Adult Renal Hamartomas

Abstract Introduction

Angiomyolipoma Leiomyoma Conclusions

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Leiomyoma

Epidemiology and Clinical Features

Adult renal leiomyomas are rare, benign spindle cell tumors that are found in approximately 5% of autopsy specimens (15,40). Despite histologic similarities between this lesion and fetal renal hamartoma (congenital mesoblastic nephroma), differentiation is usually not difficult; the latter is an infiltrative lesion that occurs in neonates, whereas the former is well circumscribed and is seen in adults.

Among those tumors found at autopsy, the average size is less than 5 mm (15). Clinically apparent leiomyomas are substantially larger but are far less common. As with angiomyolipomas, these lesions are more likely to be seen in middle-aged and older women (median age, 42 years) (40), have no metastatic potential, and are currently discovered as incidental findings in a large portion of cases. Leiomyomas share another feature with angiomyolipomas in that they occur with increased frequency in patients with tuberous sclerosis.

Steiner and coworkers (40) reviewed the literature for cases with clinical manifestations and found that more than 50% of the patients had a palpable mass or pain and 20% had hematuria. They divided leiomyomas into two groups: (a) those very small lesions occasionally discovered at autopsy and (b) the rare lesion that produces clinical symptoms (40). In view of the current widespread use of cross-sectional imaging, it is clear that there is a third group of leiomyomas, composed of lesions large enough to be radiologically detected but without clinical signs and symptoms.

Histologic Characteristics

Leiomyoma of the kidney consists predominantly of smooth muscle cells without pleomorphism or nuclear atypia. Most are considered to be hamartomatous. The fact that a large number of leiomyomas contain microscopic fat (4) raises the possibility that these tumors exist on a continuum with angiomyolipoma and lipoma. As discussed, smooth muscle is a component of all angiomyolipomas. Lipomas are very rare but often have a smooth muscle component. It has also been noted that thick-walled vessels are a frequent feature of all three lesions (2,4).

Pathologic Characteristics

Generally, leiomyomas are well-circumscribed, peripheral lesions (Figure 16, Figure 17). Steiner et al (40) reported that among clinically apparent lesions, 53% are subcapsular, 37% are capsular, and 10% are attached to the renal pelvis. These locations approximate the distribution of smooth muscle in the kidney and have led some authorities to designate the lesion as "capsuloma." As reported by Steiner and colleagues (40), lesions with a mean size of over 12 cm in diameter contain hemorrhage in 17% of cases and cystic degeneration in 27%.

Radiologic Features

Although cyst formation may occasionally be seen (40), leiomyomas typically appear as well-circumscribed, solid lesions (Figure 18, Figure 19). There may be a cleavage plane between the leiomyoma and the cortex (Figure 20), and the lesion may be extremely exophytic or attached to the cortex by only a small stalk (40,41). Irregular calcification may be seen in a minority of cases (41).

Differential Diagnosis

Renal leiomyosarcoma, a rare malignant smooth muscle tumor, lacks features that allow it to be distinguished from its benign counterpart, renal leiomyoma. More important, one must recall that, because renal adenocarcinoma is both common and characterized by a wide range of radiologic manifestations, it is the primary diagnostic consideration in the evaluation of a solid mass arising from the kidney, even if the mass is well circumscribed and homogeneous.

Treatment

It is clear from the description of the gross pathologic characteristics and the corresponding radiologic findings that differentiation of leiomyoma from renal adenocarcinoma is not possible and that the suspicion of a leiomyoma will prompt surgical exploration for diagnosis (40). However, identification of a well-circumscribed, peripheral renal mass, especially if it is small (less than 4 or 5 cm) and in a middle-aged woman, should allow one to suggest the diagnosis of a leiomyoma. Although this will not obviate surgical exploration, it may allow the surgeon to prepare for a renal-sparing operation in selected cases. The prognosis after resection of these benign lesions is excellent (40,42).

Abstract Introduction

Angiomyolipoma Leiomyoma Conclusions

Source Information References