We experienced a case of secondary CNS THRLBCL whose clinical, radiological, and pathological interpretation was inconclusive until the final diagnosis. T-lymphocytic hyperplasia in the context of immunodysregulation may be responsible for occult neoplastic lymphocytes. Further understanding of the molecular pathomechanisms of these diseases will allow us to give the condition a better clinical entity, which would be the first step in order to manage them more effectively.