The aim of this study is to investigate whether subjects with breast cancer that have certain favorable features, after performing the surgery and radiation, the chemotherapy can be safely omitted in the treatment. In addition, the investigation looks at whether the omission of chemotherapy ensures a better quality of life. Participants decide, in consultation with their treating physician, whether they choose to be treated with adjuvant chemotherapy or not.

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.](streamdown:incomplete-link)

patients with pathological stage I TNBC (pT1a/b/cN0) and a high sTIL score (defined as ≥50% for patients ≥40 years; ≥75% for patients <40 years)

no IPD plan