Urogenital ImagingRenal cell carcinoma
(RCC), the most common
renal tumour, comprising approximately 85% of all primary
malignant renal neoplasms.
Renal tumours are usually solitary, although bilateral tumours are encountered in approximately 2% of patients. RCC occurs mainly in the sixth decade; they have been found in children, but only sporadically. The development is not related to ethnic groups. Histologically, the most common type of
malignant renal cell cancer is clear cell adenocarcinoma, with granular cell
carcinoma next in frequency. Papillary
tubular adenocarcinoma defines a small group of RCC, but because of their limited blood supply, these can simulate
cystic lesions and, thus, can represent a diagnostic challenge on imaging. RCC arises from the proximal convoluted tubule. RCCs are relatively slow growing and may be large before they produce symptoms. Before widespread use of cross-sectional imaging (
ultrasound (US),
CT,
MRI),
renal cell cancer often presented as an advanced disease at the time of diagnosis. Today, however, there has been a shift to a smaller size and lower stage of
renal cell cancer at the time of diagnosis. RCC may be locally aggressive, extending into the
renal veins and inferior vena cava (
IVC) or invading adjacent soft tissue structures. RCC can metastasize by lymphatic and haematogenous routes. Common sites of haematogenous metastases include bone, liver, and lungs. Nodal metastases commonly involve pararenal and para-aortic nodes and may also include
mediastinal and
pulmonary hilar nodes.
Imaging
Imaging plays an important part in the detection, characterization and staging of renal cell cancer (Fig.1). Although the intravenous urogram (IVU) is still often used as the initial study in the search for renal masses, it has been shown that in the presence of a CT-confirmed renal mass, detection by IVU is only 21% when the lesion is smaller than 2 cm, 52% when the lesion is 2-3 cm, and 85% when the lesion is 3 cm or more in diameter. A normal IVU, therefore, does not exclude the presence of a renal mass. While US is an excellent modality for the detection and characterization of renal cysts, in the detection of solid lesions US accuracy decreases considerably (Fig.2). When compared with CT, US demonstrates detection of 60% of lesions smaller than 2 cm and 83% of lesions between 2 and 3 cm in size. Lesion detection on contrast-enhanced MRI (90-97%) equals that of CT (89 - 99%). US, CT, and MRI have all been used with varying degrees of success in the characterization of renal masses. On US, the appearance of renal cell carcinoma is variable. Approximately 86% of tumours are isoechoic, 4% are hyperechoic, and the remainder are hypoechoic as compared to the adjacent renal parenchyma. Approximately 13% of renal cell carcinomas will demonstrate increased through transmission. These tumours have usually undergone extensive liquefaction necrosis. Colour Doppler sonography using frequency shift determinations has demonstrated some utility in differentiating benign from malignant lesions. Power Doppler sonography, which is even more sensitive to flow than conventional colour Doppler imaging, may provide additional information in characterizing renal lesions. Although US is useful in characterizing renal masses, it is inadequate in staging renal cell carcinoma.
The CT appearance of renal cell carcinoma varies with tumour size and vascularity (Fig.3) (Fig.4) (Fig.5). When large enough, these tumours appear as masses that alter renal contour or intrarenal architecture. Detection of small lesions is facilitated by rapid sequence scanning techniques during administration of contrast material because abnormal enhancement may be evident even when renal contours are normal. Heterogeneous enhancement is characteristic, but after administrat weighted images. When intratumoural haemorrhage or necrosis is present, signal intensity can be heterogeneous on both T1- and T2-weighted images. Haemorrhage from a RCC may result in deposition of iron in the kidney and lower the signal intensity of the tumour on both T1- and T2-weighted images. This effect is not specific for renal cell carcinoma and may be seen with any haemorrhagic lesion or with systemic haemolysis. The differential diagnosis for a renal lesion appearing hypointense on both T1- and T2-weighted sequences also includes a fibroma, milk of calcium cysts, and other calcified renal lesions. Intralesional calcifications are not well depicted on MR imaging. Renal cell carcinomas have been well depicted on postcontrast T1-weighted fat suppressed images. On gadolinium contrast-enhanced MRI scans, tumours generally enhance to a lesser degree than the surrounding renal parenchyma. Lesion sensitivity is increased when dynamic postcontrast scanning is employed.
