• Histologically confirmed diagnosis of de novo classical Hodgkin Lymphoma (cHL). Note: Availability of either block or unstained slides plus stained slides used by the local pathologist to make diagnosis, and of all pathology reports is mandatory for the study to perform central pathology review for confirmation of cHL diagnosis and for biological biomarkers assessments. Central pathology confirmation is not required to start treatment;

• Patients \>= 65 years ineligible for frontline standard chemotherapy (mainly due to medical comorbidities);

• Treatment naïve;

• Measurable disease defined as presence of both fluorodeoxyglucose-avid nodal involvement and at least one nodal target lesion measurable in two diameters (and at least 1.5 cm in its major diameter); • Indication for systemic treatment, i.e., all stages except IA without a large tumor burden, as radiotherapy is regarded curative in those patients;

• Eastern Cooperative Oncology Group (ECOG) performance status (PS) \<= 2;

• Adequate organ and marrow function as defined below:

‣ Absolute neutrophil count (ANC) \> 109/L (without growth factor support within 7 days of ANC measurement), unless due to bone marrow involvement by lymphoma

⁃ Platelet \> 50 x 109/L (without growth factor support or transfusion within 7 days of platelets measurement) , unless due to bone marrow involvement by lymphoma

⁃ Hemoglobin \> 8 g/dL (prior transfusion is acceptable)

⁃ Creatinine clearance ≥ 30 ml/min (as estimated by the Cockcroft-Gault equation or as measured by nuclear medicine scan or 24-hour urine collection)

⁃ Aspartate aminotransferase (AST)/serum glutamic oxaloacetic transaminase, and alanine aminotransferase (ALT)/serum glutamic pyruvic transaminase ≤ 3.0 × upper limit of normal (ULN)

⁃ Serum total bilirubin \< 1.5 × ULN (or \< 3 x ULN in case of documented Gilbert's syndrome)

• Life expectancy ≥ 6 months;

• Men must agree to use effective contraception if sexually active. This applies for the time period between signing of the informed consent form and 4 months after last tislelizumab dose. The investigator or a designated associate is requested to advise the patient how to achieve highly effective birth control method, e.g. vasectomy, use of condoms or complete sexual abstinence, when this is in line with the preferred and usual lifestyle of the subject.The use of condoms by male patients is required unless the female partner is permanently sterile. Periodic abstinence (e.g., calendar, ovulation, symptothermal, post ovulation methods for the female partner) and withdrawal are not acceptable methods of contraception.

• Subject voluntarily signs and dates an informed consent form approved by an National Ethics Committee (NEC) prior to the initiation of any screening or study-specific procedures, indicating that they understand the purpose of and procedures required for the study and are willing to participate in it;

• Subject must be able to adhere to the study visit schedule and other protocol requirements, and to return to enrolling institution for follow-up (during the active monitoring phase of the study).