Autopsy case of primary myelofibrosis in which myeloid sarcoma was the initial manifestation of tumor progression

Myeloid sarcoma (MyS) is defined as an extramedullary tumor‐forming neoplasm consisting of immature myeloid cells with/without maturation. We experienced a case involving a 68‐year‐old Japanese male patient who had been followed‐up for four years with a diagnosis of chronic idiopathic myelofibrosis/primary myelofibrosis (PMF) and noticed a painful mass in his left axilla. A wedge biopsy characterized the lesion as MyS that displayed megakaryoblastic/megakaryocytic differentiation. As his complete blood count included a few myeloid blasts (1% of WBC) and a bone marrow biopsy detected fibrosis without evidence of acute myelogenous leukemia (AML), a diagnosis of extramedullary blastic transformation of PMF was made, which was confirmed later by V617F mutation in Janus kinase‐2 in both initial bone marrow biopsy and axillary tumor biopsy specimens. The patient died of pneumonia eight months after developing the axillary tumor. At autopsy, multiple MyS masses were detected in his soft tissue, but his bone marrow only contained fibrosis. Although MyS rarely develops before the leukemic transformation of PMF, no evidence of AML could be found in the patient's bone marrow at any point during the course of his disease. Thus, it is possible that the blasts in his peripheral blood were derived from the remaining MyS. Furthermore, the present case indicates that extramedullary blastic transformation, which is occasionally seen in CML, can also occur in PMF. Therefore, it is important to recognize that there is a wide variation in the pathogeneses of MyS and PMF.