Imaging plays an important role in renal cancer treatment decision. The decision as to the type of surgery (nephron sparing; simple or radical nephrectomy) that can be performed in patients with renal cell carcinoma is helped by imaging. Because surgery provides the only effective therapy and because survival depends on local and distant extent, precise staging is critical for preoperative planning and prognosis.
Staging
RCC can be staged according to the classifications described by Robson and Churchill. According to these criteria, tumours confined by the renal capsule are classified as stage 1, and lesions confined by perirenal fascia are stage 2. Stage 3 lesions are subdivided into three categories. Stage 3A tumours extend to the renal vein or inferior vena cava; stage 3B lesions extend to retroperitoneal lymph nodes; and stage 3C lesions involve both draining vessels and retroperitoneal lymph nodes. Tumours that have invaded adjacent viscera or muscle structures are classified as stage 4A, and lesions that have metastasized distantly are considered stage 4B.
Although CT has been the test of choice for staging renal cell carcinoma MRI appears to have a similar accuracy. Combined transverse and sagittal MRI planes are optimal for the evaluation of venous anatomy and the normal tissue tumour interfaces.
The particular uses of MRI staging include determination of the origin of the mass, evaluation of vascular patency, detection of perihilar lymph node metastases and evaluation of direct tumour invasion to adjacent organs. On CT and MRI, diagnosis of lymph node metastases is based on detection of lymph node enlargement with nodes measuring larger than 1 cm in diameter in short axis considered abnormal. Approximately 60% of nodes measuring greater than 1 cm in transverse diameter, in the setting of RCC, however, have been shown to be inflammatory or hyperplastic in nature rather than metastatic. Lymph node enlargement due to inflammation/hyperplasia or to metastasis cannot be distinguished on the basis of imaging findings. MRI is a sensitive tool for determining the presence and extent of tumour thrombus and for demonstrating invasion of the wall of the inferior vena cava.
In the assessment of thrombus extension into the renal vein or inferior vena cava, MRI has replaced venography. In renal cell carcinoma, thrombus extends into the inferior vena cava in 4 -10% of cases. Typically, tumour thrombi enlarge the renal vein and inferior vena cava and cause the density of these vessels to be heterogeneous. On either CT or contrast enhanced MRI care must be taken to avoid confusing artefacts caused by laminar flow of intravascular contrast material with intraluminal thrombi. Such artefacts may be seen after injections of either the arm or foot vein and may be eliminated by use of both rapid sequence scanning during injection of contrast material and delayed scanning in the region of the suprarenal inferior vena cava. The accuracy in the determination of IVC thrombus is 100% on MR imaging compared to 88% and 78% for CT and ultrasound, resp similar to slightly better than that achieved with CT. The overall accuracy of CT in staging renal cell carcinoma ranges from 67-91%. When each stage is analysed separately, the accuracy of MR imaging is similar to that of CT in patients with stage I and stage II disease, but with more advanced disease (stage III and IV), particularly involving a large tumour mass, MR staging has proven superior.
Because CT is less expensive and more widely available, CT remains the preferred cross-sectional imaging procedure for the detection, characterization and staging of renal lesions. Three-dimensional imaging and display of renal tumours using spiral CT has also recently been shown to serve as a surgical aid when planning partial nephrectomy.
MRI is reserved for those cases where CT staging is inconclusive, especially with respect to vascular extension and direct tumour invasion of neighbouring tissue or in patients with renal failure, or where there are other contraindications for the use of iodinated contrast media.
HH