Aneurysmal bone cyst, a study of ultrastructure and malignant transformation.

Four cases of aneurysmal bone cyst, of which one became malignant 7 years after irradiation, were studied by electron microscopy. The aneurysmal bone cyst was composed of four different types of stromal cells - fibroblasts, myofibroblasts, osteoblasts, and histiocytes - and osteo-clastlike multinucleated giant cells. The surface of blood spaces was devoid of specialized endothelium, which may explain the presence of large quantities of extravasated erythrocytes. Some histiocytes contained siderosomes. The malignant lesion consisted of two main types of stromal cells, of which one had electron lucent and the other electron dense cytoplasm. The stromal cells produced osteoid and the tumour was regarded as an osteosarcoma. The multinucleated giant cells resembled those observed in aneurysmal bone cysts, but the nuclei seemed to be more often spherical. It is concluded that irradiation of the aneurysmal bone cyst may cause sarcomatous transformation in a cell capable of producing osteoid.