Splenic irradiation in myelofibrosis: effect on circulating myeloid progenitor cells.

We have investigated the mechanism of splenic irradiation-induced granulocytopenia in two patients with myelofibrosis and marked splenomegaly. Serial assays were performed for circulating granulocyte-monocyte progenitors capable of colony formation in vitro (CFU-C). For comparison, similar studies were performed on two patients receiving whole brain irradiation for glioma. Splenic irradiation caused a significant decrease in circulating CFU-C in the myelofibrosis patients. There was no decrease in circulating CFU-C in the brain-irradiated patients. No radiation-induced humoral inhibitor of granulopoiesis and no increased CFU-C radiosensitivity could be demonstrated in the myelofibrosis patients. These observations, taken together with previous data on splenic blood flow and pooling, suggest that the major mechanism of irradiation-induced granulocytopenia in myelofibrosis is destruction of proliferating precursor cells in the splenic tissue and sinusoids.