Undifferentiated embryonal sarcoma (UES) is a rare and highly malignant hepatic neoplasm, affecting almost exclusively the pediatric population. It has replaced malignant mesenchymoma, under which diagnostic term the first three cases were described. A link between embryonal sarcoma and mesenchymal hamartoma of the liver (MHL) has long been proposed, because of clinicopathologic overlaps of these entities; however, until recently, this association remained tenuous. Cases of UES arising in a background of mesenchymal hamartoma of the liver have previously been reported in two teenage girls. Discovery of a similar genetic abnormality in MHL and UES has clinched the supposed link between them. There have also been two reports of UES with prominent cystification, one associated with peripheral eosinophilia, and thereby masquerading as hydatid cyst of the liver. We report a case of UES arising in a young boy with MHL, with unusual histologic features, including large mesothelial-lined cysts and ectopic adrenal cortical tissue under Glisson's capsule.

The perivascular epithelioid cell family of tumors (PEComas), defined by their co-expression of melanocytic and muscle markers, includes angiomyolipoma, lymphangioleiomyoma, and clear cell "sugar" tumors of the lung, pancreas, and uterus. We present seven cases of a unique and previously unrecognized tumor of children and young adults, which represents a new addition to the PEComa group of tumors. Culled from three institutions over a 50-year period, all cases occurred in or immediately adjacent to the ligamentum teres and falciform ligament. Six patients were female and one male; their ages ranged from 3 to 21 years (median, 11 yrs). Tumor sizes ranged from 5 to 20 cm (median, 8 cm). All cases consisted of clear to faintly eosinophilic spindled cells arranged in fascicular and nested patterns. The cells had small but distinct nucleoli and low mitotic activity. Immunohistochemically, all cases were positive with antibodies to gp100 protein (HMB-45) and negative for S-100 protein. In three of the seven cases studied immunohistochemically, the tumors expressed smooth muscle actin, melan-A, microphthalmia transcription factor (MiTF), and myosin, but not desmin. No expression of the TSC2 gene product, tuberin, was seen in three cases. One case studied cytogenetically disclosed a t(3;10). Follow-up data, available in six of seven cases (median duration, 18 mos), showed five patients to be free of disease and one to have a radiographically presumed lung metastasis. We think these tumors comprise a new entity for which we propose the term "clear cell myomelanocytic tumor of the falciform ligament/ligamentum teres." The differential diagnosis of these tumors includes clear cell sarcoma of tendons and aponeuroses, leiomyosarcoma, and angiomyolipoma.