IMAGING OF MALIGNANT RENAL MASSES

MALIGNANT RENAL TRACT MASSES
Imaging of malignant renal masses
Ultrasound, as one of the first-line investigations in patients with haematuria, is highly sensitive in detecting large renal masses above 2.5 cm in diameter and in differentiating them from renal cysts. Smaller masses may be missed with ultrasound however, as they are frequently isoechoic (in 86% of cases); CT is more sensitive in small lesion detection. MRI also detects small renal masses more frequently than ultrasound but is generally reserved for patients with equivocal CT scans as it is less widely available. IVU is also known to miss small renal masses and normally requires further characterization of any detected mass with ultrasound or CT.

Renal cell carcinoma (RCC)
Adenocarcinoma is the most common type of renal malignancy (referred to as renal cell carcinoma) occurring less commonly in the bladder and ureter. RCCs are frequently large at clinical presentation; they may occasionally be identified as an incidental finding in an asymptomatic patient.

Ultrasound appearances
The RCC is a (usually) large, heterogeneous mass which enlarges and deforms the shape of the kidney. The mass may contain areas of cystic degeneration and/or calcification. It has a predilection to spread into the ipsilateral renal vein and IVC. Colour Doppler usually reveals a disorganized and increased blood flow pattern within the mass with high velocities from the arterioverous shunts within the carcinoma. Smaller RCCs can be hyperechoic and may be confused with benign angiomyolipoma. The latter has well-defined borders whilst an RCC is illdefined: differentiation may not be possible on all occasions and biopsy or interval scan may be required. A chest X-ray and/or CT will demonstrate if metastases are present in the lungs. Liver, adrenal and lymph node metastases can be demonstrated on ultrasound but CT is used for staging purposes as ultrasound generally has a lower sensitivity for distant disease detection.

Transitional cell carcinoma
Transitional cell carcinoma is the most common bladder tumour, occurring less frequently in the collecting system of the kidney and the ureter. It usually presents with haematuria while still small. It is best diagnosed with cystoscopy. Small tumours in the collecting system are difficult to detect on ultrasound unless there is proximal dilatation. Depending on its location it may cause hydronephrosis, particularly if it is situated in the ureter (rare) or at the vesicoureteric junction (VUJ). IVU, retrograde cystography and CT are methods of diagnosis.

Ultrasound appearances
Situated within the collecting system of the kidney, the transitional cell tumour is usually small (compared to the RCC), homogeneous and relatively hypoechoic. Proximal renal tract dilatation may sometimes be present. These tumours are easy to miss on ultrasound unless the kidney is scanned very carefully, and often are, unless the case is highlighted by clinical symptoms or a high clinical index of suspicion. They can mimic a hypertrophied column of Bertin; CT may differentiate in cases of doubt. Once large, they invade the surrounding renal parenchyma and become indistinguishable from RCC on ultrasound. They frequently spread to the bladder and the entire renal tract should be carefully examined. In the bladder they are potentially easier to see as they are surrounded by urine. Invasion of the bladder wall can be identified on ultrasound in the larger ones but biopsy is necessary to determine formally the level of invasion. IVU or a retrograde cystogram are the methods of choice for demonstrating a filling defect in the PCS or ureter; CT may be useful and is also used for staging purposes.

Lymphoma
Renal involvement of non-Hodgkins or Hodgkins lymphoma is not uncommon and depends upon the stage of the disease. The ultrasound appearances are highly variable and range from solitary to multiple masses, usually hypoechoic but sometimes anechoic, hyperechoic or mixed. The masses may have increased through transmission of sound and may mimic complex fluid lesions such as haematoma or abscess. The clinical history should help to differentiate these cases. Occasionally diffuse enlargement may occur secondary to diffuse infiltration.

Metastases
Renal metastases from a distant primary are usually found in cases of widespread metastatic disease and are frequently multiple. In such cases, the primary diagnosis is usually already known and other abdominal metastases, such as liver deposits and/or lymphadenopathy, are commonly seen on ultrasound. Rarely, a single metastasis is seen in the kidney without other evidence of metastatic spread, making the diagnosis difficult (as the question arises of whether this could be a primary or secondary lesion). CT may identify the primary and frequently picks up other, smaller metastases not identified on ultrasound.