Renal Masses: Imaging and Biopsy

Plain Abdominal Radiography

The plain abdominal radiograph (KUB—kidneys, ureters, bladder) is not used to diagnose a renal mass. Loss of the psoas margin or displacement of retroperitoneal fat may occasionally suggest the presence of one, as may an opacity projected over the renal outline or a loss of the renal outline. Central calcification within a renal mass is more suggestive of malignancy than peripheral calcification (87% vs 20%–30%).

Ultrasound

Ultrasound (US) is often the first method for evaluating a patient with a suspected renal mass. US is ideally suited for children, pregnant women and patients with renal impairment. US can reliably differentiate solid masses from simple cysts, which are the most common space-occupying lesions in the kidney. A lesion that appears solid on US, or demonstrates any suspicious features, merits further analysis with computed tomography (CT) or magnetic resonance imaging (MRI). US can be used to evaluate an incidental renal lesion detected on a single-phase contrast-enhanced CT.

US is less accurate in staging renal cell carcinoma than is CT or MRI. It is poor at demonstrating lymph node disease, skeletal or lung metastases.

Computed Tomography

CT is still the investigation of choice for evaluating and characterising solid renal masses. It can accurately assess ‘pseudomasses’ and other anatomical variants and can provide attenuation values that can confirm the presence of fluid in cysts or fat in angiomyolipomas. As CT can delineate accurately the perinephric space and the retroperitoneum, it is useful in the diagnosis of complicated renal sepsis and the assessment of the extent of haemorrhage; it can also identify tumour recurrence after radical nephrectomy. Accurate analysis of renal masses requires the appropriate use of intravenous (IV) contrast medium.

The continued development of multislice CT has improved the detection, characterisation and staging of renal tumours and allows high-quality multiplanar imaging of the kidneys, which is useful for evaluating small areas of enhancement and for presurgical planning. With the increased use of laparoscopic, robotic and nephron-sparing surgery, it is essential to be able to review images using multiplanar and three-dimensional (3D) images with surgical colleagues to decide upon appropriate management.

Unenhanced CT images are essential for identifying calcification and allowing true evaluation of enhancement following IV contrast agent administration. Corticomedullary phase (25–40 seconds post-IV injection) imaging is helpful in demonstrating normal variants, pseudotumours, tumour vascularity and the renal vein. The nephrographic phase (90–100 seconds post-IV contrast administration) is best for the detection of central renal masses, as the medulla is optimally enhanced and small medullary lesions are better visualised. For optimal lesion detection and characterisation, images should be obtained in both phases; however, if only one phase is to be used, to reduce radiation dose, it should be the nephrographic phase. If surveillance imaging of a lesion is to be undertaken, the single optimal phase for detection of the mass can be used rather than repeating a three-phase examination.

Magnetic Resonance Imaging

The ability of MRI to characterise renal masses has improved with the development of phased-array multicoils, fast breath-hold imaging and the use of gadolinium-diethylenetriamine pentaacetic acid Gd-DTPA contrast enhancement. Protocols vary widely but usually include pre-contrast and post-contrast T ₁ weighted images with and without fat suppression. The coronal and sagittal planes are helpful for evaluating the extent of lesions. MR angiography can demonstrate the renal arteries and veins, and the inferior vena cava. MRI can be used as an alternative to CT for detection and surveillance of masses, although this depends upon local resources and availability of equipment. Many radiologists prefer to use CT for the initial evaluation of renal masses as it is such a reliable technique and easier to interpret than MRI. There has been recent interest in the use of diffusion-weighted imaging for the characterisation of renal masses. Malignant tumours are associated with a significantly reduced apparent diffusion coefficient (ADC) value compared with benign lesions, and this technique can be used to differentiate low- from high-grade disease, with accuracies of up to 83%.

Simple renal cysts and angiomyolipomas have characteristic appearances, but the signal from most other masses is non-specific.

MRI is an alternative to CT in patients with renal insufficiency or severe previous reactions to contrast medium. MRI is superior to CT in differentiating benign thrombus from tumour thrombus and in identifying its extent. MRI is ideally suited for monitoring patients who have a genetically increased risk of renal malignancy, such as von Hippel–Lindau (VHL) disease, who require repeated imaging for surveillance.

MRI is no more specific than CT at differentiating malignant from reactive lymphadenopathy. MRI is a more lengthy and complex investigation than CT. The role of MRI has reduced with the development of multislice CT, which allows the production of reliable and high-quality multiplanar reconstructions.

