274 CHAPTER 6 Urological neoplasia

Radiological assessment of renal masses

Abdominal ultrasound

This is considered by many to be the first-line investigation for a patient with flank pain or a suspected renal mass. The size resolution for renal masses is 1.5 cm, exhibiting variable echo patterns.

Ultrasound may also detect renal cysts, most of which are simple: smooth-walled, round or oval, without internal echoes and complete transmission with a strong acoustic shadow posteriorly. If the cyst has a solid intracystic element, septations, or an irregular or calcified wall, further imaging with CT is indicated.

CT scan

If a renal mass is detected, a thin-slice CT scan before and after contrast is the most important investigation. In general, any solid-enhancing renal mass is considered a renal carcinoma until proven otherwise. Even relatively avascular renal carcinomas enhance by 10–25 Hounsfield units.1

Occasionally, an isodense but enhancing area of kidney is demonstrated (“pseudotumor”) and may correspond to a harmless hypertrophied cortical column (of Bertin) or a dysmorphic renal segment. CT may mislead with respect to liver invasion (rare) because of “partial volume effect”; real-time ultrasound is more accurate. Lymphadenopathy >2 cm is highly indicative of metastases, but occasionally can be inflammatory or due to another process such as lymphoma.

Bosniak developed a radiological classification of renal cysts (Table 6.11).2 The classification is based on homogeneity and complexity of cystic fluid, presence or absence of septations, calcifications, or solid components; and the density of cystic fluid is determined by Hounsfield units. Hounsfield units are a measure of X-ray attenuation applied to CT scanning: –1000 units equate with air, 0 units equate with water, and +1000 equate with bone.

MRI with gadolinium contrast may be used for imaging the inferior vena cava or renal vein (MRV), locally advanced disease, or renal insufficiency, or for patients allergic to iodinated contrast. Renal arteriography is seldom used in the diagnostic setting but may be helpful to delineate the number and position of renal arteries in preparation for nephron-sparing surgery or surgery for horseshoe kidneys.

Nephrogenic systemic fibrosis (NSF) is a scleroderma-like skin disease that affects patients with renal insufficiency. There is a strong association with the development of NSF and exposure to gadolinium contrast agents used in performing MRI. Therefore, in patients with GFRs <30 mL/ min/1.73 m2, these agents should not be used.

Fine needle aspiration/needle biopsy

Ultrasound or CT-guided fine needle aspiration (FNA) or needle biopsy in the investigation of renal masses is of limited value because of the better accuracy of modern cross-sectional imaging, false-negative biopsy results

1 Barbaric ZL. (1994). Principles of Genitourinary Radiology. 2nd Edn. Thieme Medical Publishers: New York.

2 Israel GM, Bosniak MA (2005). An update of the Bosniak renal cyst classification system. Urology 66(3):484–488.

RADIOLOGICAL ASSESSMENT OF RENAL MASSES 275

(5–15%), and risks of hemorrhage (5%) and tumor spillage (rare). FNA is useful for aspiration of renal abscess or an infected cyst, or to diagnose suspected lymphoma or metastatic lesions.

Table 6.12 provides a practical radiological classification of renal masses.

Table 6.11 Bosniak renal cyst classification system based on CT findings

IBenign simple cysts; thin wall without septa, calcifications, or solid components, water density and no contrast enhancement. No further imaging is needed.

IIBenign cysts with a few thin septa; the wall or septa may contain fine calcification and sharp margins; they are non-enhancing, and usually <3 cm.

IIF Well marginated and may have thin septa or minimal smooth thickening of the septa or wall, which may contain calcification that may also be thick and nodular; no contrast enhancement. Includes totally intrarenal non-enhancing lesions > 3 cm. These require follow-up (designated by the F designation).

IIIIndeterminate cysts with thickened irregular or smooth walls or septa; enhancement present. 40–60% are malignant (cystic renal cell carcinoma and multiloculated cystic renal cell carcinoma). Other class III lesions are benign and include hemorrhagic cysts, infected cysts, and multiloculated cystic nephroma. Surgery is recommended, although additional imaging by MRI or with biopsy is supported by some clinicians.

