= CHLOROMA = Myeloid sarcoma = MYELOBLASTOMA

= uncommon extramedullary solid tumor consisting of primitive precursors of the granulocytic series of WBCs (myeloblasts, promyelocytes, myelocytes)

Peak age: 7–8 years; child >adult; M=F

Myeloid sarcoma is a rare extramedullary proliferation of immature myeloid cells occurring in 3–5% of AML patients.

Clinical setting:

1. patient with acute myelogenous leukemia (in 3–8%)
2. harbinger of AML in nonleukemic patient: usually developing within 1 year (rare)
3. indicator of impending blast crisis in CML (in 1%) / leukemic transformation in myelodysplastic syndromes (polycythemia rubra vera, myelofibrosis with myeloid metaplasia, hypereosinophilic syndrome)
4. during remission of hematologic malignancy (up to 20%)
5. isolated event

Associated with: chronic myeloid leukemia, myelodysplastic syndrome, essential thrombocythemia, polycythemia vera

• 60% are of green color (chloroma) ← high levels of myeloperoxidase (30% are white / gray / brown depending on preponderance of cell type + oxidative state of myeloperoxidase)

Location: often multifocal

1. common: bone (skull, orbit, paranasal sinus, periosteum); lymph nodes; soft tissues; skin; breast
2. less common: GU tract, GI system, head & neck, chest

Site: propensity for bone marrow (arises from bone marrow traversing haversian canal + reaching the periosteum), perineural + epidural tissue

Radiography:

• osteolysis with ill-defined margins

CT:

• soft-tissue mass infrequently with bone erosion / demineralization / periosteal reaction
• generally homogeneously isoattenuating to slightly hyperattenuating relative to muscle / brain
• homogeneous enhancement on CT / MR (DDx to hematoma / abscess)

MR:

• iso- to hypointense relative to gray matter / bone marrow / muscle on T1WI
• heterogeneously iso- to slightly hyperintense on T2WI

Prognosis: poor outcome; resolution under chemotherapy ± radiation therapy; recurrence rate of 23%

DDx: osteomyelitis, histiocytosis X, neuroblastoma, lymphoma, multiple myeloma