Does stereotactic ablative body radiotherapy (SABR) added to continued systemic therapy improve time to treatment failure compared with physician's choice of systemic therapy in oligoprogressive ER-positive, HER2-negative advanced breast cancer? Study protocol for a multicentre, randomised, open-label phase II trial in Australian tertiary cancer centres (AVATAR-II).

INTRODUCTION: Oligoprogressive disease (OPD) is a clinically significant pattern of progression observed in patients with oestrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer treated with endocrine therapy (ET) and cyclin-dependent kinase (CDK) 4/6 inhibitors. Stereotactic ablative body radiotherapy (SABR) is an emerging strategy that may ablate resistant subclones and prolong the benefit of systemic therapy. The AVATAR-II trial investigates whether the addition of SABR to continued systemic therapy can delay the need to change treatment strategy. METHODS AND ANALYSIS: AVATAR-II is a multicentre, randomised, open-label, phase II randomised controlled trial enrolling 74 patients with histologically confirmed ER-positive, HER2-negative advanced breast cancer and 1-5 sites of extracranial OPD. Eligible patients must have demonstrated clinical benefit (stable disease or partial response) from ET and CDK4/6 inhibitors for at least 6 months prior to randomisation. Participants will be randomised 1:1 to either SABR to all OPD sites with continuation of current systemic therapy (Arm A) or physician's choice of systemic therapy (Arm B). The primary endpoint is time to treatment failure, defined as progression not amenable to SABR, cessation of systemic therapy or death. The secondary endpoints include progression-free survival (PFS), PFS2, overall survival, treatment-related adverse events and patient-reported quality of life using the Functional Assessment of Cancer Therapy-Breast score. ETHICS AND DISSEMINATION: This study received ethical approval from the Peter MacCallum Cancer Centre Human Research Ethics Committee (HREC), under approval number 25/45 and HREC reference HREC/105165/PMCC. Results of the study will be disseminated via peer-reviewed presentation at scientific conferences and open-access publication. TRIAL REGISTRATION NUMBER: NCT06882499.