Extramedullary erythropoiesis in chronic subdural hematoma simulating metastatic small round cell tumor.

We report two cases of extramedullary erythropoiesis within chronic subdural hematoma that caused diagnostic confusion. In both cases, the initial favored diagnosis by the submitting pathologists was that of a metastatic malignant tumor, including lymphoma, carcinoma, and malignant melanoma. In both cases, the subdural chronic hematoma contained cohesive clusters of small round blue cells with scant cytoplasm and round hyperchromatic nuclei. In both cases, some mitotic figures were identified. There was no gross or microscopic evidence of a meningeal mass lesion. The erythroblastic nature of the cells was confirmed using immunohistochemistry for CD43, glycophorin A, and erythropoietin A. It is important for surgical pathologists to be aware of this benign process and not to overinterpret it as either a primary or metastatic malignant tumor.