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Dong Won Kim, MD
, Sang Hyeon Kim, MD
, Sang Hyeon Kim, MD
Abstract
The steady increase in imaging studies has led to the incidental discovery of many renal masses. Although most incidental solid renal masses are asymptomatic and small in size, they are mostly malignant renal cancers necessitating accurate diagnosis. Small-sized masses are more likely to be benign tumors; therefore, access is needed according to size. Because most malignant tumors are renal cell carcinoma, and most benign tumors are angiomyolipoma and oncocytoma. Knowledge of common imaging findings of these tumors is helpful for diagnosis and management. However, imaging techniques are often insufficient to characterize solid renal masses which are discovered incidentally in radiological examinations, especially small-sized masses. Herein, we describe the imaging features characteristic of incidental solid renal masses and discuss their management in cases when an accurate diagnosis could or could not be made.
Figures and Tables
Fig. 1
A 55-year-old man with classic angiomyolipoma in the right kidney.
A. Gray-scale ultrasound image shows a 9-mm, oval, hyperechoic mass (arrow) in the right kidney.
B, C. Axial unenhanced (B) and contrast-enhanced CT (C) images demonstrate a mass with fat attenuation of −54 Hounsfield unit (arrows).
Fig. 2
An 85-year-old woman with small clear cell renal cell carcinoma in the left kidney.
A, B. Gray-scale ultrasound images show an isoechoic small mass with peritumoral hypoechoic rim (A, arrow) and tiny intramural cystic portion (B, arrow).
C. Axial unenhanced CT scan reveals a focal area with slight hypoattenuation (arrow) in the left kidney.
D. Axial contrast-enhanced CT image in the corticomedullary phase shows a 1.8-cm mass (arrow) with heterogeneous early enhancement in the left kidney.
Fig. 3
Pseudoenhancement of a left renal cyst in a 40-year-old man.
A. Axial unenhanced 5-mm-thick CT image shows an ill-defined hypoattenuating mass (arrow) in the left kidney with an attenuation of 17 HU.
B. Axial, contrast-enhanced, 5-mm-thick, nephrographic phase CT image shows an oval, 8-mm mass (arrow) with an attenuation of 38 HU in the left kidney.
C. Axial, contrast-enhanced, 3-mm-thick nephrographic phase CT image shows the same mass (arrow) with an attenuation of 18 HU in the left kidney.
HU = Hounsfield unit
Fig. 4
A 53-year-old man with papillary renal cell carcinoma in the right kidney.
A. Axial unenhanced CT scan shows an isoattenuating mass (arrow) with an attenuation of 19 HU in the right kidney.
B, C. Axial contrast-enhanced CT images show extremely weak early enhancement and slightly progressive enhancement of the mass on corticomedullary (34 HU; B) and nephrographic (38 HU; C) phases (arrows), respectively.
HU = Hounsfield unit
Fig. 5
A 47-year-old woman with oncocytoma in the left kidney.
A. Axial unenhanced CT scan shows a slight heterogeneous mass (arrow) in the left kidney.
B. Axial contrast-enhanced CT image in the corticomedullary phase shows irregular central stellate scar in the mass (arrow).
C. The central stellate scar shows slightly progressive enhancement and slight isoattenuation on peripheral portion of the mass (arrow) on axial excretory phase CT image.
Fig. 6
A 63-year-old woman with oncocytoma in the right kidney.
A, B. Axial, dynamic contrast-enhanced CT images in the corticomedullary (A) and excretory phases (B). Most peripheral areas of the mass (arrows) show excellent enhancement and subsequent wash-out in the corticomedullary phase, whereas focal area of the mass (arrowheads) shows increased enhancement in the excretory phase. These two different areas of enhancement reverse between the two phases (segmental enhancement inversion).
Fig. 7
A 51-year-old woman with fat poor angiomyolipoma in the right kidney.
A. Axial unenhanced CT image shows a homogenously hyperattenuating mass with an attenuation of 49 Hounsfield unit (arrow) compared to the renal parenchyma.
B. Axial contrast-enhanced corticomedullary phase CT image shows moderate enhancement, less than that of the renal cortex. The mass has angular interface (arrow).
Fig. 8
A 58-year-old man with a hyperdense cyst in the right kidney.
