Background: Different types of central nervous system (CNS) symptoms, associated either with neoplastic infiltrations or with destructive effects of cytostatic agents and/or ionizing radiation on brain tissue are observed during the therapy of hematopoietic system dysplasias. Case Report: The paper presents cases of encephalopathy in two 11- and 12-year-old boys, treated with polychemotherapy including intrathecal methotrexate, for leukemia and malignant non-Hodgkin lymphoma without involvement of the CNS. Both patients had single convulsive episodes during the therapy. Brain CT revealed multifocal hypo- or hyperdense lesions in the cortico-subcortical region. The overall clinical presentation suggested no involvement of the CNS in the neoplastic process. The treatment of the underlying disease was continued. Diffuse lesions observed in brain CT regressed completely after 6 and 12 weeks of follow-up, respectively, and no recurrent symptoms of encephalopathy were observed. Conclusions: 1. Early and distant complications taking the form of subclinical or symptomatic CNS damage can occur in the course of chemotherapy. 2. Determination of the cause of encephalopathy is associated with considerable difficulty despite the use of various diagnostic methods. The necessity of CNS imaging by magnetic resonance, both during initial diagnostics to assess the extent of neoplasia and at the later stages of treatment, especially if neurological symptoms occur, is emphasized. 3. In case of encephalopathy occurring during oncological treatment, modification of the treatment schedule should be considered. However, the dynamics of clinical and radiological course of the disease should also be taken into account.

Keywords: acute lymphoblastic leukemia, malignant lymphoma, leukoencephalopathy