Venetoclax and Decitabine in R/R T-ALL

Inclusion Criteria: * Adults aged 19 years or older but less than 80 years * Eastern Cooperative Oncology Group Performance Score (ECOG PS) ≤ 2 * Confirmed diagnosis of T-cell lymphoblastic leukemia/lymphoma according to the 2016 World Health Organization criteria, with relapse or failure to achieve complete remission despite induction chemotherapy * Patients with peripheral blood leukocytes \<50,000/uL (reducing white blood cell count through hydroxyurea or leukapheresis prior to trial enrollment is allowed) * At the time of screening, a calculated glomerular filtration rate (GFR) ≥ 30 mL/min according to the Cockcroft-Gault formulas, or creatinine ≤ 1.4, total bilirubin ≤ 3.0 mg/dL, and AST and ALT \< x5 upper limit of normal (ULN) (however, if bilirubin elevation is due to Gilbert\'s syndrome or liver enzyme elevation is due to infiltration of leukemia/lymphoma, enrollment may be allowed even if the above conditions are exceeded) * Individuals who agree to the following contraceptive measures for a period of 3 months during treatment and for 3 months after completion: Exclusion Criteria: * Individuals in complete remission with previous treatment, if relapse or resistance is not confirmed by bone marrow examination or imaging/tissue examination. * Individuals who previously received venetoclax + decitabine treatment for T-lymphoblastic leukemia/lymphoma (participants who received venetoclax + decitabine treatment for a different type of cancer \[e.g., acute myeloid leukemia\] and have elapsed more than 1 year since the last treatment are allowed). * Pregnant or breastfeeding individuals. * Individuals who received systemic anticancer chemotherapy or participated in a clinical trial treatment within the past 2 weeks. * Individuals with active leukemia involving the central nervous system. * Individuals with a cancer type other than T-lymphoblastic leukemia/lymphoma that requires current active treatment (participants with a cancer type that has already been cured or is in a slow-progressing state without treatment, as determined by surgery/radiation/chemotherapy, may participate under the consultation of the clinical trial investigator). * Individuals with active human immunodeficiency virus (HIV) infection, hepatitis B/hepatitis C infection (participants without evidence of viral particles through PCR testing may be enrolled with the consent of an infectious disease specialist or hepatologist). * Uncontrolled bleeding. * Uncontrolled infection (bacterial, fungal, viral). * Uncontrolled mental illness. * Individuals who do not understand the informed consent or have difficulty in adequate communication, making them inappropriate for participation in the clinical trial. * Cases where the investigator judges that patient evaluation may be hindered or participation in the clinical trial is not appropriate. * Individuals who have a negative attitude towards participating in the clinical trial or who are unwilling to comply with the treatment and specimen collection schedule specified in the study protocol.