A, B. Axial, contrast-enhanced, corticomedullary phase CT image shows a 7-mm hyperdense mass with an attenuation of 60 HU (arrow, A) in the right kidney with ambiguity in the hyperattenuation or enhancement of the lesion. The HU of this mass needs to be compared to the HU on unenhanced CT image (arrow, B). Axial unenhanced CT scan reveals a 61-HU mass with no enhancement as compared to contrast-enhanced CT image.
C. The small renal mass is identified as an anechoic cyst on gray-scale ultrasound image (arrow).
HU = Hounsfield unit
Fig. 9
A 68-year-old woman with chromophobe RCC in the right kidney.
A. Axial unenhanced CT image shows a homogenously isoattenuating mass (arrow) in the right kidney.
B. Axial contrast-enhanced CT image in the corticomedullary phase shows homogeneous moderate enhancement (arrow) in the mass.
C. Axial T2-weighted MR image shows low signal intensity in the mass (arrow) compared to the renal parenchyma.
D. Opposed-phase chemical shift MR image shows no signal drop in the mass (arrow). Radiological differentiation of the mass from papillary RCC is difficult.
MR = magnetic resonance, RCC = renal cell carcinoma
Fig. 10
A 48-year-old woman with angiomyolipoma in the right kidney.
A, B. Axial in-phase (A) and opposed-phase (B) chemical shift MR images. Signal drop is observed on opposed phase in the mass (arrows).
C. Axial T2-weighted MR image shows heterogeneous high signal intensity in the center of the mass (arrow).
D. Axial fat-suppressed T2-weighted MR image shows low signal intensity in same area of the mass (arrow).
MR = magnetic resonance
Fig. 11
A 58-year-old woman with isoattenuating angiomyolipoma in the right kidney.
A. Axial unenhanced CT image shows a 1-cm mass with homogenous attenuation (arrow) in the right kidney.
B. Axial contrast-enhanced CT image in the corticomedullary phase shows heterogeneous moderate enhancement in the mass (arrow).
C, D. Although axial in-phase (C) and opposed-phase (D) chemical shift MR images show no signal drop on opposed phase in most areas of the mass (arrows), India ink artifact (arrowhead) is observed at the interface of the mass and kidney on opposed-phase (D) chemical shift MR image.
E. Axial T2-weighted MR image shows low signal intensity in most areas of the mass (arrow) and slight high signal intensity at the interface of the mass and kidney.
MR = magnetic resonance
Fig. 12
A 63-year-old man with papillary renal cell carcinoma in the left kidney.
A, B. Axial in-phase (A) and opposed-phase (B) chemical shift MR images show signal drop on in-phase (arrows) but not opposed-phase imaging. Intratumoral hemorrhage or hemosiderin is indicative of signal drop on in-phase images.
C. Axial T2-weighted MR image shows heterogeneous high and low signal intensity in the mass (arrow).
MR = magnetic resonance
Fig. 13
A 26-year-old woman with isoattenuating clear cell RCC in the left kidney.
A. Gray-scale ultrasound image shows a 2.2-cm hyperechoic mass (arrow) in the left kidney.
B. Axial unenhanced CT image shows a homogenously isoattenuating mass (arrow).
C. Axial contrast-enhanced CT image in the nephrographic phase shows homogeneous moderate enhancement of the mass (arrow), less than that of the renal cortex.
D, E. Axial in-phase (D) and opposed-phase (E) chemical shift MR images show no signal drop on opposed phase in the mass (arrows).
F. Axial T2-weighted MR image shows heterogeneous high and low signal intensities in the mass (arrow). The differential diagnosis between RCC and fat-poor angiomyolipoma was difficult by imaging. A biopsy was eventually performed.
MR = magnetic resonance, RCC = renal cell carcinoma
Fig. 14
A 55-year-old woman on active surveillance with indeterminate solid renal mass in the right kidney.
A. Axial unenhanced CT image shows a 6-mm homogenously isoattenuating mass (arrow) with an attenuation of 35 HU in the right kidney.
B. Axial contrast-enhanced CT image in the corticomedullary phase shows homogeneous mild enhancement of approximately 67 HU (arrow), less than that of the renal cortex.
C. Gray-scale ultrasound image obtained after one year shows a 6-mm hyperechoic mass (arrow) in the right kidney.
D, E. Axial T2-weighted MR image (D) and axial T1-weighted MR image (E) obtained after 2 years show slight low signal intensity and iso signal intensity without any change in the size, appearance of the mass (arrows).
HU = Hounsfield unit, MR = magnetic resonance
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