Mediastinal Myeloid Sarcoma

**Semantic type:** Neoplastic Process

**Definition:** A mass-forming malignant neoplasm that arises from the mediastinum and is characterized by the proliferation of myeloblasts or immature myeloid cells. It may present in association with or precede acute myeloid leukemia, or it may be the first manifestation of relapse of acute myeloid leukemia.