Omission of Local Therapies in Women Patients With HER2-positive or Triple-negative Breast Cancer

This is a prospective phase 2 non-randomized non-inferiority trial with two independent investigational arms. In each arm a 2-stage non-inferiority group-sequential design will be implemented; the overall sample per arm is of 76 patients, 43 of which will be recruited at the first stage. For each arm a futility evaluation is foreseen at the first stage, the results of which will be presented to an independent Data Safety and Monitoring Committee (iDSMC) (see "Statistical analyses" paragraph).

Arm A will evaluate the omission of surgery in unifocal T1-2 clinically N0, M0 patients who will demonstrate maximum response after neoadjuvant systemic treatment (i.e. whole breast irradiation only).

Arm B will evaluate the omission of radiotherapy in patients with complete pathological response proved by standard conservative surgical treatment (i.e. conservative surgery only). Patients candidate to arm A, who refuse the omission of surgery and then operated, are eligible for arm B if complete pathological response is confirmed after standard surgical treatment. Moreover, patients who refuse VABB and N1 patients with complete pathological response confirmed by partial mastectomy and surgical staging of axilla (SLNB and or TAD) are also eligible for arm B. Arm B patients must have confirmed complete pathological response by local-regional surgery. Therefore, omission of radiotherapy will be ultimately offered to a cohort of patients with T1-2, N0-1, M0 breast cancer conservatively operated, in whom complete pathological response to a neoadjuvant therapies is confirmed on the entire surgical specimen of the tumor bed and of the sampled nodes.

In each study arm non inferiority of 5-year EFS will be tested against historical data. No between arms statistical comparison is foreseen.

Inclusion criteria

• Women ≥ 18y.
• Initial diagnosis of unifocal HER2-positive (regardless of HR status) or Triple-negative (HR positivity lower than 10% is allowed) T1-2, N0-1, M0 breast cancer.
• Treated with neoadjuvant systemic treatment according to the center recommendations.
• Maximum responders and/or complete pathological response proved by surgery.
• Scheduled for breast conservation.
• Giving specific informed consent.

Exclusion criteria

• One of the inclusion criteria missed.
• Residual ductal carcinoma in situ (DCIS) at VABB and/or surgery.
• Bilateral synchronous breast cancer.
• Previous malignancy within 5 years.
• BRCA1-2, PALB2 or p53 proven mutation carrier (VUS are exclusion criteria as well).
• Patients unable to perform regular follow up.