What are the CT and MRI features of RCC?
RCC is solitary in 95% of cases and may be intrarenal, exophytic, or both intrarenal and exophytic in configuration. A solid enhancing renal mass with soft tissue attenuation on CT or with low-intermediate T1-weighted signal intensity, variable T2-weighted signal intensity, and restricted diffusion relative to renal parenchyma on MRI is typically visualized. Presence of microscopic lipid content within the renal mass, as manifested by a loss of signal intensity on out-of-phase T1-weighted images relative to in-phase T1-weighted images, is a highly specific feature of the clear cell subtype of RCC, which may be seen in up to 60% of these tumors.
RCC tends to be more homogeneous in appearance when small in size and more heterogeneous in appearance when large in size due to development of cystic or necrotic components, calcifications (seen in up to 30%), or hemorrhage. Fifteen percent of RCCs are cystic, most often the clear cell and papillary subtypes. The lesions have fluid attenuation or fluid signal intensity components in conjunction with thickened enhancing walls, thickened enhancing internal septations, and soft tissue nodular enhancing components. Soft tissue components tend to enhance more briskly and heterogeneously in the clear cell RCC but more mildly and homogeneously in the papillary and chromophobe RCCs. Washout of tumor enhancement relative to the renal parenchyma may be seen during the nephrographic or delayed phase of enhancement.
Peritumoral vascularity is sometimes seen, most often with the clear cell subtype. Associated direct extrarenal extension of tumor into surrounding tissues/organs, regional lymphadenopathy, or distant metastatic disease may sometimes also be present. Tumor extension into the renal vein or inferior vena cava (IVC) occurs in up to 20% of cases and appears as a high signal intensity filling defect on T1-weighted or T2-weighted images with restricted diffusion, enhancement, and occasional venous luminal expansion. The most common sites of distant metastatic disease include the lungs, bone marrow, liver, and lymph nodes. Metastases from RCC generally hyperenhance during the arterial phase of enhancement relative to surrounding tissues.