The activity of trastuzumab in early-stage, HER2-positive breast cancer, has been demonstrated in many studies, with meta-analyses showing that in combination with a variety of chemotherapy backbones, trastuzumab reduces the risk of recurrence by nearly half, and death by a third. However, treatment with trastuzumab can result in cardiotoxicity, including heart failure, as well as the significant cost of treatment and the requirement for patients to attend the chemotherapy unit for treatment every 3 weeks for one year. Therefore there has been increasing interest in identifying which patients can safely have less treatment. The investigators therefore propose a real-world, single arm, multicentre trial evaluating 6 months of HER2 targeted therapy, for patients with early-stage, HER2 positive breast cancer, who achieve a pathological complete response (pCR) with upfront systemic chemotherapy and HER2 targeted therapy.

The activity of trastuzumab in early-stage, HER2-positive breast cancer, has been demonstrated in many studies, with meta-analyses showing that in combination with a variety of chemotherapy backbones, trastuzumab reduces the risk of recurrence by nearly half, and death by a third. However, treatment with trastuzumab can result in cardiotoxicity, including heart failure, as well as the significant cost of treatment and the requirement for patients to attend the chemotherapy unit for treatment every 3 weeks for one year. Therefore there has been increasing interest in identifying which patients can safely have less treatment. The duration of adjuvant trastuzumab in early-stage breast cancer in the majority of studies was empirically set at 12 months, which became the de facto standard of care. Neoadjuvant treatment has become the new standard of care for patients with early-stage HER2-positive disease. While patients with residual disease benefit from further alternative treatment those with a pathological complete response (pCR) have an excellent outcome and are candidates for treatment de-escalation. The investigators therefore propose a real-world, single arm, multicentre trial evaluating 6 months of HER2 targeted therapy, for patients with early-stage, HER2 positive breast cancer, who achieve a pCR with upfront systemic chemotherapy and HER2 targeted therapy.

Inclusion Criteria:

• Early-stage (I-III, i.e. non-metastatic) as per AJCC 8th edition, ER/PR positive or negative, HER2-positive as per 2018 American Society of Clinical Oncology/College of American Pathologists human epidermal growth factor receptor 2 (HER2) testing in breast cancer guidelines.
• Treated with neoadjuvant chemotherapy and HER2 targeted therapy preoperatively with a pathological complete response (pCR) defined as the absence of residual invasive cancer on hematoxylin and eosin evaluation of the complete resected breast specimen and all sampled regional lymph nodes following completion of neoadjuvant systemic therapy at surgery. Must have received less than 6 months of HER2 targeted therapy
• Able to provide verbal consent and complete questionnaires in English or French

Exclusion Criteria:

• Residual invasive disease following neoadjuvant therapy, or metastatic disease
• Contraindication to further HER2-targeted therapy following completion of neoadjuvant treatment
• Any psychological, familial, sociological or geographical condition potentially hampering compliance with the trial protocol