Computed tomography of intracranial extramedullary hematopoiesis.

Patients with extramedullary hematopoiesis may be divided into two main groups. The first group shows paraosseous foci that may result from "herniation" of medullary tissue from the underlying bone. This is seen in hemolytic disorders where the marrow has tremendous activity. The second group shows extraosseous soft tissue foci, which may arise from multipotential stem cells. This occurs when the marrow activity is ineffectual, as in the myeloproliferative disorders or, rarely, with toxic or tumoral marrow destruction. We present a patient with myelosclerosis and four intracranial masses of hematopoietic tissue in the subdural space. We believe this is the first description of the cranial CT findings in this condition.