BENIGN RENAL TUMORS
There are several different kinds of benign renal tumors, which may originate from a wide range of cell types. Solid renal tumors, however, are generally malignant, with the probability of malignancy strongly correlating with tumor size. For example, one series found that masses greater than 4 cm in diameter were malignant in more than 90% of cases, whereas those less than 1 cm in diameter were malignant in 54% of cases. Although certain benign tumors have characteristic radiologic findings, most cannot be distinguished from malignant tumors using imaging alone. Thus most solid masses are surgically removed, with the final diagnosis rendered only after histopathologic examination. Some of the more common and well-documented benign renal tumors are presented here.
PAPILLARY
ADENOMA
Small
cortical lesions are seen in 7% to 23% of kidneys at the time of autopsy. These
are defined as papillary adenomas by the World Health Organization Classification
of Tumours when they possess papillary or tubular architecture of low nuclear
grade and are 5 mm or smaller in diameter. These masses are too small to be
reliably detected using modern imaging techniques.
PAPILLARY ADENOMA AND ONCOCYTOMA
ONCOCYTOMA
Oncocytomas
account for approxmately 5% of renal tumors in adults. They are often
incidental findings, occurring most commonly in those over the age of 50. They
are believed to originate from the intercalated cells of the collecting duct.
Classic radiographic features include a central stellate scar on CT imaging and
a spoke-wheel pattern of blood vessels on angiography; however, these findings
are unreliable and frequently absent, and angiography is now rarely per- formed
as part of the diagnostic workup. Thus oncocytomas cannot reliably be
distinguished from malignant tumors using noninvasive methods. Biopsies,
however, are also unreliable because oncocytoma-like areas can be found in
chromophobe renal cell carcinomas. Thus even a suspected oncocytoma is
generally treated like a renal cell carcinoma, with the definitive diagnosis
established only after surgical resection of the entire mass. Grossly, the
tumors appear well-circumscribed and mahogany brown, with a central stellate
scar seen in about one third of cases. There may be small areas of hemorrhage,
but necrosis should not be seen. Characteristic histopathologic findings include
round to polygonal cells that have strongly eosinophilic cytoplasm and round
nuclei, and which are arranged in nests, acini, tubules, and microcysts.
ANGIOMYOLIPOMA
Angiomyolipoma
(AML) of classic type are benign mesenchymal neoplasms composed of blood
vessels, smooth muscle, and adipose tissue. (There are rare AMLs of epithelioid
type that can exhibit malignant behavior.) About half of AMLs occur in
otherwise healthy individuals, generally in their fifth or sixth decade. Most of
the remainder occur in those with the genetic disorder known as tuberous
sclerosis (TS), which has numerous manifestations affecting multiple organ
systems. Besides renal AML, the other major clinical features of TS include
cerebral cortical tubers; subependymal nodules; retinal hamartomas; cardiac rhabdomyomas; facial angiofibromas, typically
in a malar distribution (formerly known as adenoma sebaceum); hypopigmented
macules known as ash-leaf spots; orange peel-like plaques on the lower back
known as shagreen patches; and periungual fibromas (flesh colored papules near
the fingernail bed). 70% to 80% of patients with tuberous sclerosis develop
renal AMLs, typically in their fourth decade. Benign renal cysts may also be
seen.
Like other
renal tumors, AMLs are often discovered as incidental findings on axial imaging.
Less commonly, the tumors may cause flank pain, hematuria, and a palpable
abdominal mass. In rare cases, life-threatening retroperitoneal hemorrhage may
occur, a phenomenon known as Wunderlich syndrome. AMLs can often be distinguished from other
renal masses using computed tomography (CT) because their fat content causes
them to appear as hypoattenuating lesions (less than 20 Hounsfield units). The
presence of fat, however, is not pathognomonic for AML, since certain primary
renal sarcomas (such as liposarcoma) and rare renal cell carcinomas may also
contain fat. In addition, AMLs some-times have little fat content that cannot
be visualized with CT imaging.
The optimal
management of an AML depends on tumor size and associated symptoms. Lesions
that are more than 4 cm in diameter or that cause pain or hematuria are managed
with embolization or extirpation (with a nephron-sparing technique whenever
possible).
