Pathological features and clinical associations of 58 small incidental angiomyolipomas of the kidney.

Although angiomyolipoma (AML) of the kidney is a well-described and broadly studied neoplasm, little is known about clinical and morphological features of small incidental tumors occurring sporadically. Sections from 58 small AMLs found in 37 patients from 402 consecutive unselected autopsies were studied. A female predominance and a bimodal age distribution (30-50 and >70 years) were observed. No AML was found in a prepubertal patient. Six patients had multiple and bilateral AMLs (2-8 tumors per patient). Overall, the tumors ranged in size from 0.1 to 10 mm (mean = 1.5 mm) and were variably composed of smooth muscle and fat. Abnormal blood vessels were seen in only 4 tumors (7%). Most AMLs were located just beneath the renal capsule or at the corticomedullary junction. Pure leiomyoma-like (25 tumors) and lipoma-like (1 tumor) AMLs were found in patients younger than those with AMLs consisting of a mixture of components ( P = .03). AMLs located in the subcapsular and cortical parenchyma were mainly composed of smooth muscle ( P = .004), whereas fat predominated in AMLs in the medulla and corticomedullary junction ( P = .004). No correlation between morphology and size was found. In summary, this study demonstrates that 9% of an unselected population had AMLs. Their frequent location just beneath the capsule and in the corticomedullary junction suggests that these areas might promote their development. Finally, the proportions of smooth muscle and fat appear to be associated with their location in the renal parenchyma and with the patients' ages.