OPTImaL:Optimisation of Treatment for Patients With Low Stage Triple-negative Breast Cancer With High Stromal Tumor-infiltrating Lymphocytes

Inclusion Criteria: * Female or male patients; * \>=18 years; * Written informed consent; * TNBC (defined as: invasive carcinoma; ER/PR expression 0-9%; Human Epidermal Growth Factor Receptor 2 \[HER2\] negative \[0, 1+ or 2+ on immunohistochemistry, without HER2 amplification on in-situ hybridization\]) on the diagnostic biopsy and the surgical specimen; * Pathological stage I TNBC (according to the TNM staging 8th edition): * pT1a/b/c (≤2 cm), confirmed by an invasive component of ≤2 cm on the surgical specimen (microinvasive disease \[pT1mi, ≤1 mm) is not allowed); * pN0, confirmed by absence of malignant cells in the sentinel lymph node or any other lymph node after surgery (isolated tumor cells \[N0(i+)\] are not allowed); * No evidence of nodal or distant metastases (cN0M0) on pre and/or postoperative imaging examinations (performed following local/national guidelines, but must include an 18F-fluorodeoxyglucose positron emission tomography/computed tomography \[18F-FDG-PET/CT, at least from skull base to upper legs\] or computed tomography \[CT\] of neck/chest/abdomen/pelvis \[CT only if 18F-FDG-PET/CT would not be available; 18F-FDG-PET/CT mandatory in the Netherlands\]); * sTIL score of ≥50% for patients ≥40 years at the time of TNBC diagnosis and ≥75% for patients \<40 years at the time of TNBC diagnosis on an H\&E FFPE tissue slide on the surgical specimen, according to International Immuno-Oncology Biomarker Working Group on Breast Cancer (formerly International TILs Working Group) guidelines, by local and central review * Has undergone curative breast surgery (breast-conserving surgery or mastectomy and surgical axillary staging \[including at least sentinel lymph node procedure\]); * Absence of recurrence between curative breast surgery and expression of patient preference; * Eligible for radiotherapy (if indicated). Exclusion Criteria: * Prior disease history of invasive and/or non-invasive breast cancer, or ongoing treatment for invasive and/or non-invasive breast cancer; * Multifocal, multicentric or bilateral breast cancer at the time of screening; * Administration of neoadjuvant systemic therapy; * Presence of lymphovascular invasion on the diagnostic biopsy and/or the surgical specimen; * Other invasive malignancy within 5 years prior to inclusion, with the exception of ade-quately treated non-melanoma skin cancer, localized cervical cancer, localized and Gleason ≤6 prostate cancer; * Uncontrolled severe illness or medical condition; * Any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions should be assessed and discussed with the patient before the enrolment in the in the trial.