[Extramedullary haematopoiesis as a cause of a paravertebral mass in the thorax (author's transl)].

The radiological due to extensive extramedullary haematopoiesis taking place in the posterior mediastinum in two patients are described, and the literature is reviewed. Characteristic features are a polycyclic or spindle-shaped configuration of the mass, with bilateral paravertebral localisation in the caudal and middle third of the thorax. There is absence of inflammatory or neoplastic changes in the neighbouring skeleton, but in thalassaemia there may be changes in the paravertebral portion of the ribs. Computer tomographically, the combination of a solid paravertebral tumour, with adjacent rib changes, is characteristic of thalassaemia with extramedullary haematopoiesis. In the absence of other skeletal changes, computed tomography is valuable for differential diagnosis. Lymphomas occur considerably more often and may appear identical but, because of the well know fat content of myolipoma, the scan shows contrast values between -15 and + 40 HU. Because of their high vascularity, there is a marked increase in density following intravenous contrast medium.