The HER2-RADiCAL study (Response ADaptive CAre pLan) – Tailoring treatment for HER2 positive early breast cancer

1. Female or male, age ≥ 16 2. Histologically confirmed invasive breast cancer that is HER2-positive (IHC3+, and/or ISH positive/amplified) as determined by the local laboratory in accordance with national guidelines 3. Has received neoSACT chemotherapy with concomitant trastuzumab and pertuzumab 4. pCR (ypT0/is ypN0) in breast and sampled regional lymph nodes as per local pathology reporting 5. Imaging of breast and axilla prior to initiation of neoSACT and either: a) Breast primary radiological measurement ≤ 20mm prior to neoSACT and limited nodal involvement (cN1) confirmed by axillary core biopsy or FNA (cT1N1) OR b) Breast primary radiological measurement > 20mm but ≤ 50mm and node-negative (cT2N0) or limited nodal involvement (cT2N1) 6. Multiple ipsilateral cancers are permitted provided at least one meets the tumour size and axillary node inclusion criteria and none meet any of the exclusion criteria 7. Bilateral cancers are permitted provided at least one meets the tumour size and axillary node inclusion criteria and none meet any of the exclusion criteria 8. Pre-treatment diagnostic breast tumour biopsy sample available 9. Patient must be fit to continue treatment with trastuzumab and have no concomitant medical, psychiatric or social problems that might interfere with informed consent, adherence to the reduced treatment pathway or follow up 10. Provision of written informed consent to participate in HER2-RADiCAL

1. Evidence of metastatic disease at any time since diagnosis 2. Any residual invasive disease following neoSACT. This includes isolated tumour cells in axillary nodes or tissue or evidence of lymphovascular invasion in the breast. Persistent ductal or lobular non-invasive disease (DCIS or LCIS) is permitted. Resection margins must be deemed clear of any residual DCIS according to local MDT protocol. 3. Breast conserving primary surgery where it is known that breast irradiation will not be administered (e.g. due to contraindication or patient preference) 4. Intraoperative assessment of post-neoSACT axillary SLN using one stop nucleic acid amplification (OSNA) 5. Any planned further resectional surgery for breast cancer (including re-excision, mastectomy, or axillary surgery excluding reconstructive or risk reducing surgery) 6. HER2-negative invasive breast carcinoma 7. Breast cancer with clinical stage of T≥3 at diagnosis 8. Evidence of scarring (or other pathological features consistent with previous malignant involvement) in ≥4 axillary nodes or clinical nodal stage N≥2 at any time 9. Positive SLNB pre-neoadjuvant systemic therapy as this precludes determination of pCR 10. Pregnant and/or lactating women 11. Female patient of child-bearing potential, unwilling to use an effective form of contraception during trastuzumab treatment and for 7 months after their last dose of trastuzumab (NB. See section 5.5 for further guidance) 12. Previous diagnosis of invasive breast carcinoma 13. Previous diagnosis of any other (non-breast) malignancy unless disease-free for at least 5 years and considered to be at low risk of recurrence or treated basal cell or localised squamous cell carcinoma of the skin or cervical intraepithelial neoplasia 14. Chemotherapy administered following surgery (NB. Pertuzumab and/or trastuzumab may have been continued after surgery as per local practice prior to trial entry) 15. Has received > 9 cycles trastuzumab. In the event a patient has received 9 cycles prior to study entry then consent must occur within 3 weeks of last dose of trastuzumab. 16. Clinically significant cardiac disease within 12 months of starting trastuzumab, including unstable angina, acute myocardial infarction, New York Heart Association Class III or IV congestive heart failure, cerebral vascular accident, or cardiac arrhythmia associated with haemodynamic instability 17. Left ventricular ejection fraction (LVEF) less than 50% on most recent cardiac imaging 18. History of interstitial lung disease 19. Any medical or other contra-indication to continuing trastuzumab