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.