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Although it is well known that numerical and structural chromosomal aberrations are common in leukemic cells, tetraploidy and near-tetraploidy is a rare entity in acute myeloid leukemia (AML) [1-4]. We describe a rare case of AML-M0 with near-tetraploidy and superior vena cava syndrome (SVCS) due to a large mediastinal granulocytic sarcoma (GS). A 25-year-old male was admitted to our hospital with a progressive non-productive cough, dyspnea, pain on the left upper extremity and weight loss. One month ago, he was treated with a diagnosis of venous thrombosis because of the pain and swelling on his left upper extremity. At the time of admittance, physical examination revealed dyspnea, facial edema, swelling of the neck and upper extremities and dilated jugular and superficial veins of the chest. Peripheral lymph node or organomegaly was absent. His hematological findings were: hemoglobin, 14.1 g dl−1; hematocrit, 41.6%; MCV, 87.5 fl; WBC, 15.3 × 109 l−1 with 15% polymorphonuclear cells; 22% bands; 37% lymphocytes; 26% resembling blastic cells; and platelets 284 × 109 l−1. The chest X-ray showed enlarged mediastinum and thorax computed tomography (CT) demonstrated a multi-lobar mass with a diameter of 9 × 5 cm in the anterior mediastinum which also compressed the superior vena cava (Figure 1A). Abdominopelvic CT scan was normal. Mediastinoscopy was performed for definite diagnosis. The biopsy specimens from mediastinal mass revealed diffuse malignant infiltration in lipomatous tissue, which consisted of round, large sized cells. Nuclei were large, vesicular and contained two or three nucleolus. Cytoplasm was moderate in amount and clear. The specimens showed negativity for all of the antibodies and staining except for myeloperoxidase (MPO), CD34 and TdT (Figure 1B). Bone marrow aspirate revealed the presence of large blasts with a relatively low nuclear-cytoplasmic ratio, moderate chromatin pattern, two-to-three prominent nucleoli, no cytoplasmic granules and few cytoplasmic vacuolization. Most of the cells had heterogeneous shaped nuclei. No Auer rod was seen (Figure 1C). Cytochemistry revealed negativity for MPO, Sudan black B, periodic acid Schiff, combined esterase and acid phosphatase. Flow cytometric studies of the bone marrow cells revealed positivity for CD13, CD33, CD34, CD45, CD7 and HLA-DR. Cytogenetic analysis showed near tetraploidy (Figure 1D). Results of cytogenetic analysis of 20 metaphases of the bone marrow cells were as follows; 102, XXYY, +13, +14, +14, +15, +16, +18, +19, +21, +21, +22 [6] / 104, XXYY, +X, +Y, +3, +4, −7, −8, +10, +11, +13, +13, −14, +15, +15, +19, +21, +21, +22, +22 [9] / 98, XXYY, +3, +4, −6, +8, −11, +13, +14, −16, +19, +20, +21, +21 [5]. Neither apparent structural rearrangement nor normal clone was identified. Upon establishing the diagnosis (AML-M0/near tetraploidy+/mediastinal GS+) [5], he was given an induction chemotherapy consisting of daunorubicin (45 mg m−2 per day, days 1–3) and cytarabine (100 mg m−2 per day continuous infusion, days 1–7) and bone marrow remission was achieved. Then, he received the first consolidation chemotherapy consisting of idarubicin (10 mg m−2 per day, days 1–3) and cytarabine (1000 mg m−2 per day, days 1–6), followed by second consolidation chemotherapy consisting of cytarabine (6 gr m−2 per day, days 1–3) and 1800 cGy radiotherapy on mediastinum. The symptoms of SVCS improved gradually after the 5th day of induction chemotherapy. Likewise, the mediastinal mass decreased in size by 30% after induction chemotherapy, 70% after consolidation chemotherapies and disappeared completely after the completion of radiotherapy (Figures 1E and F). Presently, he is in the 10th month of his complete remission without any recurrence in the disease or GS.