compounded by coagulopathy from the leukemia itself or from disseminated intravascular coagulation (DIC) related to sepsis Tumor Lysis Syndrome Tumor proliferation or chemotherapy can lead to rapid tumor lysis, in which the release of intracellular contents increases serum levels of lactate dehydrogenase (LDH), potassium, phosphate, and uric acid with potentially severe metabolic consequences Tumor lysis is common with acute leukemias and lymphomas but can also occur with neuroblastoma or other solid tumors with a very high tumor burden Calcium complexes with phosphate to form calcium phosphate crystals that can deposit in the renal tubules and other tissue sites This can result in renal insufficiency and hypocalcemia Urate crystals can precipitate in the acidic urine encountered in the renal tubules causing an obstructive uropathy and renal insufficiency Tumor lysi s syndrome (TLS) occurs when these electrolyte derangements occur with evidence of renal insufficiency or failure Fortunately, effective preventive strategies make clinically significant TLS a rare occurrence Screening and preemptive therapy with hydration and allopurinol is appropriate for all patients at risk for TLS The use of allopurinol or rasburicase to decrease uric acid levels is often driven by institutional protocol A full discussion of the management of tumor lysis is found in Section II Hyperleukocytosis Hyperleukocytosis is defined as WBC count above 100,000/mm3 When hyperleukocytosis is present, the clinical findings of leukostasis may develop from sludging of WBCs in the capillary beds The most vulnerable beds are those in the lungs and CNS where increased viscosity can cause either thrombosis or hemorrhage Leukostasis is much more common with myeloid leukemia than with ALL In the setting of hyperleukocytosis, hydration should be initiated immediately to reduce viscosity Transfusion of red blood cells and diuretics should be avoided to prevent further increases in blood viscosity, but platelet transfusion is appropriate to reduce the risk of CNS hemorrhage Leukocytopheresis, a technique to reduce blood viscosity acutely, should be initiated immediately in the presence of respiratory or neurologic symptoms, even if mild The use of prophylactic leukocytopheresis is controversial and should be considered only in consultation with a pediatric oncologist Since new-onset leukemia may be associated with coagulopathy, we recommend obtaining coagulation studies to determine the risk of bleeding during leukocytopheresis Extramedullary Involvement As leukemia develops, malignant cells may infiltrate nonhematopoietic tissues, producing adenopathy, hepatomegaly, and splenomegaly Anterior mediastinal masses (AMM) occur primarily with T-lineage ALL and can