Renal Cell Carcinoma
More than 85% of solid renal tumors are renal cell carcinomas (RCC), which occur bilaterally in up to 5% of the cases. Every non-fatty solid renal tumor should therefore be considered RCC unless proven otherwise. Men are twice as often affected as women are.
Due to the lack of sensitive innervation of the kidney and the renal capsule, renal cell cancer often exhibits a substantial size at the time of the diagnosis. One third of the patients already have metastases (lungs, mediastinum, bones, liver, brain, skin). Up to two thirds of the patients suffering from von Hippel-Lindau syndrome develop bilateral, often multiple renal cell carcinomas. Renal cell carcinomas exhibit calcifications in up to 50% and necrosis, while hemorrhage is less frequent.
RCC often exhibits an inhomogeneous SI on T1-weighted and T2-weighted images. In case of necrosis, mainly occurring centrally, these tumors exhibit central hyperintense signal on the T2-weighted images. Up to 15% of all renal cell tumors are iso-intense to the kidney in all pre-contrast sequences. Often a distortion of the renal architecture is the only hint for renal tumors in non-enhanced studies. Therefore, the administration of intravenous contrast agents is mandatory to reliably detect and grade renal cell carcinomas.
Small RCC manifest as round, smooth lesions, which can be clearly distinguished from normal renal parenchyma. Larger RCC however, have a more polycyclic appearance. The exact border of larger renal cell carcinomas may be hard to detect due to the invasive growth pattern and require multiplanar imaging for delineation from adjacent organs.
Non-enhanced T1-weighted images are good at demonstrating the tumor infiltration into the perirenal space. This can be even better done on fat suppressed T1- weighted images post-contrast. However, the differentiation between renal cell carcinoma, which is confined to the kidney (Robson stage I, T1, and T2), and renal cell carcinoma with spread into the perirenal space (Robson stage II, T3a) is often hard to achieve with MRI. Tumor infiltration into the renal vein or the inferior vena cava (Robson stage IIIa, T3b) can be appreciated as a moderate SI in the affected vessels that should normally show no SI at all. T2-weighted respiratory-gated images are most suitable for the detection of tumor growth into the renal vein and inferior vena cava. The reported sensitivity in the detection of (tumor-) thrombus is 93–100% for MRI and 93% for CT, while the reported specificity is 75% for MRI and 80% for CT. To differentiate a tumor thrombus from an apposition thrombus, post-contrast images are required. While the latter must not show any contrast enhancement, a tumor thrombus exhibits substantial enhancement. The infiltration of renal cell carcinoma into adjacent organs (Robson stage IV, T4) can be seen by a depleted perirenal fat space or direct infiltration into the adjacent structures.
Post-contrast T1-weighted images allow for a good delineation of necrotic and viable tumor components. The thick and irregular tumor is readily detected on contrast- enhanced scans. The delineation of the tumor from normal renal parenchyma is dependent on the exact time of image acquisition following contrast injection and on the vascularity of the tumor. The acquisition of at least two sequences after intravenous contrast administration (arterial phase 15–20 s after bolus injection and venous phase 20–30 s after the arterial phase) greatly facilitates the characterization of renal tumors. Renal tumors often demonstrate an early washout of the contrast agent compared to the normal renal parenchyma. An enhancement of more than 15% compared with the precontrast-images has been found to be very sensitive (100%) and specific (94%) for detection of renal cell carcinomas. T2-weighted sequences with fat-suppression also allow for highly sensitive detection of renal tumors.