ReviewImaging features, follow-up, and management of incidentally detected renal lesions
Introduction
The incidental finding of a renal mass on cross-sectional imaging is increasing because of the continued rise in such techniques, both in the investigation of patients’ symptoms and in completely asymptomatic individuals for private health screening purposes. Renal masses are common, with approximately 40% of patients having at least one renal cyst incidentally discovered on abdominal computed tomography (CT).1 Fortunately, the vast majority of these are easily categorized on contrast-enhanced CT as benign simple cysts, which can be safely ignored. Prevalence increases with age, from <10% under 40 years to >60% over 80 years, and they are significantly more common in males than females.1 The prevalence of renal cancer also increases with age, and 10–15% of renal cancers are cystic in nature.2, 3 The task of the radiologist is to recognize the small proportion of incidental masses that have overt or suspicious signs of malignancy, and guide the clinician to appropriate further management. Use of the Bosniak classification allows stratification of cystic lesions into those that can be ignored, those that need to be followed up, and those that require urological referral for consideration of removal.4, 5
Historically a solid renal mass is generally regarded as a renal cell carcinoma (RCC) unless specific imaging features (for example, macroscopic fat) suggest otherwise, but this view is changing. Incidentally detected solid lesions are likely to be smaller than symptomatic renal cancers.6 Approximately 50% of all renal tumours are detected incidentally, and at an earlier stage than symptomatic tumours,7, 8 which increases to 60–70% of masses under 4 cm.9, 10, 11 Increased incidental detection of small tumours has produced a rise in the incidence of renal cancer, and consequently, a rise in the both the number of treated cancers and the 5 year survival rate.12 However, approximately 20% of small renal masses are benign,13, 14 which further complicates the optimum management of such lesions. In the UK, all presumed renal cancers are discussed at a multidisciplinary team meeting (MDT) with input from urologists, radiologists, histopathologists, and oncologists to optimize treatment decisions. Recognition of the imaging features of different cystic and solid renal masses allows the radiologist to participate fully in the MDT discussion.
Section snippets
Cystic renal masses
Accurate characterization of cystic renal masses determines the management of these lesions and use of the Bosniak classification enables this (Table 1).4, 5 The Bosniak classification was developed for CT but can equally be applied to magnetic resonance imaging (MRI) or ultrasound if contrast enhancement is used.15, 16, 17 The Bosniak classification is based on the following features as seen on CT: fluid density, wall and septal thickness, enhancement, and calcification.
CT
CT plays the key role in the accurate assessment of complex cystic or solid lesions. In order to evaluate enhancement within a lesion, an unenhanced examination should be performed followed by a nephrographic phase (90–120s delay) examination. The nephrographic phase is the optimum phase to characterize renal masses.23 As there is maximal and homogeneous renal parenchymal enhancement, this enables the detection of renal masses that normally do not enhance to the same degree.24 Also, parenchymal
MRI
With the increasing use of abdominal MRI, particularly in hepatobiliary imaging, more incidental renal lesions will be discovered. Cysts will be readily apparent on the heavily T2-weighted images used to assess the biliary tree (Fig 3). A homogeneous, well-circumscribed lesion on T2W imaging represents a simple cyst (Bosniak I), and can be safely ignored.5 The authors consider that minimally complex cysts containing one or two thin septa also do not require any further imaging or follow-up,
Ultrasound
Solid renal masses will only be conspicuous if they are of different echogenicity to the normal renal cortex, or deform the normal contours of the kidney, or central echo complex. Fat-containing masses such as angiomyolipoma (AML) are highly echogenic, and will readily be seen even when only a few millimetres in size (Fig 5). Other causes of solid masses have texture more similar to normal renal cortex, which makes them difficult to discern until over about 2 cm in size.36
Siegel et al. evaluated
CT
A solid, or potentially solid, mass without macroscopic fat is one that measures >20 HU on unenhanced and contrast-enhanced CT.18 As discussed above, pre and nephrographic post-contrast imaging are optimal for the evaluation of enhancement within a renal lesion. However, most abdominal CT in the UK is performed in the portal phase, without unenhanced images, and increasingly without direct radiological supervision. Should the reporting radiologist always advise recall for a non-enhanced
MRI
Fat-suppressed sequences will identify the fat within a classical AML, although there is no advantage over CT, which will equally well demonstrate macroscopic fat. Chemical shift imaging techniques utilize the presence of fat and water within the same voxel. Fat protons precess at a lower frequency than water protons, and this results in loss of signal within a mass on opposed-phase GRE MRI, compared with the in phase. This effect can be evident macroscopically or calculated by measuring the
Management
Recent recommendations from the American College of Radiology advise intervention for masses >3 cm, and observation for masses <1 cm until they exceed 1.5 cm.18 Options for the small presumed renal cancer <3 cm include active surveillance, ablation, or surgery. The management chosen will reflect the patient’s attitude to risk, co-morbidities, and local services available. Partial nephrectomy (nephron-sparing surgery) is considered the standard of care, as the long-term incidence of renal
Role of biopsy
As discussed above, surgery is considered to be the standard of care for renal masses >3 cm, and histology of the resected lesion determines future management. Consequently, most small solid renal masses are not biopsied prior to removal. However, a considerable number of removed small renal masses are found to be benign (around 20%),9, 13, 14 and cost–benefit analysis has shown that pre-treatment biopsy will avoid unnecessary and expensive surgery in a sizeable minority of patients.67 All
Pitfalls
Non-neoplastic conditions may mimic a cystic or solid neoplasm. On ultrasound, rounded columns of Bertin can have a mass-like appearance, although are usually of the same texture as the adjacent cortex.24 Similarly pseudomasses due to adjacent scarring can be misinterpreted. Both of these are easily identified by CT or MRI, as the attenuation or signal is identical to normal renal cortex on all phases or sequences.
An acute renal infarct is painful and characteristically wedge-shaped with a lack
Conclusion
Ultrasound is useful at differentiating between simple or minimally complex (Bosniak II) cysts that do not require follow-up, and more complex cysts (Bosniak IIF, III, and IV), which require CT or MRI for accurate categorization. Incidental lesions detected at CT that cannot be adequately assessed should have a non-enhanced and renal parenchymal phase as a minimum, to further characterize them. MRI is usually reserved for lesions that remain indeterminate at CT. Percutaneous biopsy is used when
References (74)
- et al.
The prevalence of simple renal and hepatic cysts detected by spiral computed tomography
Clin Radiol
(2003) - et al.
Cystic renal cell carcinoma
Urology
(1986) - et al.
Histopathology of surgically managed renal tumours: analysis of a contemporary series
Urology
(2003) - et al.
Evaluation and management of the small renal mass
Med Clin North Am
(2011) - et al.
Solid renal tumors: an analysis of pathological features related to tumor size
J Urol
(2003) - et al.
Benign lesions after partial nephrectomy for presumed renal cell cancer in masses 4 cm or less: prevalence in Korean patients
Urology
(2010) - et al.
Renal imaging with ultrasound contrast: current status
Radiol Clin North Am
(2003) - et al.
Managing incidental findings on abdominal CT: white paper of the ACR Incidental Findings Committee
J Am Coll Radiol
(2010) - et al.
Assessment of cystic renal masses based on Bosniak classification: comparison of CT and contrast-enhanced US
Eur J Radiol
(2007) - et al.
Bosniak category IIF designation and surgery for complex renal cysts
J Urol
(2009)