The goal of this clinical trial is to determine if the omission of sentinel lymph node biopsy is safe for early-stage breast cancer patients in our institution (Clinical Hospital Centre Rijeka).

Targeted group are postmenopausal patients, older than 55 years, diagnosed with hormone-receptor-positive breast cancer, with primary tumour less than 3 cm and unaffected axilla on clinical exam and axillary ultrasound.

The main questions we aim to answer are:

• if this treatment de-escalation affects patient oncological outcomes (cancer recurrence)?
• if this treatment de-escalation affects patient clinical outcomes (early and late complications related to axillary surgery)?
• if this omission of pathological examination of sentinel lymph node affects recommendations on postoperative treatments (irradiation and systemic therapy)? Researchers will compare outcomes between participants submitted to sentinel lymph node biopsy and participants in whom the procedure is omitted to answer those questions.

Participants will be asked to:

• decide in which group they wish to participate
• visit the clinic for checkups and tests once every 6 months in first two years and yearly thereafter up to 5 years

Inclusion Criteria:

• newly diagnosed hormone receptor positive, HER2 negative invasive breast cancer or DCIS with microinvasion, histologically confirmed, in postmenopausal patients older than 55 years, with primary tumour up to 3 cm and uninvolved axillary lymph nodes on clinical examination and on axillary ultrasound performed in Clinical Hospital Centre Rijeka. Patients should be eligible for breast conserving surgery and adjuvant whole breast irradiation.

Exclusion Criteria:

• Patients younger than 55 years of age and/or premenopausal, with cN1-3 status (confirmed cytologically or histologically), or with multiple suspicious lymph nodes on axillary ultrasound, regardless of cytological/histological confirmation.
• Patients diagnosed with pure breast carcinoma in situ (DCIS) without focal invasion
• Patients with tumour larger than 3 cm or with cT4 status, with de novo metastatic disease
• Patients with a breast cancer of HER2+ or triple-negative immunophenotype
• Patients who have received any type of neoadjuvant systemic therapy before surgery and those who have a previous personal history of invasive breast cancer
• Patients with extensive multifocal or multicentric breast cancer, patients scheduled for mastectomy, partial breast irradiation or who refuse recommended adjuvant systemic oncological treatment
• Patients who do not wish to participate in a clinical trial