenal malignancy 56% (35/63) Pain 22% (14/63) Trauma 6% (4/63) Other 11% (7/63) Lesion location Exophytic 23% (17/74) Mesophytic 24% (18/74) Endophytic 53% (39/74) Mean lesion size (mm, range) 20 (10–56) Mean lesion attenuation (HU, range) 28 (21–38) Attenuation 21–29 HU 69% (51/74) Attenuation 30–39 HU 31% (23/74) Lesion location Right kidney 41% (30/74) Left kidney 59% (44/74) Reference standard None (ie, not included in primary outcome) 43% (32/74) ≥5 years of clinical follow-up 15% (11/74) Renal protocol CT 16% (12/74) Unenhanced CT 1% (1/74) MRI with and without contrast 8% (6/74) Ultrasound 11% (8/74) More than one of the above 5% (4/74)
CT, computed tomography; HU, Hounsfield unit; MRI, magnetic resonance imaging.
Denominators are 63 for subject-level data and 74 for lesion-level data.
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Discussion
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Compliance with Ethical Standards
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