Renal Arteriography

Renal arteriography is seldom used to diagnose or characterise a renal mass as the necessary information is usually provided by cross-sectional imaging. Angiography can play a role in preoperative embolisation of very vascular tumours immediately before partial nephrectomy. CT or MR angiography is usually sufficient to provide a road map for surgery and to identify the size, number and position of renal vessels.

Intravenous Urography

Intravenous urography (IVU) is no longer an appropriate method for detecting renal masses, particularly if they occur centrally rather than peripherally; cross-sectional techniques are a more appropriate for investigating a patient with a suspected renal mass.

Radionuclide Imaging

Differentiation between a definite mass and an anatomical variant that simulates a mass (pseudotumour) can be made using radionuclide imaging, although this is rarely useful in practice.

Needle Aspiration and Biopsy

Percutaneous aspiration of renal cysts is indicated in the investigation of an indeterminate cystic renal mass to diagnose an abscess or an infected cyst.

Fluid obtained at aspiration should be sent for cytological examination, although negative cytology does not exclude malignancy; this applies particularly in some cystic renal cell carcinomas, in which malignant disease is confined to the wall of the lesion. If the fluid is found to be turbid, microbiological examination should also be performed. Needle biopsy of a cyst wall can be performed to improve the diagnostic yield, although there are small but potential risks in this setting including seeding of tumour and false-negative diagnosis.

Biopsy is used to confirm the histology of a renal mass in patients with underlying non-renal malignancy or radiological features suggestive of lymphoma. Biopsy is also used to confirm the presence of malignancy before radiofrequency or cryoablation of a renal mass. Histological techniques have improved over the past 10 years and are more reliable at classifying a renal mass and differentiating between oncocytoma and renal cell carcinoma. In patients with significant other comorbidity, this may significantly alter the management of an asymptomatic renal mass. Biopsy should also be considered in bilateral masses to characterise whether the lesions represent multifocal oncocytoma or papillary tumour. This information may make such lesions suitable for attempted nephron-sparing surgery even if potentially suboptimal for this approach. There is a growing trend to perform biopsy in smaller lesions that may then be managed conservatively, or even removed using advanced complex nephron-sparing techniques.

Renal Cysts

Complicated Cysts

A classification of cystic lesions was suggested in 1986 by Bosniak, based upon CT characteristics, and is used to guide management. Class I is a simple benign cyst. Class II cysts have one or more thin septa running through them (<1 mm), thin areas of mural calcification or fluid contents of increased attenuation; they do not enhance following injection of contrast medium and are benign ( Fig. 29.1 ). These two categories of cysts are benign and do not require surgery or radiological follow-up.

Class III cysts are more complicated and contain thickened septa, nodular areas of calcification or solid non-enhancing areas. Mural enhancement can be seen in class III lesions, which are indeterminate for malignancy and should be biopsied or surgically explored. Less than 50% of these will turn out to be malignant, although there can be significant interobserver variation in how such cysts are classified ( Fig. 29.2 ).

Class IV cystic masses are clearly malignant, with solid enhancing nodules and should be treated accordingly ( Figs 29.3 and 29.4 ).

A subcategory, IIF, has been suggested for lesions with multiple class II features, and these require follow-up for up to 5 years to exclude malignancy. Surveillance of these lesions may demonstrate growth or change in calcification, but it is the development of enhancing soft tissue that should upgrade the cystic lesion and result in surgical treatment ( Fig. 29.5 ). Category IIF lesions include large (>3 cm) hyperdense cysts or hyperdense cysts that are totally intrarenal.

A simple benign cyst on US or CT requires no further investigation or follow-up. If there is wall thickening or the contents of the cyst are not of water density, the lesion is indeterminate. Haemorrhage or infection may result in cyst fluid of high attenuation but, unlike tumours, such lesions do not enhance following the administration of contrast medium. It is usually necessary to obtain unenhanced CT to adequately assess a rounded homogeneous lesion on a post-contrast CT that is not of water density to exclude low-grade enhancement signifying a renal mass (e.g. papillary tumour, rather than a benign cyst). US is helpful if the hyperdense mass satisfies the sonographic criteria of a benign simple cyst.

Thick and irregular mural calcification can be seen with both cystic renal cell carcinomas and complicated renal cysts. CT attenuation values in both lesions may be identical. Cystic renal cell carcinomas (especially papillary cystadenocarcinomas) may have fluid-range densities, while benign haemorrhagic cysts may have attenuation values much higher than those acceptable for benign cysts.