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.
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.
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 (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 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 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.
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.