Renal angiomyolipoma, fat-poor variant--a clinicopathologic mimicker of malignancy

Angiomyolipomas, composed of thick-walled blood vessels, smooth muscle, and adipose tissue, belong to the perivascular epithelioid cell neoplasms (PEComas), a family of tumors believed to be derived from perivascular epithelioid cells which co-express smooth muscle and melanocytic markers. Although most angiomyolipomas are benign, a subset of PEComas has metastatic potential. The pathologic and clinical spectrum of these tumors continues to evolve. We sought to evaluate a subset of renal angiomyolipomas with a minimal amount of fat. We studied 48 renal angiomyolipomas in 41 patients (33 females and 8 males). Based on the amount of adipose tissue, the lesions were categorized as fat-poor, fat-average, and fat-rich lesions (<25, 25-75, and >75 % of fat, respectively). Stains for smooth muscle actin, calponin, HMB-45, melanocyte-associated antigen PNL2, estrogen, and progesterone receptor were examined. Four patients (all females) had more than one lesion, four had coexistent uterine leiomyomata, two had coexistent renomedullary interstitial tumor, and males had only single lesions. Except for one woman, all lesions were sporadic. Twenty-nine were fat-poor (60 %) lesions; 8, fat-average (17 %) lesions; and 11, fat-rich (23 %) lesions. The fat content did not correlate with tumor size: the largest fat-poor and smallest fat-rich lesions were >6 and <2 cm, respectively. All lesions stained with smooth muscle actin and HMB-45; 41 % of tumors were positive for estrogen receptor (11 females and 1 male). No patient had metastases (follow-up 2-11 years). In our series, fat content in angiomyolipoma was not associated with tumor size. Fat-poor angiomyolipomas affected predominantly women and were morphologically and radiologically distinct as mimickers of malignancy. Whether they are biologically different from conventional tumors requires further studies.