Section 5.2: Malignant Renal Tumors and Nephrolithiasis / Hydronephrosis

Malignant Renal Tumors

Renal Cell Carcinoma (RCC)

Renal Cell Carcinoma (RCC), also known as hypernephroma or Grawitz tumor, is the most common primary malignant renal tumor in adults, accounting for approximately 85% of all renal neoplasms. RCC arises from the renal tubular epithelium and is twice as common in men, typically presenting in the sixth to seventh decades of life. The classic clinical triad of RCC includes hematuria, flank pain, and a palpable flank mass; however, this triad is present in only 10% of patients, and many tumors are discovered incidentally.

On grey-scale ultrasound, the sonographic appearance of RCC is highly variable. The most common presentation is a solid, vascular, heterogeneous cortical mass. Smaller tumors (<3 cm) are frequently hyperechoic, which can cause them to be mistaken for benign angiomyolipomas (AMLs). However, unlike AMLs, RCCs lack clear posterior acoustic shadowing, tend to have a hypoechoic rim (halo), and show internal vascular flow on color Doppler. Larger RCCs are typically isoechoic or hypoechoic relative to the surrounding renal cortex and often display areas of cystic necrosis, internal hemorrhage, and coarse calcifications. On spectral Doppler, RCCs demonstrate high-velocity, low-resistance waveforms due to tumor neovascularization.

One of the most critical sonographic evaluations in a patient with suspected RCC is checking for vascular invasion. RCC has a notorious propensity to invade the renal venous system. The tumor can grow directly into the main renal vein, propagate into the inferior vena cava (IVC), and potentially extend all the way into the right atrium of the heart. The sonographer must evaluate the main renal vein and the entire intrahepatic and suprahepatic IVC using grey-scale and color/spectral Doppler to detect a solid, vascularized tumor thrombus (which will demonstrate arterial flow within the thrombus on spectral Doppler, differentiating it from a simple blood clot).

Wilms Tumor (Nephroblastoma)

Wilms tumor, or nephroblastoma, is the most common primary malignant renal tumor in pediatric patients, with a peak incidence between 3 and 4 years of age. It is rarely seen after the age of 8. Clinically, infants and children present with a rapidly growing, palpable, firm abdominal flank mass, often accompanied by hematuria, fever, and hypertension (due to excessive renin production).

Sonographically, a Wilms tumor typically appears as a large, solid, well-circumscribed, homogeneous or heterogeneous echogenic mass that destroys and displaces the adjacent normal renal parenchyma. Areas of necrosis, hemorrhage, and cystic degeneration are common. Differentiating Wilms tumor from neuroblastoma (a malignant tumor of the adrenal gland or sympathetic nervous system) is a high-yield exam topic. Wilms tumor arises from the kidney itself and displaces the renal sinus, while neuroblastoma is an extrarenal mass (usually adrenal) that displaces the kidney inferiorly and laterally without invading or destroying the renal parenchyma. Additionally, neuroblastoma often encases surrounding abdominal vessels, whereas Wilms tumor tends to invade them (e.g., renal vein and IVC thrombosis).

Nephrolithiasis (Renal Stones)

Nephrolithiasis refers to the presence of crystalline stones within the urinary tract. The most common composition is calcium oxalate. Sonography is highly effective in detecting renal stones, which present as highly echogenic, bright foci with a sharp posterior acoustic shadow. To maximize the visibility of the acoustic shadow, the sonographer should:

• Use a high-frequency transducer.
• Place the focal zone directly at the depth of the stone.
• Position the ultrasound beam perpendicular (90 degrees) to the stone surface.

Two specialized sonographic signs assist in the diagnosis of nephrolithiasis:

1. Twinkle Artifact: On color Doppler, a stone will generate a rapid, chaotic alternation of red and blue pixels immediately behind it. This twinkle artifact is highly sensitive and is especially useful for identifying small stones (<5 mm) that do not produce a clear posterior shadow on grey-scale imaging, or for confirming stones located in the ureterovesical junction (UVJ).
2. Ureteral Jets: Evaluating the bladder trigone with color Doppler allows the visualization of periodic bursts of urine entering the bladder from the ureters. In the setting of complete or near-complete ureteral obstruction by a stone, the ureteral jet on the affected side will be absent or significantly diminished compared to the contralateral side.

Hydronephrosis

Hydronephrosis is the dilatation of the renal pelvis and calices, usually resulting from downstream obstruction of urine flow. If left untreated, chronic obstruction leads to parenchymal compression, ischemia, and irreversible renal atrophy. Sonography is the primary imaging modality to detect, grade, and monitor hydronephrosis:

It is important to note that a dilated renal pelvis can sometimes be a normal variant (extrarenal pelvis), which presents as a fluid-filled structure projecting medial to the renal hilum but shows no caliceal dilatation and collapses when the patient is in a prone position or has a voided bladder. Non-obstructive causes of caliceal dilatation, such as overhydration, vesicoureteral reflux, or a distended urinary bladder, must also be ruled out.