PEComa presenting in bone: clinicopathologic analysis of 6 cases and literature review.

PEComas other than angiomyolipoma, lymphangioleiomyomatosis, and clear cell sugar tumor of the lung are relatively rare, and PEComas presenting in bone are especially rare. To further characterize their clinicopathologic features, 6 cases of PEComa which first presented in bone were retrieved from the authors' consult and surgical pathology files, including both primary and metastatic lesions. Four patients were female and 2 patients were male. The age at diagnosis ranged from 35 to 71 years, with a mean of 51.5 years. As for the 3 cases known definitely to have arisen in bone, the primary sites were right tibia in 2 cases and thoracic vertebra in 1 case. In the 2 cases, presenting in scapula and femur respectively, the primary sites could not be determined with certainty. In 1 case, the lesion was first found in humerus, but the primary tumor proved to be located in the uterus. Histologically, all the tumors were composed of both epithelioid and spindle cells, showing a nested pattern with elaborate vasculature. The characteristic thin walled vessels around which tumor cells were arranged tightly, or in a radiating fashion were seen in all cases. Two or more worrisome features (tumor size >5 cm, infiltrative growth pattern, high nuclear grade, high cellularity, necrosis, and mitotic activity >1/50 HPF) were identified in 4 cases, of which 2 were primary bone tumors; these cases were classified as "malignant" histologically. We conclude that both the primary and metastatic PEComas can present in bone, although both are rare. Combining our findings with the few earlier published reports, it may be suggested that primary PEComa of bone tends to involve lower extremities (5/7 cases). Histologically, they show similar cytomorphology, nested architecture, and characteristic vessels as PEComas at other sites. In addition, a significant subset of the primary bone lesions seem to be malignant.