IV Risk of malignancy is 85–100%. Characteristics of category III cysts plus they contain contrast-enhancing soft-tissue components that are adjacent to and independent of the wall or septum. Surgery is recommended.

Table 6.12 Classification of renal masses by radiographic appearance

276 CHAPTER 6 Urological neoplasia

Benign renal masses

The most common (70%) are simple cysts, present in >50% of >50-year- olds. Rarely symptomatic, treatment by aspiration or laparoscopic unroofing is seldom considered.

Most benign renal tumors are rare; the two most clinically important are oncocytoma and angiomyolipoma. Solid masses <2.0 cm are benign in up to 30% of cases, therefore a period of observation is reasonable as many of these may not grow, especially in older or debilitated patients.

Oncocytoma

This is uncommon, accounting for 3–7% of renal tumors. Males are twice as commonly affected as females. They occur simultaneously with renal cell carcinoma in 7–32% of cases.

Pathology

Oncocytomas are spherical, capsulated, and brown/tan in color, with a mean size of 4–6 cm. Half contain a central scar. They may be multifocal and bilateral (4–13%) and 10–20% extend into perinephric fat.

Histologically, they comprise aggregates of eosinophilic cells, packed with mitochondria. Mitoses are rare and they are considered benign, not known to metastasize.

It is often difficult to distinguish oncocytoma from chromophobe RCC. There is often loss of the Y chromosome.

Presentation

Oncocytomas often (83%) present as an incidental finding, or with flank pain or hematuria.

Investigations

Oncocytoma cannot often be distinguished radiologically from RCCs; it may coexist with RCC. Rarely, they exhibit a spoke-wheel pattern on CT scanning, caused by a stellate central scar. Percutaneous biopsy is not recommended since it often leads to continuing uncertainty about the diagnosis.

Treatment

Partial nephrectomy (open or laparoscopic) is indicated whenever technically feasible, based on lesion size and location. Radical nephrectomy is rarely indicated unless the lesion is very large and partial nephrectomy is not possible or diagnosis is uncertain. Ablation is used in selected cases.

No aggressive follow-up is usually necessary.

Angiomyolipoma (AML)

Eighty percent of these benign clonal neoplasms (hamartomas) occur sporadically, mostly in middle-aged females. 20% are in association with tuberous sclerosis (TS)—an autosomal dominant syndrome characterized by mental retardation, epilepsy, adenoma sebaceum, and other hamartomas.

Half of TS patients develop AMLs. The mean age is 30 years, and 66% of patients are female. Frequently, AMLs are multifocal and bilateral.

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Pathology

AML is composed of blood vessels, smooth muscle, and fat. They are always considered benign, although extrarenal AMLs have been reported in venous system and hilar lymph nodes. Macroscopically, it looks like a well-circumscribed lump of fat. Solitary AMLs are more frequently found in the right kidney.

HMB-45 is a monoclonal antibody against a melanoma-associated antigen seen in AML and can differentiate cases from other renal cortical neoplasms.

Presentation

AMLs frequently present as incidental findings (>50%) on ultrasound or CT scans. They may present with flank pain, palpable mass, or painless hematuria. Massive and life-threatening retroperitoneal bleeding occurs in up to 10% of cases (Wunderlich syndrome).

Pregnant women appear to be at an increased risk for hemorrhage.

Investigations

Ultrasound reflects from fat, hence it has a characteristic bright echo pattern. This does not cast an acoustic shadow beyond, helping to distinguish an AML from a calculus. CT shows fatty tumor as low-density (Hounsfield units <10) in 86% of AMLs. If the proportion of fat is low, a definite diagnosis cannot be made.

Measurement of the diameter is relevant to treatment. On MRI, adipose tissue has high signal intensity on T1-weighted images and lower on T2-weighted images

Treatment

In studies, 52–82% of patients with AML >4 cm are symptomatic compared with only 23% with smaller tumors. Therefore, asymptomatic AMLs can be followed with serial ultrasound if <4 cm, while those bleeding or >4 cm should be treated surgically or by embolization.

Emergency nephrectomy or selective renal artery embolization may be life saving. In patients with TS, in whom multiple bilateral lesions are present, conservative treatment should be attempted.