Mature Type T-Lymphoblastic Leukemia/Lymphoma Presenting with Isolated Central Nervous System Symptomatology in a Patient with Giant Cell Arteritis on Long-Term Steroid Treatment

T-lymphoblastic leukemia/lymphoma (T-ALL/T-LBL) is a malignancy comprised of T-lymphoblasts that can present as one of four clinical subtypes (pro-T, pre-T, cortical T, and mature T). Clinical presentation is typically characterized by leukocytosis with diffuse lymphadenopathy and/or hepatosplenomegaly. Beyond clinical presentation, specific immunophenotypic and cytogenetic classifications are utilized to diagnose mature T-ALL. In later disease stages it can spread to the central nervous system (CNS); however, presentation of mature T-ALL by way of CNS pathology and clinical symptomatology alone is rare. Even more rare is the presence of poor prognostic factors without correlating significant clinical presentation. We present a case of mature T-ALL in an elderly female with isolated CNS symptoms in combination with poor prognostic factors including terminal deoxynucleotidyl transferase (TdT) negativity and a complex karyotype. Our patient lacked the classical symptomatology and laboratory findings of mature T-ALL but deteriorated quickly upon diagnosis due to the aggressive genetic profile of her cancer.