A Prospective, Phase II Trial Using ctDNA to Initiate Post-operation Boost Therapy After NAC in TNBC

Inclusion Criteria: * Must have histologically or cytologically confirmed triple negative (ER-/PR-/HER2-) invasive breast cancer, clinical stage II-III at diagnosis (AJCC 6th edition) based on initial evaluation by physical examination and/or breast imaging prior to study registration.ER and PR will be considered negative if ≤ 1% of cells stain weakly positive. HER2 will be considered negative if scored 0 or 1+ by immunohistochemistry (IHC) or 2+ by IHC associated with a fluorescence in situ hybridization (FISH) ratio of \< 2.0 or \< 6 copies per cell. * Must receive preoperative (neoadjuvant) chemotherapy. NOTE: Acceptable preoperative regimens include those recommended by NCCN guidelines. Participants who received preoperative therapy as part of a clinical trial may enroll. * ctDNA positive at baseline, after NAC or after surgery * Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2 * Written informed consent to provide research blood samples and tumor samples * Patients must be willing to have frequent blood tests (every 3 months ) and receive a 12 month course of tislelizumab if randomised to tislelizumab treatment on ctDNA detection * No evidence of distant metastatic disease on staging scans conducted at the time of diagnosis Exclusion Criteria: * Previously participated in other interventional trials * Previous malignancy within 3 years of breast cancer diagnosis * Pregnancy or breastfeeding * No written consent * Unable to receive standard NAC and subsequent radiotherapy(if needed) * Active autoimmune disease