| Definition | Invasion of the covering of the brain or spinal cord,  the meninges, by leukemic cells. Headache and vomiting may be prominent  manifestations. This situation may be diagnosed by examination of the  surrounding cerebrospinal fluid and identification of leukemic cells. The fluid  may be obtained by a relatively simple procedure referred to as lumbar puncture                                                         . Central nervous system (CNS) involvement was defined  as the presence of 5/µL cells in the CSF with leukemic blasts                    (cytomorphological) without major blood contamination(<50/3 erythrocytes) or as leukemic infiltration of the brain, assessed by cranial computed tomography.
 
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