Fat-poor tumors that cannot be confidently
distinguished from renal cell carcinoma should also be removed. In contrast,
patients with smaller, asymptomatic lesions that strongly resemble AML can be
monitored with serial imaging studies at 6- to 12-month intervals.
Grossly, an
AML is typically solid and well circum-scribed, although it is not
encapsulated. Patients with sporadic AML tend to have a single, large mass,
whereas those with TS tend to have multiple, small masses. Microscopically, the
mature adipose tissue varies in amount: in some cases it constitutes most of
the tumor, whereas in others only rare adipocytes are present. The blood
vessels have abnormally thick walls. The smooth muscle cells may be spindled
and grow in bundles or epithelioid with abundant eosinophilic cytoplasm.
Immunohistochemical stains are often valuable, especially in small needle
biopsy samples. The smooth muscle cells express both smooth muscle markers
(smooth muscle actin and h-caldesmon) and melanocytic markers (such as HMB-45
and Melan-A), and they are negative for epithelial markers (cytokeratin).
CYSTIC
NEPHROMA
Cystic
nephromas, sometimes called multilocular cystic nephromas, are benign tumors
most often discovered in women in their fourth or fifth decade. They should be
distinguished from cystic nephromas in children, which are considered to be
differentiated Wilms tumors. On axial imaging, cystic nephromas are well-
circumscribed and contain multiple noncommunicating, fluid-filled cystic spaces
and no calcifications. Thus, they resemble category III or IV cysts according to
the Bosniak classification scheme (see Plate 2-14), raising concern for renal
cell carcinoma. As a result, they are usually surgically resected with either a
radical or partial nephrectomy, depending on size and location. On
histopathologic examination, the septa consist of fibrous tissue lined by
cuboidal cells that may show hobnail features and flattening.
METANEPHRIC
ADENOMA
Metanephric
adenomas are rare benign tumors that are often incidental findings in middle
aged individuals, with a higher incidence among women than among men. Like
other renal tumors, they occasionally cause hematuria, abdominal or flank pain,
and a palpable abdominal mass. They may also cause secondary polycythemia. They
resemble renal cell carcinomas on axial imaging, and it is often difficult to
distinguish between the two. Thus these masses are usually surgically resected,
with the definitive diagnosis established on histopathologic analysis. Grossly,
most metanephric adenomas are 3 cm to 6 cm in diameter, and they are usually
solid with gray to tan to yellow cut surfaces. The histopathologic findings
include closely packed small, uniform round acini composed of small bland
nuclei. Psammoma bodies may be seen.
ANGIOMYOLIPOMA
ADDITIONAL
BENIGN MESENCHYMAL NEOPLASMS
In addition
to AML, several other benign mesenchymal renal neoplasms have been reported in
the kidney, including
juxtaglomerular cell tumor, leiomyoma, hemangioma, lymphangioma, schwannoma, lipoma, solitary fibrous tumor,
myxoma, and neurofibroma.
Juxtaglomerular
Tumor (Reninoma). Juxtaglomerular tumors are rare, benign,
renin-secreting masses derived from the juxtaglomerular apparatus. The classic
presentation is a young patient (25 years) with head-ache and hypertension
who is found to have elevated serum renin and aldosterone concentrations and a
low serum potassium concentration. Most tumors are unilateral, solitary, and
relatively small (2 to 3 cm). Treatment consists of surgical removal,
preferably with nephron-sparing surgery. Grossly, these tumors are
well-encapsulated, with tan to yellow solid cut surfaces. Microscopically, the
appearance is quite variable, with many tumors showing sheets of uniform round
cells.
Renal
Hemangioma. Renal hemangiomas are rare benign lesions that may cause either
microscopic or gross hematuria. Most lesions are small and cannot be visualized
using routine axial imaging of the kidney. Historically, arteriography was the
most sensitive imaging modality; however, most renal hemangiomas are now
diagnosed using cystoscopy, in which patients are noted to have unilateral
hematuria (i.e., gross blood emerging from only one ureteric orifice). Most
hemangiomas are located at the tip of a papilla and can range in size from
pinpoint to several centimeters in diameter. In the past these masses were
treated with nephrectomy or embolization; at present, however, treatment is us
ally electrocautery or laser ureteroscopic ablation.
